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Monday, June 15, 2009

A Summer Minnesota 'Snowstorm'!!!


David C. Zlesak, University of Minnesota Extension Educator

‘Snowstorm’ is the first wand flower (Gaura lindheimeri) release from the Flower Breeding and Genetics program led by Dr. Neil Anderson! Wand flower is a prized garden subject because it: blooms continually throughout the summer, is a great filler plant that mixes well with other plants in garden beds and containers, and it moves in the wind adding extra ornamental appeal. Gaura lindheimeri has grown in popularity over the past decade and is among the top 25 selling perennials in the US market today. Although this Gulf Coast native (Texas and Louisiana) is reliably hardy only to zone 6, it serves as a great annual in Minnesota.

Photo 1: Buds of 'Snowstorm' are pink and petals open white. David Zlesak

The overall goal of the University of Minnesota wand flower breeding program is to develop Minnesota hardy cultivars. Work continues towards this goal trying to generate interspecific hybrids between the highly ornamental G. lindheimeri and the hardy G. coccinea (native from Texas into Canada). Some of the G. coccinea forms in the breeding program a have wonderfully sweet fragrance and other ornamental characteristics not found in G. lindheimeri. Gaura coccinea prefers dry growing conditions and can be found in western Minnesota. Other Gaura species native to the Southern US possess diversity for foliage and color and plant habit and are being explored as parents within the breeding program as well.

Although it will take some time to continue to unlock crossing barriers between G. coccinea and G. lindheimeri, G. lindheimeri ‘Snowstorm’ is a wonderful cultivar and great addition to Minnesota gardens. ‘Snowstorm’ has proven to be a very vigorous selection with prolific flowering potential. Crowns send up multiple, wiry stems that move in the breeze and are covered with blush pink buds opening white. Plants grow to 2-3 feet tall and the airy flowering stems mix well with other neighboring plant materials. ‘Snowstorm’ is an especially nice accent mixed with other plants in medium to large-sized containers. It also works well in ground beds as a solitary accent plant, mass planting, or border.


Photo 2: Plants of 'Snowstorm' bloom prolifically throughout the growing season. David Zlesak

‘Snowstorm’ and other G. lindheimeri cultivars do best in full to part sun and planted in fertile, well drained soil. They are somewhat drought tolerant and if spent flowering stems are removed they can flower even more prolifically.

There are multiple licensed Minnesota wholesale propagators of ‘Snowstorm’ and it is currently available in many area garden centers. Don’t miss out on this summer ‘Snowstorm’!!

Mid-June Lawncare Tips


Bob Mugaas, University of Minnesota Extension Educator

While this past May went down as one of the driest Mays ever, many areas around the Twin Cities have received some rain during the first part of June. That has both been very helpful for our lawn grasses and has also provided the needed moisture for many of our weed seeds, especially crabgrass to begin actively germinating. In most areas of southern Minnesota we are past the time when preemergent herbicides for crabgrass control will be effective. However, that doesn’t mean control is not possible. Those small crabgrass seedlings can effectively be controlled with postemergent herbicides. One of the newer active ingredients for crabgrass control is quinclorac. This is a very good herbicide for controlling crabgrass once it’s germinated and the seedlings are visible. However, it is usually mixed with other broadleaf control products and marketed for home use as general weed and crabgrass killers. However, it is important to note that with these combination products, you will be applying other herbicides that are not needed or even effective at controlling crabgrass. Hence, this unnecessarily introduces these other materials into the environment; something we try to minimize doing whenever possible.

Photo 1: This mixed stand of young crabgrass plants and other broadleaf weeds is a good candidate for a combination herbicide that will selectively control the crabgrass as well as the broadleaf weeds. Bob Mugaas


Photo 2: Dead grass spots resulting from using a glyphosate containing product designed to kill all vegetation that it comes in contact with. These areas will need to be resodded or reseeded. Bob Mugaas

Where you have a mixed stand of weeds, both broadleaf and crabgrass, a combination product like the one mentioned above, can be a good alternative to buying separate products for each category of weeds. Be careful to choose products that are safe for use on your lawn. Those containing the active ingredient glyphosate will kill all grasses and broadleaf weeds leaving the lawn scarred with unsightly brown dead spots that will need to be resodded or reseeded. Remember to always follow product label directions exactly regarding a products use. Where only a few crabgrass plants are present, they can easily be hand pulled from a moist soil thereby avoiding the use of an herbicide altogether.

Even though some rain has arrived to help out our lawns and gardens, many lawns are still showing signs of the very dry conditions encountered during the month of May. Those signs range from small brown spots intermingled with green grass to larger, completely brown areas. Future rainfall patterns and amounts for the remainder of June are, of course, not known. If we continue with our cool to mild temperatures and periodically receive some rainfall over the next couple of weeks, supplemental irrigation needs for most lawns should be minimal, if needed at all during that time. Avoiding excessive traffic and play on the lawn during those times when lawns are brown and dry will help avoid additional, often more permanent injury. Likewise, herbicide applications to lawns under drought stress can cause injury to lawn grasses that would otherwise be quite tolerant of those products.


Photo 3: Typical drought stress symptom with dry, brown patches interspersed with patches of green grass. Bob Mugaas

In general, lawn grasses should be mowed at higher heights of cut from now through the remainder of the summer. Flowering of our cool season lawn grasses is winding down. As those flower heads are being mowed off, remember that those shoots ultimately die after flowering. Hence, folks will often complain about a larger amount of course, brown grass stems in their lawns making them appear thinner and even a bit unsightly. In fact, our lawns are somewhat thinner this time of year due to the dying back of those spent flower stalks. It’s really nothing to worry about. This is a natural occurrence every year. Our lawn grasses begin to regrow new rhizomes and shoots in a few weeks, which will help thicken up the lawn. This regrowth process is one of the reasons why our fall lawn fertilization is so important to the long term health of our lawns. But more on that in later article.

Rose Rust Takes Off this Spring


Michelle Grabowski, University of Minnesota Extension Educator

Cool temperatures this spring seem to be encouraging rust fungi on roses. Several different species of Phragmidium (the rust fungus) can infect both wild and cultivated roses. Gardeners should keep an eye out for two different forms of this fungus.

Bright powdery orange spores, known as uredinia, are likely to catch a gardener’s eye. These spores form in raised pustules on the underside of infected leaves, stems, or petioles (central portion of the leaf that the individual leaflets are connected to). Yellow to brown leaf spots may be noticeable from the top surface of the leaf but may not form on all rose cultivars. Infected petioles and young green stems may actually become twisted and distorted around the site of the infection. Rust fungi can infect all plant parts except the roots and gardeners may notice bright orange pustules in unusual places!

Photo 1: Uredinia on the lower surface of a rose leaf. Michelle Grabowski

If the bright orange uredinia are found on a rose plant, carefully inspect all stems that are one year or older for dark brown to black raised bumps forcing their way through the epidermis. These dark brown to black pustules are the same rust fungus in a different form. Dark brown to black teliospores fill these pustules and are tough enough to survive Minnesota’s harsh winter weather.


Photo 2: Uredinia on a young rose stem. Michelle Grabowski

Rust fungi have a complicated lifecycle that involve 5 different unique spore stages. The two described above are those most easily seen by gardeners. The bright orange uredinia can spread on the wind to infect more leaves within the plant or on nearby roses. These spores need cool temperatures, preferably 65-70F, and 2-4 hours of continuous moisture on the plant surface in order to start a new infection. If weather conditions become hot and dry, spread of the rust fungi is dramatically reduced.

Teliospores typically do not form until late summer or early fall. They are often found in the same area as the bright orange uredinia and therefore can also occur on leaves, stems and petioles. If gardeners are seeing the dark brown to black teliospore pustules now, these were formed last year and are likely the source of this year’s infections.

Photo 3: Rust fungi can infect all above ground parts of the rose including rose hips. Michelle Grabowski

The amount of damage that rust fungi can do to a rose plant varies greatly depending on several factors. Most roses are somewhat susceptible to the disease, but a few cultivars are particularly sensitive and will lose leaves in response to just a few bright orange pustules. For most roses, disease severity depends on weather conditions. If cool damp weather persists the rust fungi can grow and spread dramatically. If the summer turns hot and dry, the rust may dry up as well. A few leaf spots will not seriously affect the health of the plant but if infections on stems grow large enough to girdle the stem or otherwise restrict the flow of water and nutrients, that stem may be killed. The fungi that cause rust on roses will not infect other plants in the garden.


Managing Rust on Rose

Damage from rust on roses can be minimized by slowing the spread of the disease.

  • Examine the underside of the rose leaves and young green shoots for bright orange pustules.

  • Pinch off infected leaves and shoots. Collect diseased plant parts in a bag and remove them from the garden. Never remove more than 1/3rd of a plant’s foliage even if it means leaving a few leaf spots on the plant!

  • Carefully inspect last year’s stems for black to dark brown raised pustules. If present, prune out and destroy all infected stems.

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  • If last year’s leaves are still present around the base of the plant, rake them up and remove them from the garden.

  • Prune roses to promote good air movement in and around the plant.

  • When planting new roses allow enough space for the mature plant. Avoid crowding plants together which increases humidity in the canopy.

  • Use drip irrigation or a soaker hose instead of sprinkler irrigation to keep leaves dry.

  • Several fungicides including those with the active ingredient triforine and propicanazole will protect healthy rose leaves from infection by the rust fungi. Always read and follow all label instructions when using a pesticide! In Minnesota fungicides are rarely necessary.

Remember a little bit of rose rust will not seriously affect the health of the plant. Roses will tolerate leaf spots as long as green leaf tissue remains. Stem infections should be taken more seriously as they can affect a larger portion of the plant. In both cases use of good sanitation and moisture reducing practices are often enough to successfully manage the disease.

Photo 4: Black pustules of teliospores on last year's infected stems plus newly infected leaves. Michelle Grabowski

Water-Wise Hardscapes

Waterosity, the 2009 summer exhibition at the Minnesota Landscape Arboretum, opened on June 6 and explores and celebrates the beauty and the complex interdependence of people, plants, and water. Providing homeowners with environmentally responsible ways to manage water on their property is a key theme of Waterosity. Water-wise hardscaping includes green roofs and permeable paving and both technologies help to reduce the amount of stormwater that washes over roofs and paved areas (driveways, sidewalks, and streets), carrying sediment and pollution into lakes, rivers, streams, and wetlands. Green roofs and permeable paving displays are part of the new permanent arboretum exhibit called Harvest Your Rain.

For more information on Waterosity displays and programming, please visit the Waterosity website and Waterosity Comes to the Arboretum in the May 1, 2009 Yard and Garden News.

Permeable Paving

Peter Moe, Director of Operations, University of Minnesota Landscape Arboretum

Permeable, or porous, paving is a type of paving that allows water to drain through to the ground below. Water can then directly infiltrate on-site rather than being diverted to a drain for off site treatment. These systems protect surface water, recharge aquifers, reduce flooding and reduce the need for traditional storm sewer systems.

Photo 1: Permeable pavers. Carolin Dittmann

There are three basic types of permeable paving:

  • Porous asphalt or porous concrete mixescontain little sand or fine materials and rely on the concrete or asphalt binder to hold the stones and crushed rock together, but still leave voids for water to pass through. Both resemble a rice crispy bar.

  • Permeable paver systems typically consist of highly drainable layers below a layer of concrete pavers that are designed to allow water to pass through joints between them. They resemble typical modular pavers or bricks but have corners missing or built in spacers that allow for water infiltration.

  • Turf reinforcement systems offer support for vehicles and protects the grass from the traffic damage while allowing natural groundwater replenishment. These systems come as plastic grids with cells that are filled with a sandy topsoil mix and planted with grass. When properly installed, the turf grows over the height of the plastic reinforcing panels and is indistinguishable from a non-reinforced lawn.

Photo 2: Porous concrete. National Ready Mixed Concrete Association

General recommendations

Permeable paving should be considered on all projects and is a useful tool for stormwater management. It is particularly practical for projects with tight sites and strict stormwater run-off regulations.

Permeable asphalt and concrete and permeable paving blocks are designed for the heaviest use; they are meant for daily, low-speed use such as in parking lots, driveways, residential drives and similar areas. Generally, turf support structures are intended for infrequently used areas, such as event grounds, fire lanes, overflow or event parking, and other spaces where there is little regular traffic.

Environmental Context

The use of permeable paving decreases peak flow rates in area lakes, streams and rivers during a rain event because it allows water to infiltrate into the ground on site. This infiltration reduces the amount and speed of water leaving the site during a rain event. Permeable paving also reduces the urban heat island effect and can reduce the need for air conditioning.

Design and Installation

  • Soil type will affect infiltration rates and must be considered when designing a permeable paver system. Subsurface drain tile may be required if clay subsoils are present.

  • Subbase depth depends on the system selected, existing soils and the type of traffic the site will receive. Additional coarse, crushed rock with no fine materials may be required for temporary water storage.

  • Careful installation by trained personnel will prevent settling and result in a system that will continue to function properly and be easier to maintain.

Photo 3: Construction of the permeable paver section of the display at the Teaching Center at the Minnesota Landscape Arboretum.


Porous asphalt, porous concrete and permeable pavers should be cleaned 2-3 times each year with a street sweeper to prevent sand, dirt, seeds and other materials from plugging the surface. Sand should not be applied and typically isn’t needed since water does not pool on the surface and freeze. Snow plows and snow blowers will not damage a properly installed system.

Reinforced turf may require more watering since the sand and gravel base do not hold much water in between rain showers. Slow release fertilizer will keep the grass vigorous and allow for faster recovery after vehicle or foot traffic.


The cost of permeable paving systems varies by technology and location. The estimates provided here are based on information from several sources including, the Low Impact Development Center (,, and the Seattle Rite of Way Manual.

Paving Type
Cost/sf (installed)
$0.50 - $1.00
Porous Concrete
$2.00 - $6.50
Grass Reinforcing System
$1.50 - $5.75
Interlocking Concrete Paving Blocks
$5.00 - $10.00

Green Roofs at the University of Minnesota's Landscape Arboretum


Jonathon Hensley, University of Minnesota Graduate Student

The dense, accelerated pace of modern urban development has affected many of the earth’s natural processes. Asphalt and concrete rooftops, roads, and parking lots cover up to seventy percent of land area in dense cities like New York, while open space in sprawling cities like Phoenix, Arizona is lost to development at a rate of 1.2 acres per hour.1

Approximately 1.5% of the continental United States, an area roughly equivalent to the state of Ohio, was covered by impervious surfaces in 2004.2 This percentage continues to grow and can be as high as 75% in urban areas. Of those impervious surfaces (not allowing the permeation of water), roofs can constitute a significant percentage. Such growth in impervious surfaces can result in a variety of environmental impacts including reduced aquifer recharging, overwhelmed storm water systems, urban heating, decreased surface water quality, and increased air ozone and particulate concentrations. These negative environmental effects impact residents and municipalities by affecting: clean water availability, storm water and sewage infrastructure costs, decreased runoff water quality, decreased employee productivity, decreased wildlife habitat, and increased operating costs of buildings through increased heating and/or cooling costs. Alternative solutions to traditional impervious building methods are being sought in order to mediate these negative environmental impacts and reestablish the green spaces desperately needed in our metropolitan spaces.

Photo 1: The green roof concept. Jonathan Hensley

Green roofs are becoming an increasingly popular alternative to traditional roofs in metropolitan/suburban areas to reduce the environmental impact of impervious building roofs and to increase green spaces in downtown metropolitan areas. In fact, they are being used to increase green spaces across all areas of commercial development. The above illustration identifies the significant impacts of land development on a local hydrologic cycle; site development substantially increases surface runoff and the removal of tree canopy interception strips away the natural buffering capacities of green spaces- this increase in surface runoff and the removal of green space buffering has had far reaching consequences for our densely populated metropolitan spaces.


Photo 2: State of Maryland: Dept. of the Environment

The environmental impacts and/or benefits of green roofs in this scenario can include decreased water and nutrient run-off from developed land, reduced ambient air temperature, reduced atmospheric CO2 levels, new garden areas for people and wildlife, and reduced airborne particulates and noise pollution in congested urban environments. Financial benefits can include reduced energy costs for buildings, reduced city infrastructure needs/costs, and improved psychological well-being of citizens resulting in increased productivity among employees. As roofs can account for up to 15-30% of impervious surfaces in cities, the impact of green roofs on the urban environment and citizen well-being can be high.


Photo 3: Extensive green roof, Berlin, Germany. Ufa Fabrik

Green roofs can be built in a variety of contemporary styles; the most prominent distinction is between extensive and intensive systems. Intensive green roofs have been used in our country longer than extensive green roofs, but their reliance on deep soil media profiles and irrigation/fertilization to support traditional landscape plants has limited the progress and general utility of this style. By comparison, extensive systems rely on profiles generally not more than six inches in depth and have shown great merit for retrofitting existing buildings that do not have the appropriate structural support for an intensive system.

Extensive green roofs are composed of several material layers designed to protect a building and offer functionality in a metropolitan space. These layers are often composed of waterproofing membranes, a root impermeable layer, a drainage layer, extensive growing media, and vegetation. These systems have been shown in independent studies to positively impact urban/metropolitan environments by re-establishing green biological buffers that mediate storm water runoff, capture particulate pollution, reduce building energy costs for heating and cooling, provide sanctuary for urban wildlife, and improve citizen morale.


Photo 4: Green Roof Service LLC

1Community Cartography/New York City Planning Agency/Smart Growth America; F. Kaid Benfield, Matthew D. Raimi, and Donald D. T. Chen, Once There Were Greenfields: How Urban Sprawl Is Undermining America’s Environment, Economy and Social Fabric (New York: Natural Resources Defense Council, 1999).

2 Elvidge, C., C. Milesi, J. Dietz, B. Tuttle, P. Sutton, R. Nemani, and J. Vogelmann. 2004. U.S.

Constructed Area Approaches Size of Ohio. Eos: 85(24): 233-240.

For more information on this topic, please see the following references:

Dunnett, N., and K. Kingsbury. 2004. Planting green roofs and living walls. Timber Press,

Portland, Oregon. USA.

Snodgrass, E., and L. Snodgrass. 2006. Green roof plants: a resource and planting guide. Timber Press, Portland, Oregon.

Oriental Poppies Take Center Stage!


David C. Zlesak, University of Minnesota Extension Educator

Glowing, vibrant blooms bring oriental poppies (Papaver orientale) to center stage this time of year. The large (4-6”) flowers have crêpe paper-like petals and are held high above the foliage on strong, yet wiry stems. In a gentle breeze they almost appear to dance. The finely cut foliage makes a great backdrop to the flowers and the abundant, almost downy hairs that cover the foliage and stems set Oriental poppies apart from most other commonly grown poppies.

Native to central Asia where summers are warm and dry, these magnificent poppies have developed a unique adaptation method. When moisture is relatively abundant in spring, leaves emerge and plants quickly come into flower. As temperatures rise and moisture becomes less abundant, the foliage begins to die back and plants go dormant. As temperatures begin to cool in the fall and rains return, new foliage again emerges.

Photo 1: This double-flowered Oriental poppy is a prolific bloomer. David Zlesak

Although Oriental poppies are very cold hardy and survive routinely even into zone 3, care must be given to plant them in well drained soil to ensure survival. Excessive moisture, especially during summer months, can lead to their demise. In addition to having well drained soil, full sun down to half a day of sun is also important for success. In the right location they are very low maintenance. In fact, we can often find huge stands of Oriental poppies doing great on old farmsteads.


Photo 2: Oriental poppies have done well in this location and have multiplied. David Zlesak

The most common flower color for Oriental poppies is orange-scarlet. Extensive breeding and selection in the late 1800’s has led to Oriental poppies that come in white, pink, peach, maroon, or purple. Many Oriental poppies have a black blotch at their base. In some forms this blotch is particularly large and ornamental. Variation is also found across different varieties for petal number as well as petal margin. There are single forms and double forms and forms that have smooth petal margins and some with frilly petal margins. There are both seed propagated varieties on the market and vegetatively propagated cultivars.

Oriental poppies do not transplant well. After suffering root injury from being transplanted, they typically have stunted growth and do not bloom well during the next flowering cycle. Typically after a couple seasons they fully recover. Elite clones are typically vegetatively propagated using root cuttings. In our climate, early spring tends to be the best time to take root cuttings. This can easily be done when moving an existing plant. Often roots left behind will sprout and lead to new plants. Sections of thicker, fleshier roots from a couple to few inches in length work well. Plant the end of the root that was closest to the crown of the parent plant closer to the soil surface, or if you lost track, plant roots horizontally just below the soil surface. When shoots form at the cut end that was closer to the crown of the plant they need to be relatively close to the soil surface to be able to emerge. It takes four to eight weeks typically for new shoots to form and emerge from a root cutting.

It can be challenging to find potted Oriental poppies in the garden center because they do not respond well to typically humid and moist greenhouse production conditions. Fortunately, they can readily be purchased as dormant plants from mail order companies or started from seed.

Caterpillars on Blueberries

Jeff Hahn, Asst. Extension Entomologist

There are several caterpillars that have been detected feeding on the leaves of blueberries recently. One species is the copper underwing, Amphipyra pyramidoides. This insect, also known as the pyramidal fruitworm, is bluish green with a thin yellow stripe running the length of its body along its sides and small whitish patches. It also possesses a conspicuous hump on the end of the abdomen and grows to about 1 ½ inches when fully grown. This caterpillar feeds on a wide variety of plants in addition to blueberry, such as trees (e.g. apple, basswood, maple, oak), shrubs (e.g. lilac, viburnum, and rose), and fruit including grape, raspberry, and currant.

You can also find forest tent caterpillars, Malacosoma disstria, in your blueberries. Also referred to as armyworms, these caterpillars are easily identified by their blue and black bodies, the distinctive white footprint shaped spots on their back as well as hairs that stick out along the sides of their body. These caterpillars are about two inches long when fully grown. Despite their name, they do not make conspicuous webs in trees. They commonly feed on many deciduous trees, including aspen, birch, maple, crab apple, apple, ash, oak, and elm.

Another caterpillar feeding on blueberry leaves is the linden looper, Erannis tiliaria. This caterpillar, a type of inchworm, is brown down the top of the body, has a bright yellow stripe along its sides, 10 thin black stripes, and a grayish white underside. A linden looper is over an inch when fully grown. Besides blueberries, it is commonly found on hardwood trees and shrubs, including linden, rose, elm, maple, oak, ash, serviceberry, and cherry.

These caterpillars generally do not occur on large numbers on blueberry but they are capable of minor up to severe defoliation. If management is necessary, handpick and destroy small numbers of caterpillars. If an insecticide is desired, low impact products include Bacillus thuringiensis, insecticidal soap, and spinosad are effective. Other insecticides include permethrin. Be sure that blueberries are listed on the label of any product you wish use on them.

Click Beatles


Jeff Hahn, Asst. Extension Entomologist

People have been finding click beetles lately in their yards and around their homes. They are generally between 3/8 - ½ inch long, are dark brown or black with an elongate oval and flattened body. The prothorax, the area behind head, appears ‘loose’ with the rest of the body. The back corners of the prothorax are prolonged back into sharp points.

Click beetles are found commonly on foliage and flowers as well as under bark. They are also attracted to lights. A click beetle is unique because it can right itself when it is on its back. It does this by arching the area between the prothorax and mesothorax (where it looks loose) and then snaps it back, usually producing an audible ‘click’. This action will cause it to jump up, often allowing the insect to regain its feet. If it remains on its back, it will keep trying until it succeeds.

Some residents, upon finding a click beetle, have been concerned that they have discovered an emerald ash borer, especially if they find it on a tree or under bark. Although they are similar shape, emerald ash borers are a shiny iridescent green while click beetles are a dark color. Also emerald ash borers lack the corners of their prothorax being extended back to points like click beetles possess.

Occasionally, click beetles are found inside homes. They are attracted to moisture, e.g. from water and leaves in gutters, and then can enter buildings through available cracks and spaces. It is typical to find them on the ceilings in rooms. Fortunately they are harmless and are short-lived indoors. If you find a click beetle indoors just physically remove it, no other controls are needed. They are not usually not found in homes much past June.

Monday, June 1, 2009

What's Up With That?!


Michelle Grabowski

Powdery orange golf balls (or larger spheres) have been spotted on pine trees across Minnesota. What are these strange growths? They are galls, a woody tumor like growth that is part plant, part fungus. In this case the pine trees have been infected with one of two different gall forming rust fungi: pine-pine rust (Endocronartium harknessii) or pine-oak rust (Cronartium quercuum). The diseases get their names from the trees they infect. Pine-pine rust only infects 2-3 needled pine trees like Jack pine, Scots pine and Ponderosa pine. Pine-Oak rust lives half of its life in galls on pine trees and the other half on the leaves of oaks like northern pin oak or bur oak. The galls are present year round. In the spring, both fungi release powdery yellowish orange spores, drawing attention to otherwise discrete brown woody galls.

Extending the Season with High Tunnels, Gardening in a New World


Terrance T. Nennich, University of Minnesota Extension Educator

The gardening season in Northern Minnesota is brutal and harsh to say the least. Lack of heat units, freezing temperatures in early June and late August, very cool nights and high winds are very challenging to even the most experienced and patient gardeners. The long period of times that plants are wet from dew or prolonged rain can make disease control nearly impossible some years. Gardeners in Northern Minnesota are usually very optimistic people, continually telling themselves that next year things will be much better and the weather will be much more cooperative to help produce that lush, bountiful harvest that we all hope for. Then about every five years, somewhat ideal conditions come together and that super abundant crop is produced. And so the cycle goes.

High tunnels can help gardeners produce that great crop every year with little risk. High tunnels can lengthen the growing season as much as 5-6 weeks in the spring and also in the fall. What are high tunnels?

Photo 1: High tunnels are simple structures that can dramatically extend the growing season in Minnesota. Terry Nennich


While high tunnels resemble greenhouses in appearance, this is the only similarity.  High tunnels do not use electricity, do not use artificial heat (except in emergency situations), use only a single layer of plastic, and achieve ventilation from natural airflow by rolling up the sides instead of using electric fans.  Drip irrigation is used to water the crops.

Crops in high tunnels are typically grown in the ground, as is the case for typical garden crops; however container gardening in high tunnels is very possible.

Photo 2: Plants are typically planted directly in the soil in high tunnels in a way similar to a typical garden bed. David Zlesak

Compared to typical garden-grown crops the yield and quality of produce and flowers are usually far superior in high tunnels.  Additionally, Minnesota research has indicated that high tunnels have greatly aided in the control of diseases and in reducing common vegetable and flower pest problems.  High tunnels provide an excellent tool for organic production in Minnesota since diseases and other pests can be controlled without chemical intervention.


High tunnels can either be permanent or on heavy skids and movable. Some gardens use high tunnels to start early flower beds and pull the high tunnel away in early summer, allowing flowers to bloom several weeks earlier. Most vegetable gardeners prefer permanent structures as they are more weather resistant, especially in high winds.

Photo 3: A minimum size is needed for high tunnels in order to efficiently take advantage of solar radiation. David Zlesak

While high tunnels can be constructed in many shapes and sizes, they must be of a minimum size to be effective and utilize solar radiation efficiently. While the jury is still out on an exact minimum size, I recommend at least 10-12 foot wide,6-8 ft high at the peak and about 20 ft long.


Photo 4: There are often less disease problems when growing plants within high tunnels because high tunnels minimize free moisture by shielding plants from rain and reducing dew. Terry Nennich

Before constructing a high tunnel, gardeners should raise the soil level at least 4-6 inches so the floor of the high tunnel is above the surrounding ground level. This will allow excess water from heavy rains to flow away from the tunnel.

While is it recommended to use a plastic that is six mil and UV treated greenhouse clear,  that will last four to six years, it is possible to start out with a inexpensive four mil clear plastic that will usually last only one year. However to get that very early spring start it is recommended to have that plastic already in place.  Therefore, it is recommended to leave the plastic on the entire year.

Photo 5: Trellising and other space saving growing techniques used outdoors can also be used in high tunnels. Terry Nennich

While high tunnels have many great advantages, gardeners must be aware that there is a learning curve to using high tunnels. Management concerns include: not letting the tunnels get too hot, supplying enough soil fertility, supplying timely irrigation through the drip tape and not letting the weeds get out of control.


High tunnel web sites that I recommend include . This is a national site and has some simple designs using PVC and other materials.  is the University of Minnesota High Tunnel Web Site and contains the online version of the Minnesota High Tunnel Manual, along with several recent powerpoint presentations and a list serve.  Gardeners interested in purchasing a hard copy of the Minnesota High Tunnel Production Manual may do so by calling 763-434-0400. The cost is 25.00 plus shipping.

Tips for creating successful window box planters


David C. Zlesak, University of Minnesota Extension Educator

The popularity of creatively combining different plant materials in window boxes has soared in recent years. Combining plants of various color, texture, and plant habit to design what in essence become miniature landscapes provides gardening enjoyment from both outdoor and indoor vantage points.  Designing and having window boxes can become a very fun, creative, and rewarding endeavor. The diversity of plant materials to choose from and effects that can be generated seem almost limitless.  Many people choose to try different combinations and overall themes each year.  This makes it exciting for neighbors and passersby as they anticipate what will be next.

Photo 1: Window boxes can help to make outdoor spaces inviting. David Zlesak

A decade ago when I was a greenhouse grower at an independent nursery, designing and raising our own combination planters was one thing that helped set us apart from the competition. Combining different species or cultivars of plants that grow in harmony with each other throughout both the production period and then as they matured after purchase is challenging and requires looking beyond just what looks good together at the time of planting. For instance, dark foliaged sweet potato vine looked fantastic in hanging baskets and window boxes with trailing verbena and bacopa at first.  However, as the season went along, the sweet potatoes being produced under the soil eventually crowded out the other plants if the containers were relatively small. Much of the fun is to experiment and discover combinations that work. Familiarity with the cultural needs and growth habits of different plant species and cultivars can go a long way to better choose combinations that will work well over the long run.


Photo 2: Experimenting with plant materials to see what combinations work well together is part of the fun. David Zlesak

Group plants with similar cultural requirements.


Combine plants that share similar light requirements. Plants that do well with half a day or more of full sun include ageratum, marigold, petunia, and zinnia. Plants that need to be shaded most of the day include common impatiens, tuberous begonias, and fuchsia. Higher and lower light-requiring plants can sometimes be successfully combined. For example, combine larger, higher light-requiring plants that cast shade over smaller, more shade-tolerant plants. Temperature preference is often closely allied with light requirement. Heat loving annuals like gazania and moss rose also prefer full sun and are also more tolerant of drying out between waterings.


Photo 3: Moss roses (Portulaca sp.) do well in full sun and relatively dry conditions. David Zlesak


Combine plants with similar moisture needs. Plants that can tolerate or prefer being kept on the dry side include gazania, moss rose, verbena, and zinnia. Those that prefer to have a consistent supply of moisture include coleus, tuberous begonias, and fuchsia.


Often not thought about, the pH of the growing medium influences nutrient availability and other aspects of plant growth. In addition to the starting pH, pH can change over time in response to water quality and type of fertilizer used. Often the pH of city water is elevated to a pH of over 7.5 with sodium hydroxide or other bases to protect metal pipes from slowly deteriorating. High water pH can elevate potting medium pH over time. The rate of pH increase is dependant on some properties of the water (dissolved minerals) and the particular planting medium and its pH buffering ability. Knowing your pH will aid in selecting better adapted plants. For instance, geraniums, celosia, and marigolds prefer a higher pH (alkaline) and petunia, pansy, and bacopa a lower pH (acidic). It often becomes clear in mixed plantings of for instance petunia and geranium if the pH is shifting more towards the acidic or alkaline side based on which is performing better.


Photo 4: Bacopa does best in acidic soil. David Zlesak

Combine plants considering their growth and flowering traits


Using plants with similar growth rates helps to prevent slower growing plants from being smothered by vigorous neighboring plants. A very vigorous spreading petunia cultivar, for instance, may choke out a neighboring compact-growing ageratum or geranium. One factor that influences growth rate is response to fertilization. Many of the more expensive, specialty annuals like spreading petunias, bacopa, and sun or solar coleus, for instance, are heavy feeders and can grow quickly in a nutrient-rich environment.


Photo 5: 'Tidal Wave Silver' petunia is a very vigorous spreading petunia and is overtaking its slower growing petunia neighbor and growing out into the walkway. David Zlesak

Growth habit

Some plants have looser, more open, airy growth habits and tend to intermingle and flower better with each other, while other plants have denser growth and more difficulty growing with other plants. Growth habits also vary between upright, mounding, or trailing. Loose-growing trailing verbena and bidens cultivars can intermingle and flower well together, while geraniums, tuberous begonias, spreading petunias and bacopa cultivars tend to make a thick, mounded plant or a relatively dense mat of growth capable of crowding out neighbors. Upright, taller growing plants are often placed towards the center or back of a container, while spreading or trailing plants are placed near the edge of the container where they can grow over the containers edge. Understanding growth patterns of plants can help in determining placement in the window box and how close to plant different species so each is showcased, not smothered.  Window boxes typically have cascading types of plants planted near the edge that can spill over, while more mounded or upright plants are planted closer to the window.


Photo 6: Cascading lamium and moneywort are planted near the edge of this window box and more upright and mounded plants are planted towards the center and window. David Zlesak

Flowering time

Some plants flower continually, while others have more limited periods of bloom. For instance, osteospermums (cape or sunscape daisies), snapdragon, and pansies reduce or stop flowering during the heat of summer, but flower again once the temperatures cool down. Mixing plants so that something is blooming at all times during the season can help enhance season long beauty.

Vast plant selection options


Annual bedding plants are traditionally used for window boxes since plants are typically discarded after each season and replanted in the spring. For many traditional annuals there are newer, compact versions of old favorites that are more amenable to confined spaces. For instance, the profusion series of zinnias are more compact and more amenable to confined places than the old fashioned taller zinnias.  The Profusion and Pinwheel series of zinnias are crosses between the small, disease resistant Mexican zinnia (Zinnia angustifolia) and the traditional large flowered garden zinnia (Z. violacea, Aka  Z. elgans).  More colors and doubled flowered versions have been added to these series.


Photo 7: Ageratum is a common sun loving annual bedding plant used in window boxes. David Zlesak


Many plants used as annuals in Minnesota are actually perennials in their native climate. Examples include geraniums and fuchsias. Also consider using hardy Minnesota garden perennials in containers. At the end of the season they can be nestled into a perennial bed to extend their value, or discarded as an annual.  Due to window boxes being exposed to the air from all sides during winter, typically Minnesota hardy plants may not overwinter if left in a window box.  Root are typically less hardy than stem tissue and the strong insulation ability of soil and snow prevents roots from experiencing winter air temperatures. Frequently-used, Minnesota-hardy perennials for containers include lamium (Lamium maculatum, L. galeobdolon - plants are low and spreading and variegated foliage looks great even when not in flower) and golden-leaved creeping Jenny or moneywort (Lysamachia nummularia ‘Aurea’). Compact versions of standard perennials that have a long flowering season make great choices as well and include astilbe ‘Sprite’, heliopsis ‘Tuscan Sun’, and scabiosa ‘Butterfly Blue’.


Compact versions or carefully pruned specimens of an especially ornamental woody plant can make a great accent or focal point in a container. Some woody plants have especially interesting flowers, foliage, or texture. One potential option includes the new compact, variegated weigela, ‘My Monet’.


Photo 8: Dichondra 'Silver Falls' has a uniquely shaped, silver leaf and vigorous training habit. David Zlesak

Uncommon foliage

Foliage color, shape, texture, and size all can add unique elements to a design, even if the plant never blooms. In addition to traditional foliage plants such as dusty miller, coleus, alternanthera, and dichondra, consider using herbs and vegetables with interesting foliage color and texture such as fennel, parsley, and rosemary. Gold, purple-black, and variegated cultivars of ornamental sweet potatoes have become very popular and are quite versatile in containers. One unique consideration with sweet potato is that in relatively small window boxes the tubers it produces can easily crowd out other plants as the season progresses.

Creating one or more window boxes dedicated entirely to herbs is also a great option, especially window boxes conveniently located outside a kitchen window or bedroom or other window where the fragrances can particularly be enjoyed.  Many herbs have a compact and/or spreading growth habit and would work well in a window box and include lavender, creeping rosemary, basil, some of the mints (like chocolate mint), and thyme.


Photo 9: Commercial potting medium often contains peat moss, perlite (white pieces), and vermiculite (tan pieces). David Zlesak

Basic container garden growing recommendations

Make sure window boxes have holes for drainage and are also a suitable size to accommodate your chosen plants as they grow throughout the season. Often too small a container is chosen or too many plants are put in a container at planting time which leads to overcrowding problems by midsummer. Look beyond how full, colorful, and proportional the pot looks at planting time and envision the container and size of the plants throughout the rest of the growing season. The larger the container typically the better (heavier containers will need stronger anchoring). Larger containers is beneficial for added root development and also allows for greater reserves of moisture and nutrients and allows for extended time between thorough waterings.

Potting medium needs to have a balance of both moisture retention as well as porosity so oxygen can get to the roots. If the medium does not have good aeration and frequently becomes waterlogged, then roots can be deprived of oxygen, slow in growth, and more likely rot. On the other hand, a medium that is too porous can be challenging to water frequently enough to keep moist. Most standard, commercially sold mixes for containers are high in organic matter (often containing peat moss, compost, and/or bark) and may contain perlite or vermiculite for added porosity. Instead of filling up a window box with planting medium, another option is to place a series of potted plants into an empty window box.  This allows great flexibility to replace plants and to have a spot to set some plants that typically serve as houseplants during the summer months.


Photo 10: Window boxes can add a lot to enhance ones home and can be as simple as a row of geraniums. David Zlesak

Containers can dry out quickly, especially on hot windy days and after plants grow in size and their demand for moisture increases. To help with watering needs, leave at least half an inch from the surface of the media to the rim of the container when planting. This will provide space for water to puddle and then slowly soak in without quickly running off when you water.

Water crystals or beads (hydrogels) are commonly found for sale as a means to help hold moisture in potting media. Water beads are dry crystals that can absorb multiple times their weight in water. They can be hydrated and mixed into the medium before planting. Although water beads or crystals can hold water, research conducted by Dr. Jeff Gillman at the University of Minnesota and others suggest that they may be holding onto water too tightly and not benefiting plants because the water they are holding may not be readily available to plant roots. Work continues to compare different types of hydrogels.

The options for developing beautiful window boxes seem almost limitless, just like our garden beds, and they provide a wonderful, creative, and versatile accent to our home. If you haven’t tried window boxes yet, hopefully this article will inspire you to give them a try this season.

Tuesday Evening Classes Held Throughout the Summer at the Master Gardener Education & Research Display Garden in Rosemount, Minnesota


If you are looking to get fresh ideas for your garden, to see the latest in University flower, fruit, and vegetable varieties and accompanying research for yourself, or just to experience a beautiful, accessible garden, you will want to visit the 6-acre Master Gardener Education & Research Display Garden! There are over 25 different display and trial gardens to see.  They are well-labeled and a fantastic resource for Minnesota gardeners.

The garden is free to the public and open daily (sunrise to sunset) with plenty of free parking available. It is located on the South side of Highway 46 in Rosemount, Minnesota, just two miles East of Highway 3. It is within and part of the 5,000 acre UMore Park (University of Minnesota Outreach, Research, and Education Park). The garden started in 2001 with the purpose of being a display garden serving the public through education and research.  It is located in the Southeast metro and has a much different flavor and a unique purpose compared to the Minnesota Landscape Arboretum located in the West metro. 

Photo 1: Tuesday Evenings in the Garden classes begin at the main gazebo. David Zlesak

In 2003 Master Gardeners became involved in the project and led the way to establish and bring to fruition the many wonderful theme gardens that support the mission of public education.  Much of the plant materials and hardscape supplies within the garden have been possible from the generous donations of many individuals and businesses.  The 6 acre demonstration / trial garden serves as an outdoor learning center.  Approximately 130 Master Gardeners are currently involved in the ongoing development and maintenance of the garden. This is the first garden of its kind in the nation and it has already won prestigious national awards. Learn more about the garden.


Photo 2: The herb garden has a beautiful formal design and look. David Zlesak

One of the multiple educational programs at the garden is Tuesday Evenings in the Garden.  Each Tuesday night throughout the growing season there is a class from 6:30-8:00PM.  Classes begin at the gazebo. Click here for registration information.  Pre-registration is recommended to help instructors prepare an appropriate number of handouts and have enough supplies ready for hands on projects.

The 2009 Tuesday Evenings in the Garden Classes are:

May 26th Iris – A Kaleidoscope of Color – Lee Kaibel

This hands-on class will focus on dividing bearded iris and comparing bearded and beardless iris and how they are used in gardens.  Participants will learn how to care for each of these beautiful plants and view many of the approximately 200 varieties of iris in various stages of bloom in the display gardens at UMore. $10.00

June 2nd Luscious Tomatoes – David Zlesak

Growing delicious, fresh tomatoes at home is easy and can be richly rewarding.  Come and learn about basic types/categories of tomatoes and proven cultural techniques to help us get the most out of them.  There will be tomato plants to take home! $10.00


Photo 3: A living willow fence provides a creative barrier for the kitchen garden from the highway. David Zlesak

June 9th Herbs by the Kitchen Door – Shari Mayer

Spice up your cooking by creating a potted herb garden you can keep by the kitchen door. All supplies are included. $25.00

June 16th Rain Gardens – Joanne Sabin

Learn how to create a beautiful garden of native plants that captures rain runoff, protecting our lakes and rivers. $10.00

June 23rd Fruit Trees – Bob Condon and Sally McNamara

Get the know-how you need to grow productive fruit trees in Minnesota. $10.00

June 30th Got the Blueberry Blues? – Warren Banks

Warren covers the cultural techniques needed to grow this handsome shrub, both for fruit production and landscape appeal. $10.00

July 14th Wildflower Gardens – Roxanne Beseman and Corinne Johnson

Learn to grow wildflowers and add a touch of delicate beauty to your garden. $10.00


Photo 4: There are hundreds of hardy landscape roses throughout the garden. David Zlesak

July 21st Nonstop Lilies – David Zlesak

There are several classes/types of lilies we can successfully grow in Minnesota.  With a little care we can select types/varieties of lilies so as to have lilies in flower during most of the growing season!  Come learn about different lily categories and cultural techniques to make the most out of these amazing garden showstoppers. $10.00

July 28th For the Birds – Suzanne Hamann and Deb Morrison

This class covers plantings that will attract a wide variety of birds to your garden. $10.00

August 4th Toadstool Garden Art – Cheryl Mann

Create a lightweight hypertufa toadstool and add a whimsical touch to your garden. $25.00 (includes supplies)

August 11th Square Foot Gardening – Chris Johnson

Learn how to grow the maximum number of vegetables in the smallest space. $10.00

August 18th The Dirt on Soil – Elizabeth Spedaliere, Deb Snow and Kathy Bonnett

Want to know what’s happening beneath the soil surface? This class covers soil types, nutrients, and water retention. $10.00


Photo 5: The vegetable display gardens draw a lot of attention. David Zlesak

August 25th Stepping Stones – Joyce Clarin

Create a cement stepping stone with a unique, natural design. $25.00 (includes supplies)

September 15th Fall Pots – Kathy Bonnett, Deb Snow and Elizabeth Spedaliere

Replace faded annuals with corn stalks, dried flowers and grasses. No watering required! (Supplies included. Bring a soil-filled pot.) $25.00

Early June Lawncare Tips


Bob Mugaas, University of Minnesota Extension Educator

In some areas of the state, lawns have received ample moisture this spring to maintain good growth and color while others have been quite dry; Twin Cities included.  In fact, supplemental lawn watering has already begun in many areas around the Twin Cities.  In most cases, it will take about 1.0 inch of water per week to keep Kentucky bluegrass lawns green and actively growing through the summer months.  One question frequently asked by homeowners is “How do I know how much water my sprinkler or irrigation system is putting out?” 

Photo 1: Typical oscillating sprinkler. Bob Mugaas

Determining how much water is being supplied by a sprinkler is not a complicated process.  Simply place several small flat bottomed containers within the distribution pattern of a sprinkler.  Tuna fish cans work well for this measurement. A simple rain gauge or two placed within the water pattern will also work so long as the gradations are close enough to measure small amounts of water.  Turn on the sprinkler as you would to normally water the lawn and leave it on for an hour.  After an hour, measure the depth of water accumulated in the bottom of the container.   For example, if the average accumulated water in the cans were ¼ inch for that hour, then you can assume that your set-up is putting out about a ¼ inch of water per hour.  Looking at the amount accumulated in each container will also be helpful in determining the uniformity of water distribution and may suggest the amount of overlap needed to ensure all areas receive equivalent amounts of water.  With automated systems, place the containers within the distribution pattern and measure the amount of water accumulated after a cycle for that part of the yard is completed.  This will give you the amount of water be applied for that particular cycle. 


Photo 2: Black medic (Medicago lupulina). Bob Mugaas

By early to mid-June, many of our annual broadleaf lawn weeds will be starting to germinate.   One of the best times for controlling these weeds is in this early growth stage before they have started to mature.  Examples of our common annual broadleaf weeds include prostrate or spotted spurge, erect and prostrate knotweed, lambsquarter, redroot pigweed and purslane.  The clover like plant, black medic as well as yellow woodsorrel are usually considered annuals although some plants may overwinter and behave like a biennial or short lived perennial.   Taking a few minutes to walk your lawn in order to determine when some of these weeds may be germinating allows one to easily remove them by hand and/or treat them with an herbicide.  Remember, removing and/or treating plants with an herbicide is much more efficient when they are small, even at the lower application rates stated on the product label. 


Photo 3: Yellow Woodsorrel (Oxalis stricta). Bob Mugaas

For those sites that are irrigated, regular mowing will be required throughout the growing season.  Maintaining heights at about 2.5 inches should be adequate for most home lawns.  For those lawns not receiving irrigation, mow only as needed and required by the growth of the plant.  Try to follow the general rule of thumb to not remove more than 1/3 of the height at any one mowing.  For example if you are maintaining a height of cut at about 2.5 inches, then mowing should be done when the height reaches about 3.5 – 4.0 inches. 

While most walk-behind rotary mowers adjust mowing heights by resetting the four wheels to the desired height, it is occasionally a good idea to see how close that setting really is to the actual height of cut.  The easiest way to do this is to simply take a ruler and gently push it through the turfgrass canopy until it rests firmly on the lawn/ground (may not be actual soil) surface.  Then look across the grass plants just in front of the ruler and see what the height is.   For example, if the ground is firm then mower wheels will ride higher and consequently the height setting will more closely approximate the actual cutting height.  However, where the ground is soft or there is a significant thatch layer present, the wheels will sink more deeply into the lawn and hence the mowing height is actually less than the wheel settings would indicate.  In fact, where there is significant thatch present mower wheels can ride so much lower that the lawn surface between the wheels is actually scalped.  Remember to take the time to adjust your mower correctly, periodically verify that the mower height settings are actually providing the desired height of cut, and always mow with a sharp blade. 

Seeing Spots


Michelle Grabowski, University of Minnesota Extension Educator

As the season warms up and new leaves reach their full size a few gardeners are noticing unsightly spots in a wide variety of plants.

What are these spots?

Many of the spots that gardeners are seeing are caused by fungal or bacterial pathogens infecting the leaf tissue. There are a few look a likes, however, so gardeners should pay attention to the details. Fungal and bacterial leaf spots are typically randomly scattered across the leaf. There are often several sizes of leaf spots because the spots get bigger as the pathogen grows. In addition leaf spots caused by a pathogen start out on one leaf and eventually spread throughout the plant or to other plants. A gardener may notice fluffy white cobweb like fungal growth, powdery spores or other signs of the pathogen in the spots themselves.

Photo 1: Notice the random pattern of the leaf spots and the different sizes of spots caused by this fungal leaf spot disease or iris. Michelle Grabowski


Spots that are all one size, do not grow and spread, or occur in a specific pattern are not likely caused by fungal or bacterial pathogens. Many gardeners confuse leaf damage caused by fourlined plant bug with a fungal leaf spot disease. See the article, Fourlined Plant Bugs Are Now Active, in this issue for more information about fourlined plant bug. Notice how the spots caused by fourlined plant bug are all the same size.

Photo 2: This Munstead lavender is for sale and comes with a free fungal leaf spot disease! Michelle Grabowski

Where did these spots come from?

Fungal and bacterial leaf spot pathogens can come into the garden from a variety of places. Some are blown in on the wind. Others come in on infected plants. Most survive from season to season in last years infect leaves. Splashing rain or spring winds carry the pathogen onto the newly emerging leaves.

What to worry about?


Leaf spot diseases are not a serious threat to the health of the plant if they do not result in major leaf loss or discoloration. If the leaves are mostly green and still attached to the plant, they can still undergo photosynthesis and provide food for the plant despite a few spots. Many leaf spot diseases fall into this category and require no management.

Photo 3: This plant of Rosa foetida 'Austrian Copper' has been seriously defoliated by black spot, a fungal leaf spot disease. Michelle Grabowski

Leaf spot diseases that result in defoliation or discoloration of the majority of the leaves should be taken more seriously. For example black spot on rose can seriously injure a plant because in addition to spots, the black spot fungus causes leaves to turn yellow and fall off. This reduces the energy available to the plant and can result in reduce growth, flowering and winter survival. Gardeners should consider using a combination of management strategies to slow the spread of these pathogens.


What can you do?

There are several basic practices that a gardener can implement to reduce the amount of leaf spots that occur in the garden. Most fungal and bacterial pathogens need moisture or very high humidity on the leaf surface in order to start a new infection and to produce new spores or bacteria. Many management strategies focus on reducing leaf wetness.

Photo 4: Powdery mildew on phlox can be very severe. Plants that are seriously infected each year should be replaced with a resistant variety. Michelle Grabowski

  • Tolerate leaf spots if the majority of the leaf is green and no major leaf loss is occurring.

  • Use drip irrigation or a soaker hose to water plants instead of sprinkler irrigation.

  • If sprinkler irrigation is the only option, water in the morning so that leaves dry quickly in the sun.

  • Space plants for good air movement in the garden. This will help leaves dry quickly after rain or dew.

  • Split dense, overgrown perennials to improve air movement through the garden.

  • Mulch garden plants to reduce humidity.

  • Do not work in the garden when the leaves are wet. Wait until plants have dried to prevent accidentally moving the pathogen on your hands or tools.

  • Remove last year’s plant debris from the garden.

  • If only a few leaves are infected, pinch off infected leaves and remove them from the garden. Never remove more than 1/3rd of a plants leaves or you will do more harm than the pathogen!

  • If a plant is severely infected, look for a disease resistant variety to replace it.

  • If all of the management strategies above do not reduce the leaf spot disease to an acceptable level, consider protecting plants with a fungicide. Always read and follow all instructions on the label any time you apply a pesticide!

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