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Wisconsin Yard and Garden Tips

May, 2015

Plant Hardiness Zones Map | Last Spring Frost Map | First Fall Frost Map
UW Extension County Offices
By Sharon Morrisey, Consumer Horticulture Agent, Milwaukee County UW-Extension
Wisconsin Yard and Garden Tips is updated monthly by Milwaukee County UW-Extension. Applicability in northern Wisconsin counties may be delayed one to two weeks in spring, and advanced a like period in fall.

Spring is like riding a roller coaster. April has lots of little ups and downs being warm and wonderful one minute then cold and dark with a little snow the next. In this metaphor, May is the big thrill. This month the trees go from a greenish haze to full leaf and shadows on the ground. Perennial flowers go from tiny shoots poking out their heads to poppies and peonies by Memorial Day.

The Earth is back from the dead.

It is still a month of preparations for summer. Early May is still too soon to plant most annual flowers like impatiens, begonias, coleus, and marigolds and so on. Itís a perfect time for perennials, on the other hand. In southeastern Wisconsin, find new plants at the Perennial Plant Sale of the UW-Extension S.E.W. Master Gardeners. It will be on Saturday, May 16th from 8:00 a.m. to 1:30 p.m. at State Fair Park in the DNR Natural Resources Park at the southwest corner of the grounds. To celebrate their 20th year of the sale, there will be special gifts and door prizes. For more information visit SEWMGPlantSale on facebook.

First Week

Scout the lawn for bare patches. Rake up dead plants and debris and work up the soil. Sow grass seed now so it can fill-in and outcompete the crabgrass seed that will germinate soon, too.

If you choose to use a crabgrass preventer instead of reseeding, spot treat only problem areas rather than using combination fertilizers with crabgrass preventers. Crabgrass will not grow in shade or anywhere the lawn is healthy. Treatment of the entire lawn is therefore seldom necessary.

If your lawn needs dethatching or core aerating accomplish these now while the grass is growing rapidly due to the moist, cool spring weather. Core aerating is the preferred treatment for lawns on heavy clay soils that suffer from compaction, lumpiness, thinning and thatch.

A thin lawn can be overseeded right after core aerating or topdressing. Alternatively, use a slit seeder to get the seed into the soil where it is less likely to dry out. Be sure to keep it watered until all the seed has germinated which is about two weeks for bluegrass.

Donít roll to smooth out a lumpy lawn, core aerate or topdress instead. Rolling can compact the soil especially if it is heavy clay. You may need to core aerate again this fall if the lumpiness is severe.

You can still start seeds indoors of Brussels sprouts, okra, pumpkin, cucumber, winter squash, melons, eggplant, pepper, and tomato. Sow vine crops in individual peat pots since these do not transplant well if roots are disturbed.

Outdoors you can sow seeds directly into the garden for beets, carrots, chard, kohlrabi, late cabbage, leaf lettuce, mustard, collards, turnips, radish, spinach, onion sets, onion seeds for bunching onions, peas, and potatoes.

Purchase and plant trees and shrubs now. Prepare planting holes for balled and burlapped and container-grown plants twice as wide as the root ball. Do NOT disturb the soil at the bottom of the hole, however, to prevent settling later on. Backfill with the same soil you removed - do not mix with soil amendments such as peatmoss. Wait until the end of the second year to fertilize new plantings. Mulch the entire rootzone with 4" of organic matter.

Crabapple and apple varieties that are not resistant to apple scab disease tend to get dark green leaf spots and drop most of their leaves sometime in August every year. If possible, replace with a resistant variety. Some hawthorns and Mountain Ashes are also prone to scab. When the new young leaves are expanded only about one-quarter inch is the perfect time to begin a fungicide program.

Examine fruit trees and ornamental crabapples and cherries for clusters of tiny, hairy, Eastern tent caterpillars. Remove and destroy or prune out limbs where they are nesting. Or use the botanical insecticide B.t. (Bacillus thuringiensis) while they are still small.

Second Week

Although fall is the best time to control broadleaf weeds like dandelions, clover, thistle, and plantain, herbicides containing 2,4-D or three-way herbicides will be beneficial now if there is a serious weed problem or if you need to spot treat. Wait until late September to use a weed and feed product since that is a better time for both the weed and the feed treatments.

Watch for creeping Charlie, also called ground ivy, to start flowering in the lawn and garden. Hand-weeding is very helpful but seldom completely controls this vigorous weed. Weedkillers containing 2,4-D (Weed-B-Gone and many others) have proven effective if applied at full bloom and again after the first frost.

Begin hardening-off frost tender plants now including vegetables, herbs, perennial and annual flowers that have been started indoors as well as summer flowering bulbs such as tuberous begonia, canna, dahlia, and caladium.

Plants of broccoli, cauliflower, early cabbage, Brussels sprouts, head lettuce, onions, and parsley can tolerate some frost and can be moved to the garden now. Select smaller rather than larger plants of the cole crops (broccoli, cauliflower, cabbage, Brussels sprouts) since overly mature plants exposed to low temperatures early in the season tend to bolt into flower too early.

Look for iris borer larvae tunnels in new foliage. Destroy by hand if infestation is light or if this is the first year they have been present. In older plantings that were not thinned last fall, dig and destroy infested plants. If you can find it, there is now a biological control product for iris borer control that consists of nematodes. These microscopic soil inhabitants prey upon the borers.

Fertilize perennial flowers now as growth is beginning. Most will only need fertilizing every three years and only at this time of year.

Fertilize roses with one tablespoon of a complete, low nitrogen fertilizer after pruning. Repeat monthly or only after the first flush of blooms sometime in June. Repeat later in the season only if foliage is pale.

Institute control recommendations for anthracnose and spur blight of raspberry now if diagnosed with this disease last year. (UWEX bulletin A2128-"Raspberry Pest Management For Home Gardeners".)

Hang phermone traps designed to attract codling moths in apple trees to begin monitoring them. How many are trapped will determine if chemical control is necessary.

To control broom-forming leaf-folding aphids on honeysuckle, prune out dried "brooms" and begin spraying new growth with insecticidal soap. Better yet, take the plunge this year and replace your honeysuckles with something less susceptible to pests and less invasive in our natural areas.

Examine Spindletree Euonymus and Eastern Wahoo for clusters of small Euonymus tent-forming caterpillars that will cover the tree and strip it of leaves within a couple of weeks if it is not controlled. While not deadly to the plant, repeated annual defoliation will weaken it. Remove, prune-out, and destroy or spray with the botanical insecticide B.t. (Bacillus thuringiensis) while the caterpillars are small.

Third Week

Sow seeds outdoors of beans, okra, pumpkin, sweet corn, and watermelon. Plant only partial rows of beans and sweet corn so that successive plantings can be done every week or two. Sweet corn should be planted in paired rows for good pollination.

Remove flowers as they fade from spring flowering bulbs like tulips and daffodils. Fertilize and allow the leaves to grow until they turn yellow and die. They are required for next year's flower buds to be formed.

Easter lily plants can be planted into the garden once the temperatures have stabilized. Remove the spent flower but do not cut off the stem and leaves. Plant bulbs 6 - 8" deep in a well-drained, sunny, protected site.

Gladiolus corms may be planted now. Plant a few every 7 - 10 days until early July to assure a continuous cutting.

Fertilize peonies. For larger blossoms, pinch off the secondary flower buds as they form. Insert 2 - 3 foot long twiggy sticks near the clump's center to provide support to the developing stems.

When chrysanthemums and asters reach 6 - 7 inches tall, start pinching off one-half inch of each stem. Continue pinching until July 1st to produce a bushy plant with lots of flowers.

Plant strawberries, grapes, and raspberries. Pinch-off flowers from newly planted strawberries this year to strengthen them. (UWEX bulletin A1597-"Growing Strawberries in Wisconsin".)

Check expanding leaves of honeylocust trees for signs of damage from honeylocst plantbugs. This insect almost completely defoliates many trees in some years. The trees often recover from this early infestation especially if sprayed with insecticidal soap.

Ash plantbugs cause small, light-colored spots on new leaves. Insecticidal soap can be used to limit the damage if the leaves become disfigured.

Pine needle scale insects are tiny, white, raised specks attached to needles of pines, especially mugho, the most common dwarf pine. If possible, prune out and destroy infested branches. Insecticidal soap is effective, too, but only to kill the tiny, reddish "crawlers" that can be seen only with good eyes or a hand lens. Dormant oil in late winter or early spring smothers the adults but it is too late for that now. Ultra-fine or superior oil is a product that can be used to smother insects during the growing season without damaging most plants. Read the label for exceptions.

Look for the tiny yellow circular spots on newly emerging birch leaves which indicate feeding of the birch leaf miner. This early generation of the insect causes stress on the tree that can render it more susceptible to a more serious pest, bronze birch borer. Several different pesticides can be used to control it. Merit can be soil injected in the fall. Di-syston granules can be placed in holes punched into the rootzone in early spring. Or one of several chemicals can be sprayed onto the leaves when the spots are first seen.

Bronze birch borer will feed just under the bark of susceptible, white barked birches. Preventative insecticide treatments should be made annually around mid-May, mid-June, and mid-July.

Fourth Week

Transplant Brussels sprout, late cabbage, cucumber, and melon plants to the garden now. Seeds of summer and winter squash and cucumber may be sown directly into the garden.

Tuberous begonia, dahlia, canna, and caladium plants may be transplanted outdoors. Begonias and caladiums require shade while dahlias and cannas thrive in the sun.

Gather materials and equipment to make what should be your first fertilizer application to the lawn on June 1st. If you have been consistently fertilizing your lawn for 15 years, you can skip the July 4th application. The third and final application should be around Labor Day. If you want to use a weed and feed product to kill broadleaf weeds like dandelions and creeping Charlie, wait until late September but no later.

Pines, early spring flowering shrubs, and trees that tend to ooze a lot of sap can be pruned now. Do NOT prune anything after June 1st since this can stimulate new growth that will not mature sufficiently to survive the coming winter.

Oaks, however, should never be pruned after April 15th because of the risk of contamination by the oak wilt fungus which is carried by picnic beetles.

It is finally safe to plant almost everything outdoors - tender annual flowers like impatiens as well as tomatoes, peppers, and eggplants. Houseplants, too, can be moved to a shady spot in the yard for their summer vacations.


For more information, contact Home Horticulture Agent Sharon Morrisey.

Plant Hardiness Zones Map | Last Spring Frost Map | First Fall Frost Map | UW Extension County Offices

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