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May Gardening Tips, Tasks, & Problems

Trees & Shrubs
  • Apples, crabapples, and hawthorns susceptible to rust disease should have protective fungicidal sprays applied beginning when these trees bloom. Continue spraying at 7-day intervals until the galls stop producing spores. 
  • Fertilize azaleas after bloom. Use an acid-lover, slow-release fertilizer if a soil test indicates. 
  • If required in severe cases, apply control for holly leaf miner (Phytomyza ilicicola) by mid-May on the new foliage.
  • Continue monitoring pines, especially Scotch and Mugo, for pine sawfly (Hymenoptera) activity on new shoots. 
  • Scale crawlers (Hemiptera) are active now. Infested plants should be treated at this time while the crawlers are young. 
  • Check pines and junipers for signs of tip blight. Sphaeropsis tip blight (Diplodia pinea) and Kabatina blight (Kabatina juniperi) are common in the St. Louis area. Prune out branches that are infected with Kabatina blight, and spray fungicide on branches infected with Sphaeropsis tip blight in early May. 
  • Keep an eye on boxwoods for boxwood leaf miner (Diptera) and boxwood psyllid (Psylla buxi). Prune back and dispose of the leaves before boxwood leaf miner adults emerge in late April/early May, as control after is more difficult.
  • Kousa dogwoods (Cornus kousa) and tulip trees (Liriodendron tulipifera) are blooming this month. 
  • Continue spraying new growth on roses with a fungicide to prevent black spot (Diplocarpon rosae) disease as needed.
  • Rose slugs (Hymenoptera) may be feeding on roses this month. Damage will first show as tan spots, and as the larvae grow, holes will appear as the damage becomes heavier.
  • Euonymus and other rose family shrubs may become infected with crown galls (Agrobacterium tumefaciens). Prune out infected branches, being sure to sterilize equipment after each cut, and/or destroy the infected plant. The bacterium can live in the soil for up to 2 years without a host plant. 
 

Annuals & Perennials
  • Don't prematurely remove spring bulb foliage or next year's flower production will decline. Allow the foliage to die down naturally or wait until most of the foliage has yellowed. Once foliage has died, bulbs can be moved or divided as needed.
  • Begin planting non-hardy bulbs such as gladiolus, caladiums, dahlias, cannas, and elephant ears as the ground warms and night temperatures consistently reach 50oF or higher. 
  • Plant hardy water lilies in tubs or garden pools. 
  • Peonies, false indigo, and irises are blooming this month. 
  • Begin planting warm-season annuals around mid-May. Mother's Day is usually a safe time to plant all warm-season plants and move overwintering tropical plants outside. Once planted, begin fertilizing as needed.
  • Pinch back mums to promote bushy growth. 
  • Prune plants that are prone to flopping over before they bloom. Examples include penstemon, monarda, and asters. 
 

Lawns
  • Keep bluegrass cut at 1.5 to 2.5 inches in height. Mow tall fescue at 2 to 3 inch height. 
  • Mow zoysia lawns at 1.5 inch height. Remove no more than 1.5 inches at each mowing. Zoysia lawns may also be fertilized at this time. Apply no more than 1 lb of lawn fertilizer per 1,000 sqft. 
  • Warm-season weeds are starting to show up now. Manually remove or apply an appropriate post-emergence as needed.
  • Keep an eye out for sod webworm (Lepidoptera) emerging now. 
  • Dethatch zoysia lawns as new growth begins to keep the lawn vigorous and reduce disease problems.
 

Fruits & Vegetables
  • Stone fruit trees with a history of peach tree borer (Synanthedon exitiosa) problems should receive their first spray in the first half of the month.
  • Place cutworm (Lepidoptera) collars around young transplants. Collars are easily made from cardboard strips.
  • Keep harvesting asparagus (3 years or older) to continue spear production. Stop as spears start to become thin. Scout for and control as needed for asparagus beetle (Crioceris spp.).
  • Watch out for striped and spotted cucumber beetles (Coleoptera) which may spread wilt and mosaic disease to squash and cucumber plants.
  • Warm-season crops can be set out and planted as the ground warms around mid-May: edamame, eggplant, okra, pepper, sweet corn, sweet potato, and tomato. 
  • Direct-sow seeds of warm-season vegetables early-mid May: basil, cucumber, melon, pumpkin, summer squash, and winter squash. 
  • Do not spray any fruits while in bloom. Refer to your local extension for information on spraying fruits (MU Extension). 
  • Prune unwanted shoots as they appear on fruit trees. 
  • Herbs planted in average soils need no extra fertilizing. Too much fertilizer may reduce flavor and pungency at harvest. 
  • Do not prune apples or other plants susceptible to fireblight (Erwinia amylovora), or trees in the plum family that are susceptible to black knot (Dibotryon morbosum) at this time of the year. These diseases can be easily spread at this time. 
 

Miscellaneous
  • Birds eat many insect pests. Attract them to your garden by providing good nesting habitats.
  • If spring rains have been sparse, begin irrigating. Especially for plants growing in full sun, most gardens need 1-2 inches of water a week between April and September.
  • Take houseplants outdoors when nights will remain above 50oF. Be sure to slowly increase light levels as exposure to direct sun immediately could result in sunburn damage. Most houseplants, when moved outdoors, prefer bright and indirect light, or direct morning sun. 
  • Cankerworms (Paleacrita vernata) rarely cause permanent damage to ornamentals. Use BT if control is deemed necessary. 
  • Slugs hide during the day, if plants are getting eaten place a board nearby and the slugs will likely gather under it during the day. Dispose of any slugs found. 
  • Now is the time to repot any overcrowded houseplants. Be sure to only go 1-2 inches up in pot size as using too large of a pot can harm the plant. 
  • In late May, young bagworm (Thyridopteryx ephemeraeformis) larvae begin emerging from the mother's bag and start forming their own. Collect and dispose of them as they are located.
  • Galls may be noticeable on various plants such as oaks, hackberry, and witch hazel. Galls are a normal part of plants and usually do not harm plants. Eriophyid mites are commonly responsible for these galls and do not require any control.