May Gardening Checklist:

Prepared by the DeKalb County Master Gardeners

Outdoor Gardening

• Plant warm-season flowering annuals, vines, herbs and vegetables after our area's average last frost date of May 15. Cautious gardeners often wait until Memorial Day before setting out cold-sensitive plants such as tomatoes, peppers, eggplants and squash. Pinch back one-third of new growth to encourage stocky habit (except vines). Be sure newly purchased annuals have been hardened off properly before planting them outside. Water transplants with a starter fertilizer to promote root development.
• Continue to plant new perennials, ornamental grasses and containerized roses. If roots are encircling the pot, make four cuts into the bottom of the root ball with a sharp tool, and flare the sections outward when planting.
• Stake tall perennials before they reach 6 inches. Begin to regularly pinch back fall-blooming perennials such as chrysanthemums and asters. Pinch once a week until the middle of July. This promotes stocky growth.
• Continue to check peonies for botrytis blight or other foliar fungal problems. Peonies that suffered from botrytis last year should be sprayed regularly this year starting when the plants are between 2 to 4 inches tall. Cage plants when they reach 10 inches to support flowers.
• Let spring bulb foliage yellow and wither before removing it. The leaves manufacture food that is stored in the bulb for next year's growth.
• Spray emerging lily shoots with anti-rodent spray if rabbits and deer have been a problem. Be sure to reapply after rainfall.
• Monitor all annual plantings in window boxes and containers. On warm days, hanging baskets can require water every day. Always water the soil before adding dilute quarter-strength fertilizer to containers. Consider incorporating water-conserving granules into container soil.
• Plant tender water lilies and lotus when water temperature is over 65 degrees F.
• Plant summer and fall flowering bulbs such as Asiatic and Oriental lilies, dahlias, peacock orchids (Acidanthera), cannas, tuberous begonias, caladium, crocosmia, freesia, gladioli, montbretia and calla lilies after May 15.

Woody Plant Care

• Trees and shrubs, including balled and burlaped evergreens, can still be planted this month. Plant on a cloudy day, early in the morning, to prevent heat and transplant shock. Water thoroughly and gently at planting time and continue with 1 inch of water a week, for the first year. Mulch root zones to conserve moisture. Keep mulch a few inches away from tree trunks and plant crowns or it will cause rots. Extend mulch out one to two feet beyond the planting hole.
• Prune spring-flowering shrubs and ornamental trees immediately after they bloom. These include forsythia, viburnums, lilacs, small magnolias, rhododendrons and azaleas. Prune to the ground old canes of forsythia and lilacs, or wait until late winter when plants are dormant. Not removing the spent blooms on rhododendrons after flowering will reduce flowering next year.
• Deadhead (or lightly prune) spent lilac blossoms to increase flower production. Lilacs are heavy feeders and can benefit from a second application of fertilizer after flowering. Broadcast a balanced granular formula 10-10-10 and water in well. Fertilizer with excessive nitrogen can encourage foliage at the expense of flower production.
• Fertilize roses with a liquid 20-20-20 solution when flower buds are set.
• Monitor roses for insects and diseases. Check every day for black spot, especially in wet weather. Do not handle rosebushes if foliage is wet and infected. Wait until leaves have dried before removing them and spraying. Monitor for rose slugs (small white caterpillars with black heads) and their damage (tissue-like patches in foliage).
• Succulent new green growth is particularly susceptible to aphid attack. Monitor newly planted shrubs, small flowering trees and perennials for signs of aphids–curled, distorted tip growth. Spray strong stream of water on damaged foliage to remove pests.

· Watch for the web-making Eastern Tent Caterpillar in crabapple and other flowering fruit trees. The dark caterpillar is two inches long with a white stripe on its back. Caterpillars return to the web in the evening. Prune out the branch the web is on or knock caterpillars into bucket of soapy water.

Lawn Care

• Mow lawn at 2 to 2-1/2 inches, removing one-third or less of the leaf blade. Leave clippings to return nutrients to the soil or add to compost heap, especially if clippings are thick and wet. If you are applying grass seed, do not use a pre-emergent weed killer in the same area.
• Fertilize lawn in mid-May if necessary. Late fall is an alternative time to fertilize. Monitor for weeds and hand pull or spot treat accordingly.

Fruit & Vegetable Gardening

• Be prepared for late spring frosts. Cover tender plants with row covers, cardboard, blankets, hot caps, or newspaper. Metal or plastic can conduct cold to plants. Average frost free day in this area is May 15.
• Thin carrots, beets and late lettuce.
• Harvest green onions, lettuce and radishes. Begin harvest of asparagus and rhubarb.
• Spread several inches of aged compost on vegetable and herb beds, if not done yet.

If you have questions or comments about this article, home gardening or about the Master Gardener program, please call the Master Gardeners c/o University of Illinois Extension, DeKalb County office at 815-758-8194 or email dekalb_mg@extension.uiuc.edu .