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Missouri Environment & Garden


Missouri Botanical Garden

May Gardening Calendar

Missouri Botanical Garden

Published: April 1, 2012


  • Weeks 1-4: Apples, crabapples and hawthorns susceptible to rust disease should have protective fungicidal sprays applied beginning when these trees bloom.
  • Weeks 1-4: Pinch azaleas and rhododendron blossoms as they fade. Double flowered azaleas need no pinching.
  • Weeks 1-4: Fertilize azaleas after bloom. Use a formulation which has an acid reaction.
  • Weeks 1-2: Canker worms (inch worms) rarely cause permanent damage to ornamentals. Use B.T. if control is deemed necessary.
  • Weeks 1-2: Don't remove spring bulb foliage prematurely or next year's flower production will decline.
  • Week 1: Continue monitoring pines, especially scotch and mugo, for sawfly activity on new shoots.
  • Week 1: Begin planting gladiolus bulbs as the ground warms. Continue at 2-week intervals.
  • Week 1: Plant hardy water lilies in tubs or garden pools.


  • Weeks 2-4: Apply post-emergence broadleaf weed controls now if needed.
  • Weeks 3-4: Zoysia lawns may be fertilized now. Apply no more than 1 pound of actual nitrogen per 1,000 square feet.
  • Week 4: Watch for sod webworms emerging now.


  • Weeks 1-4: Place cutworm collars around young transplants. Collars are easily made from cardboard strips.
  • Weeks 1-4: Growing lettuce under screening materials will slow bolting and extend harvests into hot weather.
  • Weeks 1-4: 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.
  • Weeks 1-2: Plant dill to use when making pickles.
  • Weeks 1-2: Keep asparagus harvested for continued spear production. Control asparagus beetles as needed.
  • Weeks 1-2: Begin planting sweet corn as soon as white oak leaves are as big as squirrel ears.
  • Week 1: Isolate sweet, super sweet and popcorn varieties of corn to prevent crossing.
  • Week 1: Thin plantings of carrots and beets to avoid overcrowding.
  • Week 1: Control caterpillars on broccoli and cabbage plants by handpicking or use biological sprays such as B.T.
  • Week 1: Set out tomato plants as soils warm. Place support stakes alongside at planting time.


  • Weeks 1-4: Mulch blueberries with pine needles or sawdust.
  • Week 1: Don't spray any fruits while in bloom. Refer to local Extension publications for fruit spray schedule.
  • Weeks 2-4: Protect bees and other pollinating insects. Do not spray insecticides on fruit trees that are blooming.
  • Week 4: Prune unwanted shoots as they appear on fruit trees.


  • Weeks 1-4: Birds eat many insect pests. Attract them to your garden by providing good nesting habitats.
  • Weeks 2-4: Herbs planted in average soils need no extra fertilizer. Too much may reduce flavor and pungency at harvest.
  • Weeks 3-4: Take houseplants outdoors when nights will remain above 50 degrees. Most prefer only direct morning sun.
  • Weeks 3-4: Watch for fireflies on warm nights. Both adults and larvae are important predators. Collecting may reduce this benefit.
  • Weeks 3-4: 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 29, 2012