Gardening by Month - May
Monthly Tips and Tasks

Category
 
Week
 
Activity 
  1 2 3 4  
 Ornamentals x x x x Apples, crabapples and hawthorns susceptible to rust disease should have protective fungicidal sprays applied beginning when these trees bloom.
  x x x x Pinch azaleas and rhododendron blossoms as they fade. Double flowered azaleas need no pinching.
  x x x x If spring rains have been sparse, begin irrigating, especially plants growing in full sun.
  x x x x Fertilize azaleas after bloom. Use a formulation which has an acid reaction.
  x x     Canker worms (inch worms) rarely cause permanent damage to ornamentals. Use Bt if control is deemed necessary.
  x x     Don't remove spring bulb foliage prematurely or next year's flower production will decline.
  x       Continue monitoring pines, especially Scotch and mugo, for sawfly activity on new shoots.
  x       Begin planting gladiolus bulbs as the ground warms. Continue at 2-week intervals.
  x       Plant hardy water lilies in tubs or garden pools.
    x x x Scale crawlers are active now. Infested pines and euonymus should be treated at this time.
    x x   Plant summer bulbs such as caladiums, dahlias, cannas and elephant ears.
    x     Begin planting warm-season annuals.
      x x Begin fertilizing annuals. Continue at regular intervals.
      x x Trees with a history of borer problems should receive their first spray now. Repeat twice at 3-week intervals.
      x x Bulbs can be moved or divided as the foliage dies.
        x Pinch back mums to promote bushy growth.
 Lawns x x x x Keep bluegrass cut at 1.5 to 2.5 inch height. Mow tall fescue at 2 to 3.5 inch height.
    x x x Mow zoysia lawns at 1.5 inch height. Remove no more than one-half inch at each mowing.
    x x x Apply post-emergence broadleaf weed controls now if needed.
      x x Zoysia lawns may be fertilized now. Apply no more than 1 pound of actual nitrogen per 1000 square feet.
        x Watch for sod webworms emerging now.
 Vegetables     x x x x Place cutworm collars around young transplants. Collars are easily made from cardboard strips.
  x x x x Growing lettuce under screening materials will slow bolting and extend harvests into hot weather.
  x x x x Slugs will hide during the daytime beneath a board placed over damp ground. Check each morning and destroy any slugs that have gathered on the underside of the board.
  x x     Plant dill to use when making pickles.
  x       Keep asparagus harvested for continued spear production. Control asparagus beetles as needed.
  x       Begin planting sweet corn as soon as white oak leaves are as big as squirrel ears.
  x       Isolate sweet, super sweet and popcorn varieties of corn to prevent crossing.
  x       Thin plantings of carrots and beets to avoid overcrowding.
  x       Control caterpillars on broccoli and cabbage plants by handpicking or use biological sprays such as B.t. 
  x       Set out tomato plants as soils warm. Place support stakes alongside at planting time.
    x x   Place a stake by seeds of squash and cucumbers when planting in hills to locate the root zone watering site after the vines have run.
    x x   Remove rhubarb seed stalks as they appear.
    x     Watch for striped and spotted cucumber beetles now. Both may spread wilt and mosaic diseases to squash and cucumber plants.
      x x Set out peppers and eggplants after soils have warmed. Plant sweet potatoes now.
        x Make new sowings of warm-season vegetables after harvesting early crops.
 Fruits x x x x Mulch blueberries with pine needles or sawdust.
  x       Don't spray any fruits while in bloom. Refer to local Extension publications for fruit spray schedule.
        x Prune unwanted shoots as they appear on fruit trees.
 Miscellaneous x x x x Birds eat many insect pests. Attract them to your garden by providing good nesting habitats.
    x x x Herbs planted in average soils need no extra fertilizer. Too much may reduce flavor and pungency at harvest.
      x x Take houseplants outdoors when nights will remain above 50 degrees. Most prefer only direct morning sun.
      x x Watch for fireflies on warm nights. Both adults and larvae are important predators. Collecting may reduce this benefit.
      x x Sink houseplants up to their rims in soil or mulch to conserve moisture. Fertilize regularly.
May Pests and Problems


Hold off planting warm season vegetables, such as tomatoes, eggplants, vine crops; herbs, and warm-season annuals until the soil warms, usually in mid to late May.
 
 
 

Continue to inspect for and treat for holly leafminers as new leaves emerge.
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Holly Leafminer
Tunneling damage in holly (Ilex) leaf caused by holly leafminer, a fly maggot (Diptera) 
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Holly Leafminer
Holly leafminer maggot (Diptera) between layers of leaf tissue on inkberry holly (Ilex glabra
 

Collect and dispose of bagworms as well as cedar-apple galls on junipers before the orange spore-producing structures emerge from the galls.
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Bagworms
Female bagworms (Lepidoptera) move bag and all to some very odd places and then die leaving behind bags of eggs.These bagworms probably moved to this fire hydrant from the juniper nearby. 
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Cedar-Apple Rust
Galls of cedar-apple rust on juniper (Juniperus
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Cedar-Apple Rust
When orange gelantized telia appear on the cedar host (Juniperus virginiana), basidiospores from it are infecting the apple host (Malus) of cedar-apple rust 

Apply protective sprays on apples and hawthorns for rust disease if warranted.
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Cedar-Apple Rust
Pustules of cedar-apple rust on hawthorn leaf (Crataegus
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Cedar-Hawthorn Rust
Hawthorn leaves (Crataegus) showing pustules of cedar-hawthorn rust 
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Cedar-Quince Rust
Cedar-quince rust on hawthorn fruit and twigs (Crataegus

Do not prune apples and other plants susceptible to fireblight at this time of year as you can spread the disease. This also applies to black knot, which affects plums and other member in the Prunus genus.
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Fireblight
Fireblight canker and twig dieback on crabapple (Malus
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Black Knot
Black knot on plum (Prunus
 

Check pines and junipers for signs of tip blight. Spaeropsis tip blight and Kabatina blight are common in the St. Louis area. Cankerworms may be observed at this time of year but they usually do little damage and do not warrant control.

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Sphaeropsis Tip Blight of Pines (Diplodia Tip Blight)
Dying branch tips on Austrian pine (Pinus nigra) caused by sphaeropsis tip blight of pines 
 
 

Monitor pines for sawfly damage. Also check for crawlers of euonymus and pine scales.
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Pine Sawflies
European pine sawfly (Hymenoptera) rearing up in characteristic defensive posture on pine (Pinus
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Euonymus Scale
Euonymus scale on variegated euonymus (Euonymus
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Pine Needle Scale
Heavy infestation of pine needle scale (Hemiptera) on mugo pine (Pinus mugo

Treat for borers if found in dogwood, ash, lilac, apple, and peaches. Apply a preventative insecticide spray to the trunks of peach trees and other stone fruits in the first half of the month to control peachtree borers. Apply again in August.
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Dogwood Borer
Damage at root collar on flowering dogwood (Cornus florida) from dogwood borer (Lepidoptera); note white larvae (caterpillars). J. Solomon, USFS, Bugwood.org 
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Lilac Borer and Ash Borer
Sap flow from a borer hole (Lepidoptera) in trunk of a summit ash (Fraxinus pennsylvanica 'Summit'
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Peachtree Borer
The thick, gummy or jellylike sap mixed with frass at the base or crown of this peach tree (Prunus) is caused by a heavy infestion of peachtree borers (Lepidoptera) 

Scout for slugs, which chew holes in leaves and cutworms, which can cutoff young plants at ground level. The rose slug can cause damage to roses.
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Slugs and Snails
Slug damage on hosta leaves (Hosta
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Rose and Pear Slugs (Sawflies)
Rose slug, a sawfly larva (Hymenoptera), and feeding damage on upper leaf surface of rose (Rosa) leaves 
 

Watch for sod webworms towards the end of the month.
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Sod Webworm
Sod webworm adult (Lepidoptera) on lawn grass 
 
 

Treat broadleaf lawn weeds if warranted while they are still actively growing. Weeds are more difficult to kill with herbicides as their growth slows when weather warms.
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Perennial Broadleaf Weeds in Lawns
Dandelion in bloom, a common perennial weed in lawns and gardens 
 
 

Monitor azaleas for early signs of lacebug damage.
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Lace Bugs
White flecking on upper leaf surface on azalea (Rhododendron) caused by feeding of lace bugs (Hemiptera); heavy infestation 
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Lace Bugs
Heavy infestation of lace bugs (Hemiptera) on underside of azalea (Rhododendron) leaves; silvery white adults, dark nymphs, dark spots of excrement 
 

Scout for eastern tent caterpillars and destroy webs if found. The forest tent caterpillar is on the increase in the St. Louis area. It feeds on many common tree species. It does not form tents like the eastern tent caterpillar despite it's name.
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Eastern Tent Caterpillar
Eastern tent caterpillar (Lepidoptera) inside their web on crabapple (Malus
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Forest Tent Caterpillar (FTC)
Forest tent caterpillar with quarter for size. 
 

If boxwood leaves are infested with leafminers, some control can be obtained by pruning the plants back and disposing of the leaves before the adults hatch in late May. The boxwood psyllid causes cupping of leaves.
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Boxwood Leafminer
Damage to boxwood (Buxus) caused by boxwood leafminer (Diptera), a fly maggot; later stage 
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Boxwood Leafminer
Close-up of boxwood leafminer (Diptera), a fly maggot, exposed in the tunnel it has eaten between the layers of a boxwood (Buxus) leaf 
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Boxwood Psyllid
Characteristic cupping of leaves on boxwood (Buxus) caused by the boxwood psyllid (Hemiptera). 

Other problems to be on the lookout for are anthracnose of sycamore and the taxus mealybug.
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Sycamore Anthracnose
Sycamore anthracnose on sycamore leaf (Platanus); note that the spots are along the veins and angular in shape rather than round 
 
 

Dethatch zoysia lawns as new growth begins to keep the lawn vigorous and reduce disease problems.
 
 
 

Indoor plants moved outside for the summer are very susceptible to sunburn and wind whipping. Acclimate plants gradually to outdoor conditions to avoid setting them back or damaging tender new growth.
 
 
 

Be on the lookout for asparagus beetles.
 
 
 

Do not harvest and eat rhubarb that has been damaged by a late frost.