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Extension > Yard and Garden News > April 2016

Friday, April 29, 2016

Wise watering practices for newly planted trees and shrubs

Kathy Zuzek, Extension Educator

It is prime time for planting trees and shrubs in Minnesota. If planting is on your list of spring chores,
remember that proper watering is the most important practice to ensure survival and establishment of
your woody plants. No matter what planting stock you choose - bare root, containerized, balled and burlapped, or tree spaded – your new tree or shrub has a smaller root system than an in-ground, established plant. Frequent watering allows an under-sized root system to supply all the water needed by the plant while the root system expands and grows to a normal size. Under-watering and over-watering are both detrimental to plant health so it is important to know when to water, where to water, and how much water to apply. Wise watering practices for newly planted trees and shrubs can be found here.

Thursday, April 28, 2016

Boxwood leafminer: A rare insect in Minnesota

Jeff Hahn, Extension Entomologist

Boxwood is a shrub or small tree that usually grows well up to hardiness zone 5. They are marginally hardy in Minnesota (hardiness zones 4 and 3). Despite that, boxwood can be found occasionally here in the landscape. Where boxwood is commonly grown in landscapes in states further south, the most serious insect pest is boxwood leafminer. In the upper Midwest this insect is rare. A resident in a Minneapolis suburb recently reported the presence of this insect in her boxwood.

As an adult, boxwood leafminer is a small delicate orangish mosquito-like fly belonging to the gall
Boxwood leafminers: The blistering damage by the leafminers
can be seen in the top leaf while damage and larvae are seen
in the bottom leaves.  Photo: John A. Weidhass.
midge family (Cecidomyiidae). This insect spends the winter as a larva inside boxwood leaves. It becomes active when the weather warms in the spring and eventually pupates and emerges as an adult. Adults lay eggs inside the boxwood leaves which hatch later in the summer where they remain through the winter. There is just one generation a year.

The feeding of the larvae causes irregularly shaped swellings in the leaves. These damaged areas yellow at first and eventually turn brown. Despite this appearance, this feeding is generally just cosmetic, just affecting the plant’s appearance. In most cases, it is best to tolerate boxwood leafminer feeding as it is likely causing no lasting injury to plant health. Keep in mind that this is a rare insect in Minnesota and not widespread.

It is not clear where this insect occurs in Minnesota. If you live in Minnesota and have a boxwood, I would be interested to hear whether you have encountered this insect or not. Contact me at hahnx02@umn.edu.

Wednesday, April 27, 2016

Water wisely: grapes in the home garden

'King of the North' grapes
Photo: Julie Weisenhorn
By Matthew Clark, Assistant Professor
Grape Breeding and Enology
Department of Horticultural Science. enology.umn.edu. grapes.umn.edu

Grapes are a welcomed addition to home gardens. Successful grape breeding by universities and grape growers has resulted in a very good selection of varieties that perform well in our northern landscapes. These fruits can be grown for wine, jams and jellies, and as table grapes for fresh eating. University of Minnesota Grape Varieties

Some growers may make claims that a stressed grape plant will produce better wine. In reality, grapes need to have adequate water to enable systems like photosynthesis, evaporative cooling, and nutrient transport to work properly. Common symptoms of drought stress in grapes wilting leaves, change in leaf angle (away from the sun), and shriveling fruit. Drought stressed plants are less likely to survive Minnesota winters.  Winter injury itself can cause symptoms of drought stress and allow vines to become infected with crown gall.  Careful observation of trunks and canes of wilted plants will help to identify if the symptoms are caused by vascular damage, disease, or drought.


Soak bare root vines
3-4 hours before planting
Photo: Julie Weisenhorn
Grapes can be purchased as bare-root stock or in pots. Bare-root plants are dormant and have an   When watering the young vines, the root zone should be well saturated. Applying 5 gallons of water over a 3 x 3 foot area is a good estimate for 1 inch of water.
established root system that is in balance with the shoot. They must be soaked in water for 3-4 hours prior to planting, but will be less vulnerable to drought stress at planting than potted vines. Plants grown in pots require regular watering until the roots become established and the leaves have acclimated to growing outdoors. It will be worthwhile to monitor these plants daily to ensure they do not suffer drought stress. Young grape vines need regular watering to ensure successful establishment especially at the time of planting and the following two years. Older vines seldom need any watering unless on sandy or other very well drained soils.

According to Growing Grapes for Home Use, grapes are best planted in spring and grown in well-drained soil. The amount of organic matter in the soil may influence the growth and vigor of vines by providing access to additional nutrients.  Dig a hole large enough to spread out the root system. Cover the roots completely with soil. Create a slight depression around each vine. This will allow water to pool and soak into the soil, saturating the root area and preventing run-off.  Mulching is not recommended for grapes because mulch will moderate the soil temperature, often keeping it cooler in warmer months, and grape vines grow best in warmer soil.

Young grapes require about ½ to 1 inch of water per week, depending on rainfall, for the first two years during the growing season. By the end of the second growing season, a trunk should be established and your vine is likely to not need additional watering intervention unless specific soil conditions (sandy, well drained) or prolonged drought dictate the need. Apply water only to the root zone. Avoid getting grape foliage wet as this can encourage many grape diseases. Reduce watering young vines going in the fall to encourage the plant to harden off its canes to prepare for winter.

Managing weeds near grapevines is critical as weeds compete for water and nutrients as well as harbor potential insect and vertebrate pests. Weeds can be managed by hand-pulling, cultivation (hoeing) or through the use of well-timed herbicide applications. Cultivation and herbicide application before planting is most effective.  Combining a pre-emergent herbicide and a broad-spectrum herbicide like glyphosate before planting, and while plants are dormant in subsequent years can be a very effective strategy.  Make sure to not apply contact herbicides after the buds begin to grow as grapes are very sensitive and easily damaged by herbicides. Grow tubes can help protect the young, and sensitive vines and are a great tool when is establishing a planting.  They can even minimize damage from deer.

Additional Extension resources on grapes:


Thursday, April 21, 2016

Weed management: creeping charlie

Julie Weisenhorn and Sam Bauer, Extension educators

Treating aggressive perennial weeds requires proper timing, an effective product and cultural
practices to help deter re-establishment. Creeping Charlie (Glechoma hederacea) is consistently a problem weed though some homeowners have come to tolerate it because it attracts and provides food for pollinators (one of its few redeeming qualities).


Creeping charlie in bloom
Also called ground ivy, creeping charlie is part of the Mint family. Like all mints, it spreads on top of the soil via stolons (surface roots) and will regrow from very small pieces of vegetation left behind in the soil after removal. It is a very adaptable plant and grows well in moist soil in part shade / full shade sites. Creeping charlie can be found where lawn grass is thin and not very robust. This may be in compacted soils, shady locations, and weedy sites. Therefore, treating with an appropriate product, maintaining a healthy lawn, utilizing shade tolerant grasses, and choosing alternative ground covers in areas not suitable for growing lawn grasses will help manage weeds including creeping charlie. 

Best cultural practices are recommended to encourage healthy growth and vigor of lawn grasses. Proper selection of grass varieties for the site, fertilization and watering practices that encourage deeply rooted plants are important to a healthy lawn that can out-compete weeds. 

Most lawns are comprised Kentucky bluegrass and perennial ryegrass and require full sun, proper nutrition, and non-compacted soils for optimal growth. A soil test can provide recommendations for soil health and fertilizer. As shade increases, it becomes less and less favorable for sustaining a lawn in this area, and more favorable for weeds like creeping Charlie. Pruning to increase sunlight and aeration may be options for improving growing conditions; however, if the area is too shady, give up on trying to grow grass in this area and consider planting other ground covers or a shade garden of perennial flowers. 

Creeping charlie and dandelions are
both perennials weeds in turf
Timing of herbicide treatment is key to successful control of perennial weeds like creeping charlie. As always, read and follow all instructions and guidelines on the label of any product, synthetic or organic, including proper clean up and storage. Note that herbicides are only effective in managing weeds if applied properly. The herbicide label is the law and misuse constitutes illegal application. Autumn is the best time of year for systemic herbicide applications when creeping charlie is actively taking up nutrients from the soil to sustain the plant through the winter. Spring is a second option when the plant is actively growing.

Some considerations for choosing an herbicide:

  • If you have a large area of creeping charlie or if your lawn is more than 50% creeping charlie, you may want to consider killing off the entire area with glyphosate (the active ingredient in the non-selective herbicide Round-Up®) and re-seeding. Note this product kills all plants including lawn grasses. Depending on the product formula, re-seeding can occur within a few days after application. People and pets may re-enter the area when dry.
  • If you want to treat small areas where creeping charlie is growing in lawn grasses, you can use a a selective herbicide like 2,4-D, Dicamba or triclopyr, or combinations of these products. Triclopyr will be the most effective option for creeping Charlie. These are systemic, selective broadleaf herbicides. They are taken up by the plant and kill the entire plant from roots to flowers. Note they do not kill lawn grasses if applied properly. These products are effective and usually require only 2-3 applications per year depending on proper use and timing.
  • Chelated iron burns creeping charlie foliage and its stolons. A maximum of four applications may be applied annually. Lawn grasses may show some burning on the blades, but will recover. They also will turn a deep green due to the absorption of iron. Because of this, chelated iron should not be used for spot treatments as it will result in deep green spots throughout the lawn. Treated areas may be re-seeded the next day, and people and pets may re-enter the area when dry. Note that chelated iron can be expensive, and may stain equipment, sidewalks, driveways, etc. Here is a publication from the University of Maryland with more information

In the past, borax (boron) was a product recommended for eradicating creeping charlie. However, research has shown that the addition of boron to soil, even in very small amounts, can create an unfavorable growing environment, and make it difficult to re-establish lawn grass. Also, this is an illegal application.  Any product used in this fashion must be specifically labeled for the weed you are trying to control.  Therefore, borax it is no longer recommended for eradication of creeping charlie.

Going forward, regular and well-timed cultural lawn care practices will help keep weeds like creeping charlie managed and turfgrass healthy. The Upper Midwest Lawn Care calendar is a good tool for helping time applications of fertilizer and pre and post-emergent herbicides and when to perform tasks like aeration to reduce compaction. For more on healthy lawns: http://www.extension.umn.edu/garden/yard-garden/

Round-Up® is a trademark of Monsanto Corporation.

Garden offenders: get a jump on four common problem plants

Julie Weisenhorn, Extension educator

Typically people contact Extension with the question "How can I get get rid of  __________?" (fill in with a troublesome plant). Unfortunately, these offenders are well-established by the time we usually get this call. So here are four of the most common problem plants and what they look like now in their emerging stage, so you can get a jump on getting them out of your yard and garden!

Buckthorn in springtime
Buckthorn (Rhamnus cathartica)
Public enemy #1, buckthorn was introduced as a hedge plant. Buckthorn was a favorite of urban homeowners for its interesting cherry-like bark, tolerance to many growing conditions and its receptiveness to pruning. Today, common or European buckthorn as well as glossy buckthorn (Frangula alnus Mill.)  are listed on the Minnesota Department of Agriculture Restricted Noxious Weed List. Buckthorn is leafing out right now and can be easily spotted by its silvery-gray park with white lenticels, bright green, rounded young leaves and the sharp thorn on the ends of branches. Guidelines for removal can be found on the Minnesota Department of Natural Resources buckthorn webpage.
Silvery buckthorn bark
with white lenticels

Thorn on young buckthorn

Garlic mustard (Alliaria petiolata)
Also listed on the Minnesota Department of Agriculture Restricted Noxious Weed List, this biennial
Young garlic mustard plant
 plant is stealthy. Its cute, tidy rosette of scalloped leaves can sometimes be mistaken for more desirable ornamentals like hollyhocks. Beginning gardeners or gardeners new to the area may be afraid to remove it.  An easy identifying feature: garlic-scented leaves. (Plan to throw away your gloves after pulling these plants). As a biennial, garlic mustard puts its energy into leaves and roots the first season, and blooms the second season. If left alone, the small clusters of white flowers result in hundreds of tiny seeds, making is a prolific invader of our woodlands. This is one plant that warrants chemical spot treatments.
For more: Minnesota Department of Natural Resources garlic mustard page.


Creeping charlie (Glechoma hederacea)
Early blooming creeping charlie
Not a Smart Gardens radio show goes by without a creeping charlie question - even in the winter! A member of Mint family and like other mints, it spreads by means of stolons (surface roots). Though not a registered noxious weed the Minnesota Department of Agriculture, it is considered invasive by the Minnesota Department of Natural Resources and can be a tough plant to eliminate once it has become established though some homeowners have come to tolerate it because it attracts and provides food for pollinators (one of its few redeeming qualities). It is a very adaptable plant and grows well in moist soil in part shade / full shade sites where lawn grass is thin and not very robust. This may be in compacted soils, shady locations, and weedy sites. Therefore, treating with an appropriate product, maintaining a healthy lawn, utilizing shade tolerant grasses, and choosing alternative ground covers in areas not suitable for growing lawn grasses will help manage weeds including creeping charlie. More on creeping charlie

Creeping bellflower (Campanula rapunculoides)
Creeping bellflower emerges early
Creeping bellflower forms a mat
Campanula cultivars are favorite perennials of gardeners, and Campanula rotundifolia, harebell, is a Minnesota native wildflower and a tough plant that grows in rocky, dry soil. However, its European cousin, the creeping bellflower (C. rapunculoides) is an aggressive plant and not so well-liked. Creeping bellflower forms mats of thick, tuber-like rhizomatous roots. It is a prolific seeder and quickly outcompetes more desirable plants for moisture and nutrients. This plant leafs out early, making it easy to spot treat with a broadleaf herbicide and easier to remove by hand while desirable plants are still emerging. For more details: Minnesota Wildflowers,
Wisconsin Department of Natural Resources

Tuesday, April 19, 2016

Soil temperatures needed for germination

By Beth Berlin, Extension Educator-Horticulture
University of Minnesota Extension - Stearns & Benton Counties

ST. CLOUD, Minn. - Many gardeners were ready to get a jump start on their gardens when it was so warm back in early March. Although the frost is out early this year, soils need to reach a specific temperature in order for seeds to germinate. Click here for a list of common vegetables and the minimum and optimal soil temperatures for seed germination ...

Friday, April 15, 2016

Do your trees and shrubs need fertilizing this spring?


Kathy Zuzek

The most beneficial time to fertilize trees and shrubs is from early spring when soil becomes workable until the time in April or early May when woody plants move into active growth. Fertilizing at this time provides woody plants with nutrients just as they put on their main flush of growth during spring.

Tuesday, April 5, 2016

Water wisely: annual and perennial flowers


An interview with Steve Poppe, Senior Horticulture Scientist
U of MN West Central Research and Outreach Center Morris MN

Text by Julie Weisenhorn, Extension Educator.
Photos by Esther Jordan, Communications Specialist.

Horticulture Display Garden
U of M West Central Research & Outreach Center
Morris, MN
If anyone knows about watering annual and perennial flowers wisely, it’s Steve Poppe. Poppe is a senior horticulture scientist and head of the Horticulture Display Garden at the University of Minnesota West Central Research and Outreach Center (WCROC) in Morris, MN. Established in 1968, the 5-acre garden has been one of North Central regional sites of All American Selection (AAS) annual and plant trials since the 1990s along with University of Minnesota sites in Grand Rapids and the Minnesota Landscape Arboretum. Poppe became certified as an AAS judge in 2012, one of 60 judges in North America. He and his staff trial 350-500 annuals and 150 perennial each year (16,000 plants). Poppe observes and rates them weekly from greenhouse to garden until frost, and sends his data to the AAS organization where it is reviewed. If a plant receives a high ranking by all AAS judges, it becomes an AAS winner. Usually only 1-2 plants make this rank each year. Annual Flower Trial Results

Y&G: What kind of soil and watering system do you have at the Horticulture Display Garden?
Poppe: We are blessed with well-drained, silty loam prairie soil with a pH of 7.2. Every three years, we add composted animal manure from WCROC. The Garden has a full irrigation system and is equipped with in-ground water tensiometers (moisture sensors) installed 2-3” below the soil surface (in the root zone).  We water every plant by hand immediately after planting so we can be certain the transplants are well-watered and any transplant stress minimized. Hand-watering also avoids wetting the soil of nearby beds before they are planted.

Y&G: How do the moisture sensors work?
Poppe: In general terms, the in-ground moisture sensors or tensiometers measure soil water tension.
New Guinea impatiens trial
A lower number means the plant root area is fully saturated. Our threshold for watering is around 30 centibars. However, because soil can vary between planted areas within an irrigation zone, we don’t water based only on the moisture sensors. We also take in consideration the plant types, wind, air temperatures, humidity, etc. and try come to a compromise that will provide enough water, but not too much.

Y&G: What are some problems you’ve seen from too much water, not enough water?
Poppe:  It’s important to be familiar with each plant’s water needs. For example, if you have annuals that don’t require much water like vinca or geranium, and you are using an automated irrigation system, locate these plants in a drier site. If they get too wet, they will likely contract root rot. Likewise, a plant that isn’t getting enough water will suffer drought stress. For our perennial trials, we use drip tube irrigation instead of overhead sprinklers and monitor irrigation needs with water sensors. Drip tube irrigation reduces disease issues from splashing water. We have to be careful not to overwater perennials that are mulched and planted in the Display Garden. We are also always worried about frost and thus may wet down plants with water if frost is predicted.

Y&G: To mulch or not to mulch …?
Mulch in general is a great asset to garden beds to retain moisture and keep weeds under control. We do mulch some of the perennial beds in the Display Garden. However, gardeners may want to consider waiting until the soil warms up before applying mulch. In the spring, exposed black garden soil will retain heat, warming the soil and the plant roots. Mulch can also be overwhelming for delicate new transplants, so I recommend leaving the beds exposed until the plants are about 6” tall, then apply 3” of mulch to control weeds, retain soil moisture and provide a nice finished look to the garden. Warm season vegetables like tomatoes and peppers will also benefit from not applying mulch too early.

Y&G: Would you recommend home gardeners put down a slow-release nitrogen fertilizer before applying mulch? What about before and after planting?
Poppe: Yes. As wood mulch breaks down, it will use nitrogen in the soil, so it’s a good idea to add some supplemental nitrogen fertilizer before applying wood mulch. Prior to planting our annual flower trial, we apply a 20-20-20 granular fertilizer, and, after annuals are transplanted in the garden, we fertilize with ammonium sulfate (21% nitrogen and 24% sulfur). We apply this in mid-July to give plants a boost prior to our annual Horticulture Night, and toward the end of the season when they are looking a little tired.

Y&G: Do you ever have issues with deer?
Poppe: A lot of people experience deer problems including us! Our perennial trials are grown in a
Shade Garden
Horticulture Display Garden
deer enclosure because we can’t tolerate any animal browsing on these plants. At the Horticulture Display Garden, we put down deer repellent right after planting to protect the annual flowers. We rotate repellents because deer will become accustomed to a product. Spray repellents are washed off by irrigation and rain and must be reapplied, so we most often apply a granular product called Deer Scram around the outer perimeter of the garden beds. The deer don’t like the smell and it lasts about 30 days. If they start to browse on some plants, we’ll treat with a systemic repellent. A systemic is applied to the plant foliage and taken up by the plants, so it isn’t washed off by irrigation or rain.

Y&G: Are there any insects or diseases that you regularly encounter? Do you have Japanese beetles? Downy mildew? Soil-borne problems?
Poppe: No, fortunately we don’t have Japanese beetles, and the downy mildew on impatiens hasn’t been a problem. Most of our insect pest issues occur in the greenhouse – fungus gnats, spider mites, aphids, etc. Being on the prairie, it tends to be windy, so the plants have good air circulation which helps dry off leaves and reduce foliar diseases. Gardeners should remember it’s an important cultural practice to space plants based on their mature size and plant in an area that has good air circulation  to encourage the plants to dry off.

Y&G: What are your top 3 best practices for watering annuals and perennials?
Poppe: Watch the moisture needs of newly planted transplants. Hand-water plants at the roots immediately after planting. This reduces transplant stress while it is getting established and building its root system. I recommend hand watering for home gardeners so you know for certain the water is getting right to the roots.

Look at the plant. When you’re selecting plants for purchase from a greenhouse, avoid choosing plants that are too tall or spindly. Pass over the plants that show signs of too little water (wilting, tipping over) or too much water (yellow leaves, leaf drop, saturated soil). Choose healthy, compact plants with good root structure. Once transplanted, these plants will be able to easily take up water and nutrients from the soil. To check the root structure, gently remove a plant from the container and look at the root mass. The roots should be bright white, firm and plentiful. If the roots are brown and mushy, don’t buy that plant.

Feel the soil. Water sensors are great tools, but we still feel the soil with our hands to see if it’s too dry, or too wet. Just grab a handful down about 3”. Is it wet? Dry? Moist? Feeling soil is the best gauge!

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