Yard & Garden Line News May 15, 2005

Yard & Garden Line News
Volume 7 Number 7 May 15, 2005

Features this issue:

A New Entrance for the Display and Trial Garden--Part 1
Spiny Elm Caterpillar
Prepare For Bean Leaf Beetles
Pearleaf Blister Mites
T.S. Eliot Was Wrong
May Garden Calendar
Editorial Notes

A New Entrance for the Display and Trial Garden--Part 1:
Julie Weisenhorn, Teaching & Technology Specialist --Landscape Program

The site.

Building the block wall.

Setting pavers.

The crew.

View from the back
of the garden
Photo credits: Julie Weisenhorn

The Dept. of Horticultural Science's Display and Trial Garden, on the University's St. Paul campus, is a popular destination for garden clubs, master gardeners, students, and the public during the growing season. This year's landscape design students created an entrance to the garden at the corner of Gortner and Folwell Avenues."We've always wanted to establish a welcoming entrance that also serves as a meeting location for tour groups and classes," explained Brad Pedersen, professor and landscape design faculty member. "This new entrance unifies the Display and Trial Garden and the new Plant Growth Facility across Gortner Avenue. The student designers considered the design of the Plant Growth Facility main entrance when designing the Garden's entrance. It was also important that there be handicap access through the new entrance and plenty of room for groups to congregate as well."

The students were part of the spring semester class, HORT 5021 Landscape Design, Implementation and Management II. As part of the coursework, students could choose three projects from a selection of nine. Four students chose the new entrance as one of their projects. As a team, they did the site analysis and worked with University Building Codes personnel and created a base plan for the site from which they would all develop their own design solution. While one student's design was selected as the main solution, features from the designs by the rest of the team were incorporated to create one final design.

As part of the course, the students installed the new entrance over a three-week period (about 30 hours). They demonstrated great problem-solving and some pretty impressive resolve throughout the project--even when our Minnesota climate ran the gamut from sun to wind to rain and even snow. Materials were donated by Anchor Block Company, Bloomington, MN, and Damon Roth, Tier One Landscaping, served as visiting contractor and supervisor of the students.

The site was excavated and irrigation removed. Students set the 6"x 6" green treated posts 42" down in concrete footings and back-filled with gravel. The walls and pillars were constructed of pre-fabricated block. The wall forms seating areas and the pillars will eventually feature lighting and large planted containers. The entrance was constructed with pavers, also donated by Anchor Block Company. Prior to paving, the students put down gravel base with fines. After compacting it to the proper depth of 8 ¼", the students laid down 1" of sand and screed it smooth using a screed board and rails supplied by Roth. The students selected a complicated pattern consisting of four different pavers. To maintain quality control, the students worked in teams to lay the pavers with some students doing the paving and others supplying them with the correct-sized pavers. Still other students were responsible checking for errors in the pattern and making sure the direction stayed on course.

With 28 students working, the paving went quickly. Sand was swept over the pavers and worked into the cracks to tightly hold the pavers in place. This type of construction also allows pavers to easily be replaced in the event they should crack. New soil was hauled in for the adjacent beds and the irrigation replaced. The students celebrated their project with a BBQ the last day of class.

With the spring weather finally cooperating, the next steps are construction of an arbor that will support Minnesota-bred grape varieties, and to plant the adjacent beds and several large containers that will soften and key the pillars of the new entrance. These projects will be done by staff and students working in the Garden over the summer. A new garden sign will also be installed later this month. Watch for developments in future issues Yard & Garden News!

Spiny Elm Caterpillar
Jeffrey Hahn, Assist. Extension Entomologist

Spiny elm caterpillar Photo credit
Jean-Pierre Bérubé

Mourningcloak butterfly Photo credit: Tom Peterson

You might encounter the spiny elm caterpillar, Nymphalis antiopa, this spring. This caterpillar prefers to feed on the leaves of elm and willow but you may also find it on a variety of other hardwood trees including birch, hackberry, linden, cottonwood, and poplar.

This caterpillar is the immature form of the familiar mourningcloak butterfly. This moderate sized butterfly has a wingspan of two to three inches. The wings are dark brown with a row of blue spots and a creamy yellow band along the edge of them. This is one of the few butterflies that overwinters as an adult so it is one of the first you will see in the early spring. This year, there were a number of sightings of mourningcloak butterflies during early April.

Once this butterfly is active in spring, it lays hundreds of eggs on a small twig. After the young larvae hatch, they feed gregariously, i.e., in non-social groups. It's common for these caterpillars to defoliate one branch first before moving to the next one. The spiny elm caterpillar feeds for about five to six weeks.

This striking looking caterpillar grows to be two inches in length when full grown. It has a black body with many tiny white spots. It can be distinguished from other caterpillars by the seven or eight conspicuous red spots running down the top of its back, red prolegs on its abdomen, and branched spines circling its body.

After pupating, adult butterflies appear in June and July. Look for caterpillars from a second generation in July and August. The larvae finish feeding at the end of the summer in time for the larvae to pupate and emerge as adults. These butterflies find sheltered areas, e.g. under leaves, to spend the winter.

Spiny elm caterpillars are common but usually do not develop high enough numbers to be considered a pest. It is possible that they can become very abundant on a local level and severely attack a small number of trees. In most cases, spiny elm caterpillar feeding does not seriously injure trees even if it is severe. As long as trees are healthy and well-established, they can tolerate even complete defoliation in a single season by these caterpillars.

However, if these caterpillars attack a recently transplanted tree (within the last few years) or a tree that is already stressed or unhealthy, severe feeding could injure the tree. If it's necessary to protect your trees from spiny elm caterpillars, it's best to treat them as soon as caterpillars are noticed. They smaller the caterpillars are when you treat them, the more effective the insecticide application will be while also reducing the feeding damage.

There are many insecticides registered for treating caterpillars. Check the label to find a product that lists the particular tree species you wish to treat. There are several environmentally friendly insecticides that are effective against caterpillars, such as Bacillus thuringiensis, insecticidal soap, and spinosad. If spiny elm caterpillars are approaching two inches in length when you first see them, just ignore them. They are finishing or are done feeding and treating them does not help the tree's health.

Prepare For Bean Leaf Beetles
Jeffrey Hahn, Assist. Extension Entomologist

Bean leaf beetles Photo credit: Kathleen Bennett

(Note: This article was adapted from a fact sheet prepared by K. Bennett et. al. It can be found on the VegEdge web site, http://www.vegedge.umn.edu/vegpest/BLB.htm)

Gardeners should be on the watch for bean leaf beetles, Cerotoma trifurcata. This beetle is an occasional pest of snap beans in home gardens. They are also a concern to farmers as these also eat soybeans, clover and dry edible beans.

This beetle is about 1/4 inch long, oval-shaped with the head visible from above. Most bean leaf beetles in Minnesota are yellowish-green with four black spots. However, you may also find some that are red and some that lack spots. Despite these differences in coloration, you can always recognize a bean leaf beetle by the black triangle at the top of its wing covers.

Bean leaf beetles overwinter as adults and become active sometime from mid-May to early June. They feed, mate and then lay eggs. Larvae hatch one to three weeks later and feed on the bean roots. Despite this feeding, they are not known to seriously damage plants. They feed for about two to three weeks and then pupate in the soil in earthen cells.

Adults emerge from mid-July through September. There is typically only one generation per year in Minnesota. It is possible that two generations can occur in the southern part of the state in which case the first generation appears in July and the second generation appears in late August - September.

Adult beetles prefer to eat young, tender plant tissue. They feed primarily on the undersides of leaves, creating round, 1/8 inch diameter holes. High populations of adults can defoliate the first true leaves and kill young seedlings. Extensive feeding can reduce the vigor and yields of bean plants. When pods form later in the season, adults will also feed on the outer surface of them, although this generally results in just cosmetic damage. Although bean leaf beetles are known to vector some plant diseases, this is generally not an issue in the home garden as most snap bean varieties are not considered susceptible.

If you have had a persistent problem with bean leaf beetles in the past, you can minimize damage from them by planting your snap beans a little later than normal. In southern Minnesota, plant snap beans in early to mid-June to allow them to escape damage from overwintering adults. Plant correspondingly later in central and northern Minnesota.

If you can not or do not wish to delay planting your snap beans or you don't have a history of beetle problems, just monitor your garden occasionally for bean leaf beetles. It's especially critical early in the season when the plants are more susceptible to feeding injury. The best time to look for beetles is in the afternoon between 12:00 and 4:00. If you find moderate or severe injury on 10% or more of your plants, you should protect your snap beans, especially if the first set of true leaves are present. As the snap beans put out more leaves, they are more tolerant of defoliation.

If it's practical, handpick the bean leaf beetles you find in your garden. Just knock them into a pail of soapy water. You can also spray your beans with an insecticide. Use a garden product containing esfenvalerate, permethrin, or carbaryl.

Pearleaf Blister Mites
Jeffrey Hahn, Assist. Extension Entomologist

Pearleaf blister mite damage. Photo credit: Jeff Hahn

Blister-like deformations are noticeable on the leaves of some pear trees now. They may appear to be the result of a disease but in fact are caused by tiny eriophyid mites called pearleaf blister mites. These mites are only 200 - 230 microns long which is approximately 1/125th inch long. You would need high magnification from a dissecting microscope to see them.

These mites overwinter under bud scales and when spring arrives they become active moving to developing leaves. As they feed on the undersides of leaves, small pimple-like growths appear. As more mites feed, these growths expand into reddish or pinkish blisters. As the summer progresses, they become larger and can merge with adjacent growths. Eventually, these blisters turn brown or black. Blisters that do not contain mites are green.

Despite this damage, these mites generally do not harm trees. It's possible that plant growth can be slowed on trees that are heavily infested but severe populations would probably need to occur over several consecutive years. For the most part, you can ignore the damage from pearleaf blister mites.

If you wish to protect your tree, you should make a dormant oil application in the spring. This will suffocate the overwintering mites before they attack the leaves. Miticides are not effective once you see blistering on the leaves.

T.S. Eliot Was Wrong
Janna Beckerman, Extension Plant Pathologist

Frost damage on native nannyberry. Photo credit:
Janna Beckerman

This year, May was the cruelest month. I can't say I didn't see it coming. In fact, I warned everyone (Yard and Garden News, Spring Garden Prescriptions, April 15), including myself. And even though I said it, deep down I knew that it could happen and kept my mulch mostly on the perennials, and continued about my garden, in complete and total denial. The fact that the warmest April on record came to a screeching halt on a 21 degree night in May isn't even that surprising-This is Minnesota. It just stinks, and there is nothing any of us can do about it, except maybe relish the development of global warming.

How Frost Damages Plants
At some point, you've probably complained about a hot summer day and said "It's not the heat, it's the humidity." Similar things can be said about this cold snap: It's not the frost, but the freezing and thawing that injures plant cell walls, resulting in damaged flowers, leaves, shoots, and even killing tender plants. Damage is often more severe in variegated, red, or golden-leaved plants. Frost damage reports are arriving, and include everything from potatoes, to Viburnum, hosta, impatiens, and flowering apples. Symptoms vary from complete death to scattered damage on leaves. In my own yard, my native, Minnesota provenance nannyberry (Viburnum lentago) had mildly, frost-damaged leaves, whereas my wayfaring tree (Viburnum 'Onondaga') 30 feet way didn't suffer a single lost flower. Rosa 'Scabrosa' suffered severe burns on a few leaflets, and a mystery rugosa hybrid may need to be removed, while nearby roses, R. rugosa cultivars 'Purple Pavement,' 'Snowy Pavement,' 'Blanc Double de Coubert,' R. gallica 'Tuscany Superb,' and R. 'William Baffin' were unaffected-and within 10 feet of the affected plants. Not only is May cruel, but frost damage appears somewhat capricious.

Fruit crops that were flowering during the hard frost may be the biggest losers, with reports of frost damaged apricots, and early flowering apples. If the frost damage resulted in blackened pistils, the flowers will not produce fruit. Both eating and ornamental cherries are showing signs of leaf wilting, followed by blackening and death. Many cherries (tart cherries, Nanking cherries) suffered severe blossom damage, so don't be surprised by a reduced harvest later this summer!

Frost damage on R. 'Scabrosa.' Nearby roses were unaffected. Photo credit:
Janna Beckerman

Diagnosing Late Frost Damage
For tender annuals, like impatiens, petunia, and begonia, the damage may appear as dark green, watersoaked areas, to blackening and death. Woody ornamentals show a variety of symptoms beyond what I've shown here. The Canadian Ministry of Forests has a great photo website on late frost damage on woody plants at: http://www.for.gov.bc.ca/hfp/forsite/pest_field_guide/frost.htm.

Managing Frost Damage
If your plants regularly suffer frost damage on certain sites, it may be time to reconsider your planting positions and your fertilizer practices. When siting plants, determine if frost pockets exist, and avoid planting early flowering woody plants. Frost pockets occur where cold air settles or becomes trapped (like the bottom of slope, or ditch, or between buildings). Late season applications of nitrogen-rich fertilizers stimulate succulent growth that is particularly susceptible to cold damage. Covering plants can protect them in a light frost, but may not help if plants aren't somewhat hardened off by a few preceeding days of cooler temperatures. Our cool weather leading up to the hard frost may have saved many of bush fruit crops, and other flowering ornamentals from more severe damage.

To prevent frost damage to apple, cherry, apricot or pear blossoms, do not plant trees with a southern exposure thereby preventing early flowering and decreasing the likelihood of frost damage. With all the lakes in Minnesota, growing early flowering fruit crops near a lake results in slowing down the spring warm up, thereby preventing early blossom development. If frost has occurred, an early morning watering prior to the flowers thawing may prevent the most severe damage from occurring.

For those of us with frost injured plants, when the threat of frost has passed, prune out any damaged growth, cutting back to an undamaged bud or sideshoot. Severely damaged annuals may need to be removed, and damaged perennials can be cut back. In between, don't forget to stop and smell the lilacs, and hope for better luck next spring.

Please check out the diagnostics web pages at
http://www.extension.umn.edu/projects/yardandgarden/diagnostics/

May Garden Calendar
Compiled from conversations with Bob Mugaas, Nancy Rose, Patrick Weicherding Regional Extension Educators

'Flair' tulips. Photo credit:
Beth Jarvis

We seem to be waiting for spring to start and stay started.

Lawns:
There's still time to put down pre-emergence herbicides for crabgrass control. The window is also wide open for post-emergence control of thistles, dandelion, creeping Charlie, etc.

Research indicates now, bloom time, is the second best time to treat creeping Charlie with traditional herbicides. We don't know why this is, fall is still the best time, but it's something resezarches have discovered.

Grass grows more rapidly in spring. Mow it when it needs it, about an inch taller than desired height, not because it's Saturday. Too close mowing, AKA "scalping" is never wise.

It's still ok to do aerification as it doesn't injure grass plants as much as vertical mowing/power raking does. It's too late to risk vertical mowing. Apply preemergence herbicides afterward as aeration brings weed seeds to the surface.

Trees and Shrubs:
Replenish mulch around trees and shrubs.

Do not prune elms and oaks yet.

Hold off on all pruning chores until later as trees and shrubs aren't fully leafed out. Ash seem to be leafing out slowly. Silver maples are full of maturing seeds and will leaf out more once they drop their seeds.

If your landscape trees or shrubs are showing deficiency symptoms, apply a slow release high nitrogen fertilizer. They need a greater amount of N than P or K.

We've all been taught and do teach that the rootballs of potbound trees and shrubs should be sliced, spread or otherwise re-directed. Based on research conducted in 2004 on extrememly pot-bound Niobe willow and little leaf-linden, cutting into the rootball doesn't seem to help. The U research is confirming research at other universities. Other genera will be guinea pigs in 2005.

Scout for carry-over Dutch elm disease infections. Watch for flagging/wilting or dead branches high up in the canopy.

Green ash are leafing out slowly this spring.

Flowers and Food:
Seed early vegetables and direct sown flowers. Warm weather crops should wait until late May.

It's hard on plants to make the transition from inside you house or a sunny nursery to the great outdoors. Any plants, from home-started tomato seedlings to a Mother's Day potted plant, need to harden off to make a successful transition.

Set plants outside in a shaded (eg: north side of the house) site for increasingly longer periods to time and over a week or so, move them into more light and for lkonger periods of time. Hardening off takes a week or so.

Mother's Day gifts and other houseplants can be moved outside for the summer around Memorial Day, providing it's warmed up.

Editorial Notes

'Daydream'

Mystery tulips Photo credits:
Beth Jarvis

The 'Daydream' tulips have been a real people stopper here at the Arboretum. They bloom yellow and mature to apricot. I thought Arboretum Landscape Gardener Duane Otto was repeating the open "one color-mature to another" routine with the tulips in front of the Oswald Visitors Center. I found a few in peach and the rest in pink, When I asked Duane about it, he said they'd been shipoped the wrong bulbs. They were supposed to be peach. He'd love to know what the pink ones are named.

Charlie Rohwer, a Horticulture grad student, has written an article entitled "How do those bugs eat?" It will run soon. Patrick Weicherding has agreed to write about what happens to trees when lightning strikes. Julie will continue the landscaping project story in a future issue.

Please feel free to cut and paste any of the articles for use in your own newsletters. All we ask is that you give our authors credit.

Back issues Yard & Garden Line News are on the Yard & Garden Line home page at www.extension.umn.edu/yardandgarden/.

Deb Brown answers gardening questions on Minnesota Public Radio's (MPR) "Midmorning" program on the first Thursday of every month at 10 a.m. The program is broadcast on KNOW 91.1 FM, and available state-wide on the MPR news radio stations. (Scroll down for map.)

For plant and insect questions, visit http://www.extension.umn.edu/askmg. Thousands of questions have been answered, so try the search option in the black bar at the top left of the board for the fastest answer.

If you would like to receive an e-mail reminder when the next issue of the Yard & Garden Line News is posted to the web, just send an e-mail to: listserv@lists.umn.edu (note: the second E in listserve is omitted), leave the subject line blank, then in the body of the message, type: sub yglnewslist or to unsubscribe, enter: unsub yglnewslist

Happy gardening!

Beth Jarvis
Yard & Garden Line Project Coordinator

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