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Wednesday, June 21, 2017

SWD out now!

Jeffrey Hahn, Extension Entomologist

The first spotted wing drosophila (SWD) adults were detected in Minnesota last week, collected in
Adult male spotted wing drosophila (note the black spot at the
tip of the wing). Bob Koch, Univ. of MN Extension
traps at the Rosemount Research and Outreach Center. If you are growing susceptible fruit in your gardens, including raspberries, blackberries, blueberries, cherries, strawberries, or grapes, be sure to set out traps to determine when SWD is first active in your area. SWD is a very destructive invasive insect pest that can severely damage untreated susceptible fruit crops.

The best approach to managing SWD is through detection, sanitation, and insecticide treatments. For more information on SWD see Spotted wing Drosophila in home gardens.

Tuesday, June 20, 2017

Dutch elm disease active now

M. Grabowski, UMN Extension

Leaves turn yellow, then brown and wilt on a Dutch
elm disease infected elm tree.
M. Grabowski,  UMN Extension 
Although not as common as they once were, many American elm trees can still be found throughout Minnesota.  In late spring and early summer the first symptoms of Dutch elm disease begin to appear. Gardeners with elm trees on their properties should watch for leaves that wilt, turn yellow, and then brown. This may happen to leaves on just one branch or on multiple branches throughout the canopy. Leaves may fall off the tree and be scattered on the lawn below.

It is important to react quickly if symptoms of Dutch elm disease appear. The infection can be pruned out if the fungus has not yet reached the main trunk of the tree. This requires pruning out the infected branch 5 to 10 feet below symptoms of the infection to be successful. Gardeners that suspect Dutch elm disease should contact a certified arborist to inspect the tree and submit a sample for diagnosis to the UMN Plant Disease Clinic.

Young Accolade elm in a landscape. This tree
is Dutch elm disease resistant.
M. Grabowski, UMN Extension 

There are many varieties of elm that are resistant or tolerant of Dutch elm disease that are now available in nurseries. Gardeners looking for a new shade tree should consider planting an elm with Dutch elm disease resistance. Elms are fast growing, resilient trees that tolerate many common stress factors found in urban sites. 

Thursday, June 15, 2017

After the storm

M. Grabowski, UMN Extension Educator

Strong winds and hail have caused significant damage to plants large and small in Minnesota landscapes. Here's what gardeners need to know about treatment and recovery of storm damaged plants.


Storms often reveal hidden rot that
weakens trunks or large branches.
M.Grabowski, UMN Extension
Storm damaged trees may be unstable and can be a hazard to people and property. Contact a certified arborist to evaluate and treat large trees that have suffered from storm damage.

Smaller branches that have been damaged by the storm, are low in the canopy, and accessible from the ground can be treated by the gardener. Prune off branches that are split, cracked, or torn at a point where the wood is undamaged. A tree will heal over small scattered wounds from hail but severely hail damaged branches should be pruned out. A clean pruning cut at a branch union allows the tree to naturally heal over the wound site. A proper pruning cut does not leave a branch stub or remove bark beyond the branch collar. Painting over pruning wounds is unnecessary unless the tree is an oak tree.

Minnesota is currently in the high risk season for oak wilt infection. Sap beetles are attracted to fresh wounds in oak trees and can introduce spores of the oak wilt fungus, which are attached to their bodies. If oak trees have been damaged in recent storms, prune to remove damaged branches and then immediately paint over the wound with water-based paint, a pruning/wound sealer, or shellac.
Hail damage on hostas.
M. Grabowski, UMN Extension

Flower garden

In the flower garden, broad leaved perennials like hostas were most severely damaged by recent hail storms. Long tears in the leaf are signs of hail damage. The plants will tolerate leaf damage and produce new leaves as long as the growing point of the plant is undamaged. Gardeners can prune out the most severely damaged leaves. Do not remove more than one third of the plants foliage however, as the plant will need energy produced by its leaves to recover and produce a new flush of leaves.

Hosta leaves torn by hail
M. Grabowski, UMN Extension
Stems of annual or perennial plants that have been broken by the storm should be pruned back to the next undamaged leaf. Many flowering plants can produce new shoots from buds on the side of the damaged stem or from the crown of the plant. Annual flowering plants that were broken off at the soil level are unlikely to recover at this point however as the plants are not well established this early in the growing season. These plants should be removed and replace.

Vegetable garden
Eggplant leaves with hail damage
M. Grabowski, UMN Extension

Storm damage to vegetables in the garden will vary based on how well established the plants are. Spring greens like spinach, lettuce, bok choy, and Swiss chard likely suffered torn leaves from hail and wind. Plants will recover as long as the growing point is undamaged. Young newly emerged seedlings may have escaped damage due to their size or have been killed completely by a direct hit. Gardeners should evaluate vegetable plants over the next week. Young seedlings that produce new leaves are likely to recover. Seedlings that fail to produce new growth should be removed and replaced.

The apples will bear scars of
hail damage even at harvest
M. Grabowski, UMN Extension
Transplants with a central stem need to be evaluated carefully. A tomato or broccoli plant will tolerate and recover from tearing of the leaves. If the stem breaks near the soil level, however, it is best to remove and replace the plants. Tomatoes, peppers, and eggplants that suffer a break in the stem higher up may sprout from buds on the side of the stem and recover. Broccoli, cauliflower, and cabbage that lose their central growing point however may recover but will likely produce multiple small heads as opposed one large head. Reseeding of broccoli, cauliflower and cabbage can be done in early July where storms have significantly damaged the spring crop to produce a fall crop.

Disposal of plant material
In Minnesota yard waste cannot be sent to the landfill. Place all plant material in a backyard compost bin or bring it to a municipal compost facility.

Friday, June 2, 2017

Creeping charlie: Management and value to pollinators

By: James Wolfin and Phoebe Koenig, University of Minnesota Bee Lab 

Range of creeping charlie (Glechoma hederacea L.)
in North America. Credit: USDA Plants database
Creeping charlie (Glechoma hederacea L.), also called ground ivy, is an herbaceous perennial native to the British Isles and present in North American landscapes for nearly 200 years. While some consider creeping charlie to be a weedy species, others consider it to be naturalized, and some seed providers will sell this flower as a form of ornamental ground cover. Creeping charlie is in the Mint family and an early spring bloomer (April-May) easily recognized by its small, pale violet flower. It grows well in shaded areas with fine-textured soils that are damp and slightly acidic to slightly alkaline (pH 5-7.5). Like other mints, creeping charlie spreads rapidly through stoloniferous growth, where stems grow at the soil surface and spread laterally. These stems are commonly referred to as “runners” and allow creeping charlie to grow in its easily identifiable mat-like form of ground cover (Hutchings and Price 1999).

Creeping charlie can form
a thick mat in lawns
Creeping charlie and lawns
Creeping charlie is considered by many to be a nuisance weed in lawns and will infiltrate areas that   An allelopathic plant will produce biochemicals that deter the fitness of surrounding plants. One study (Rice, 1986) found that flowers growing alongside creeping Charlie experienced decreased seed germination and faster rates of root and shoot growth.
have been neglected or otherwise poorly managed. Once established within a lawn, creeping charlie may suppress the growth of surrounding plants, due to a characteristic called “allelopathy”.

Fine fescue coverage in a heavily
shaded environment.
An effective way to exclude creeping charlie from your home lawn is by practicing responsible lawn species selection. In shaded areas where creeping charlie often thrives, it is important to utilize grasses like the fine fescues (Festuca spp), which are known to perform well in Minnesota. In the northern climates of the United States, Kentucky bluegrass is the most common species of turfgrass. While Kentucky bluegrass is a high quality turfgrass that performs well in terms of winter hardiness, this species of grass struggles in shady areas. Both tall fescue (Festuca arundinacea) and fine fescue (Festuca spp.) are cool-climate turfgrass species that do well in shady areas. Mixing fescues with Kentucky bluegrass is an effective way to ensure that strong turfgrass density is present despite the shade provided by common yard trees. In addition to employing careful turfgrass species selection, it is important to utilize best practices in lawn care including mowing, irrigation, and fertilizer to ensure proper turfgrass health and density. management. The first thing to consider when establishing a home lawn should be

When creeping charlie is present in low numbers, it may be possible to control the weed via hand weeding. When hand weeding, it is critical to remove the roots as well as the above ground portion of the plant to ensure that there is no re-emergence.  Due to the aggressiveness of creeping charlie, it may be necessary to weed the area several times to remove all creeping charlie vegetation and use proper lawn care practices to promote a healthy, thick lawn instead of creeping charlie.  A study by Price and Hutchings (1986) noted that creeping charlie is less abundant when turfgrass is present, as the grass provides competition for sunlight and soil nutrients. Increasing access to sunlight will improve turfgrass growth, limiting the ability of creeping charlie to spread through the turf lawn. 

When creeping charlie has overtaken turf
There are several options for eradicating creeping charlie when it has taken over a lawn. Typically use to cut sod for lawn renovation, a sod cutter may also be used to quickly remove strips of weeds, like creeping charlie, from a lawn. This will result in an area of bare soil within the lawn. Be sure all creeping charlie vegetation is eradicated to prevent re-invasion. If any piece of plant material is not removed, there is potential for creeping charlie to re-establish within the lawn. After all creeping charlie vegetation is removed, the areas of bare soil should be seeded with high quality turfgrass suited to the site conditions (soil type, amount of sun/shade, plant hardiness zone) to ensure dense, uniform germination throughout the area.  Lawns with dense turfgrass coverage are less susceptible to weeds including creeping charlie. 

Solarization strip installation
Credit: Xerces Society
A second option for removing a large area creeping charlie from a lawn is through solarization, a Xerces Society.
method for killing plant material and soil pests that involves anchoring clear plastic sheeting over the area during the warm months of spring and summer. The clear plastic sheet captures heat and sunlight, raising temperatures to the point where grasses and weeds can no longer survive. Solarization is best fit for sunny, flat sites that are less than 1/4 of an acre in size. Note: In cooler, shaded areas (where creeping charlie is likely to be found) the process of solarization takes the better part of a growing season, typically 5-6 months, or up to a full year in some instances. Additional information on solarization can be found through the

The plastic should be removed in the late fall when soil temperatures are 35-55°F so that the area can be dormant seeded. Dormant seeding is done at these temperatures because the soil is too cold for germination to occur, but the ground is not yet frozen. This ensures that the target species seeded into the newly prepared area will germinate in the spring when temperatures have reached the proper number of degree days. If seeded properly, the new turfgrass area should be dense, uniform, and free of weed pressure.  

An additional strategy for removing a creeping charlie infestation is the use of chemical herbicides, specifically herbicides that include glyphosate and/or triclopyr as active ingredients. While this can be a less ecologically-friendly solution, it is an effective form of weed management. Applications of herbicides  are most effective on warm, calm days when plants are actively growing. It is important to avoid windy days to ensure that herbicide applications do not drift onto non-target plants. Glyphosate treatments are most effective when two treatments are applied, with the second treatment taking place 7-10 days after the initial treatment. After the creeping charlie has been removed, high quality grass seed should be overseeded onto the turf lawn.   

IMPORTANT: A pesticide label is a legal document. Always follow the pesticide label directions attached to the pesticide container you are using. Pesticide labels may change frequently. Internet labels may not match the label on the container you are using. The site of use or plant to which the pesticide is to be applied must be listed on the label or the pesticide cannot be used. Remember, the label is the law.

Value to bees and other pollinators
Bumble bee foraging on creeping charlie flower
For people that live in an area where letting creeping charlie grow is not a problem for neighbors, creeping charlie employs a unique strategy to attract some bee visitors, such as sweat bees, bumble bees, and honey bees, that is tied into how the flower produces nectar. The flowers have a unique strategy for rewarding visitor pollinators, commonly referred to as the “lucky hit” strategy.  Creeping charlie flowers produce an average of 0.3 ml of nectar per flower, but the amount of nectar in any one flower varies greatly, ranging from 0.06-2.4mL.  When 805 creeping charlie flowers were sampled for nectar quantity, it was found that only 8% (64/805) of these flowers had a large volume of nectar, and the rest had almost none (Southwick et al. 1981).  The availability of nectar also varies throughout the day. As the morning fades into afternoon, “lucky hits” become less frequent, as creeping charlie flowers do not replenish their nectar throughout the day. Most flowers produce their nectar at night or in the early morning, so it is believed that all the “lucky hits” available in the afternoon are ones that were missed by bees earlier in the day.  One researcher (Southwick et al. 1981) found that bees foraging on creeping charlie for 5.9 minutes obtained enough nectar from the flowers to make foraging energetically profitable.
creeping charlie can serve as a nectar source.

While creeping charlie can be a good nectar source, we are not recommending that you let it take over your lawn. In addition to the issues associated with nectar production, pollen (the main protein source for bees) from creeping charlie is not readily available to visiting bees and other insect pollinators. Bees need a variety of food sources, and the best lawns have many kinds of flowers, hopefully with a range of bloom times. Creeping charlie is invasive, and can prevent you from growing additional flowers in your lawn. Instead, if you are looking to promote pollinator health in your lawn or garden, we recommend planting a diversity of flowers that produce high quality nectar and pollen consistently. That being said, if your lawn/garden is already overrun with creeping charlie, and you have not had a chance to eradicate it yet, you can at least take pleasure in seeing the bees buzzing around it, and note when they spend extra time on one bloom. They are likely hitting the jackpot!

Dickinson, R. and Royer, F. Weeds of North America, University of Chicago Press, Chicago, IL. 2014

Foord, K. 2016. The value of lawn weeds for pollinators: Not all weeds are created equal. Yard and Garden News, University of Minnesota Extension. Accessed 06/19/17 

Hultén, E. 1971. The Circumpolar Plants. II. Dicotyledons. Almquist & Wiksell, Sweden.

Hutchings, M.J., Price, E.A.C. 1999. Glechoma hederacea L. (Nepeta glechoma Benth., N. hederacea (L.) Trev.)

Rice, E.L. 1986. Allelopathic growth stimulation. The Science of Allelopathy (eds A.R. Putnam & C.S. Tang), pp. 34-40. Wiley, Chichester, UK.

Southwick, E. E. "Lucky Hit" nectar rewards and energetics of plant and pollinators." Comparative Physiology and Ecology 7.2 (1982): 51-55.

Southwick, Edward E., Gerald M. Loper, and Steven E. Sadwick. "Nectar production, composition, energetics and pollinator attractiveness in spring flowers of western New York." American Journal of Botany (1981): 994-1002.

Monday, May 29, 2017

Late spring flowers for pollinators

You can support pollinators by providing flowers throughout the growing season. Spring flowering plants like willows, maples, plums, and gooseberries provide bountiful nectar and pollen for bees that are active in spring. Many bee species in Minnesota are only out flying as adults during the first month of spring, taking advantage of these abundant floral resources to gather pollen to feed their young, which will be developing within the nest the rest of the year. After the early spring flowers have faded there is a gap before the next bounty of long-blooming flowers appears in early to middle summer. Flowers such as bee balm, purple prairie clover, and joe-pye weed, will continue sustaining bees throughout most of the summer.

In many landscapes, there is a lull between these two heavy blooming periods. Although the early spring bees have gathered all the food they need, there are other bees active throughout the growing season like bumble bees, sweat bees, small carpenter bees, and the European honey bee. These bees need continually blooming flowers from April through September. For bumble bees, late spring/early summer flowers are crucial for colony development as colonies start from scratch each year and the fledgling colonies need pollen and nectar to grow.

What can we provide for bees during this lull in bloom?

Here are a few great choices to fill in this gap:
Virginia waterleaf, Hydrophyllum virginiana, an easy to grow plant that can fill in shady understories. Two-spotted bumblebee, Bombus bimaculatus, photo by Heather Holm.

Wild lupine, Lupinus perennis, a great provider of nutritious pollen. Mason bee, Osmia sp., photo by Heather Holm.

Friday, May 26, 2017

Check your garden for flea beetles

Jeffrey Hahn, Extension Entomologist

Flea beetles are very small, 1/16th – 1/8th inch long. They are usually dark colored although some can have red or yellow on them. An easy way to identify flea beetles is that they can jump. Now is a
Flea beetles and their damage on turnips. Note the beetles'
small size.  Photo, J. Hahn, U of MN Extension
good time to check your garden for their presence.

Flea beetles attack a variety of vegetables, including beans, broccoli, cabbage, potatoes, squash, and radish. Flea beetles chew shallow pits and small holes into leaves. This feeding can be particularly damaging to seedlings and cole crops.
 For more information on flea beetles, including management, see Flea beetles in home gardens.

Tuesday, May 23, 2017

Powdery mildew covered shoots

M.Grabowski, UMN Extension Educator

Ninebark shoot covered in powdery mildew
M. Grabowski, UMN Extension
As the buds of trees and shrubs open and young shoots begin to grow, some will emerge already infected with powdery mildew. Powdery mildew, a common fungal disease of many garden plants, is easily recognized by powdery white growth on the surface of infected leaves, shoots, and other plant parts. Gardeners often describe infected plants as dusted by flour or having cobweb like growth on leaves.

Some powdery mildew fungi survive Minnesota’s harsh winter by colonizing young plant tissue within dormant buds. When the buds open in spring and the new shoot emerges, it will be covered by the characteristic white growth of powdery mildew. From that one severely infected shoot, spores are blown throughout the plant canopy starting new infections on healthy leaves from neighboring shoots.

Powdery mildew leaf spots that
originated from windblown spores
M. Grabowski, UMN Extension 
This type of winter survival is common on woody ornamental plants like ninebark, hawthorn, currant, and rose. Gardeners should carefully inspect young shoots of woody shrubs. If a powdery mildew infected shoot is found, it should be pruned out, and buried in the compost pile promptly.  Although spores can be blown in from other locations later in the season, removing a concentrated source from right within the plant canopy will help to delay the start of disease and slow the progression of symptoms.

Powdery mildew fungi steal nutrients from their host plants, but rarely cause significant damage to the health of the plant. By delaying the start of disease, gardeners can reduce this stress even further. 
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