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Missouri Environment & Garden


Missouri Botanical Garden

May Gardening Calendar

Missouri Botanical Garden

Published: April 17, 2020


  • Apples, crabapples and hawthorns susceptible to rust disease should have protective fungicidal sprays applied beginning when these trees bloom.
  • Pinch azaleas and rhododendron blossoms as they fade. Double flowered azaleas need no pinching.
  • If spring rains have been sparse, begin irrigating, especially plants growing in full sun.
  • Fertilize azaleas after bloom. Use a formulation which has an acid reaction.
  • Canker worms (inch worms) rarely cause permanent damage to ornamentals. Use Bt if control is deemed necessary.
  • Don't remove spring bulb foliage prematurely or next year's flower production will decline.
  • Continue monitoring pines, especially Scotch and mugo, for sawfly activity on new shoots.
  • Begin planting gladiolus bulbs as the ground warms. Continue at 2-week intervals.
  • Plant hardy water lilies in tubs or garden pools.
  • Scale crawlers are active now. Infested pines and euonymus should be treated at this time.
  • Plant summer bulbs such as caladiums, dahlias, cannas and elephant ears.
  • Begin planting warm-season annuals.
  • Begin fertilizing annuals. Continue at regular intervals.
  • Trees with a history of borer problems should receive their first spray now. Repeat twice at 3-week intervals.
  • Bulbs can be moved or divided as the foliage dies.
  • Pinch back mums to promote bushy growth.


  • Place cutworm collars around young transplants. Collars are easily made from cardboard strips.
  • Growing lettuce under screening materials will slow bolting and extend harvests into hot weather.
  • 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.
  • Plant dill to use when making pickles.
  • Keep asparagus harvested for continued spear production. Control asparagus beetles as needed.
  • Begin planting sweet corn as soon as white oak leaves are as big as squirrel ears.
  • Isolate sweet, super sweet and popcorn varieties of corn to prevent crossing.
  • Thin plantings of carrots and beets to avoid overcrowding.
  • Control caterpillars on broccoli and cabbage plants by handpicking or use biological sprays such as B.t.
  • Set out tomato plants as soils warm. Place support stakes alongside at planting time.
  • 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.
  • Remove rhubarb seed stalks as they appear.
  • Watch for striped and spotted cucumber beetles now. Both may spread wilt and mosaic diseases to squash and cucumber plants.
  • Set out peppers and eggplants after soils have warmed. Plant sweet potatoes now.
  • Make new sowings of warm-season vegetables after harvesting early crops.


  • Birds eat many insect pests. Attract them to your garden by providing good nesting habitats.
  • Herbs planted in average soils need no extra fertilizer. Too much may reduce flavor and pungency at harvest.
  • Take houseplants outdoors when nights will remain above 50 degrees. Most prefer only direct morning sun.
  • Watch for fireflies on warm nights. Both adults and larvae are important predators. Collecting may reduce this benefit.
  • Sink houseplants up to their rims in soil or mulch to conserve moisture. Fertilize regularly.

Gardening Calendar supplied by the staff of the William T. Kemper Center for Home Gardening located at the Missouri Botanical Garden in St. Louis, Missouri. (www.GardeningHelp.org)

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REVISED: May 11, 2020