Begin to harden off warm-season transplants, moving them into a cold frame or protected area.
Gradually move houseplants outside to protected areas. Large houseplants in plastic pots should be slipped into larger heavier pots to prevent them from falling over in wind. Guard against overexposure to afternoon sun. Carefully monitor for insects during their time spent in the garden.
Overwintered tender annuals or tropicals (e.g., hibiscus, gardenia, geranium) may be pruned, fertilized, and taken outside once night temperatures reach 40 degrees.
Amaryllis bulbs (in their pots) can be moved to a protected spot in the garden where they receive morning sun. Fertilize twice a month with a dilute 15-30-15 mix.
Plant warm-season flowering annuals, vines, herbs, and vegetables after the Chicago 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. Avoid fertilizing newly planted annuals for two weeks.
Continue to plant new perennials, ornamental grasses, and roses in containers. If plant roots are root-bound (encircling the pot), make four cuts into the bottom of the root ball with a sharp tool, and flare the sections outward when planting.
Provide a gentle water drip for migrating birds. The May migrants — warblers, tanagers, orioles and buntings — are all attracted to shallow pools and the slight pinging sound of dripping water.
Stake tall perennials before they reach 6 inches. Begin to regularly pinch back fall-blooming perennials such as chrysanthemums, asters and tall sedums. Pinch once a week until the middle of July. This promotes stocky growth.
Continue to direct the growth of perennial vines on their supports. Climbing roses should be encouraged to develop lateral, flower-bearing canes.
Continue to check peonies for botrytis blight or other foliar fungal problems. Peonies that suffered from botrytis or bud blast last year should be sprayed regularly, starting when the plants are between 2 to 4 inches tall. Cage or provide support for peony blossoms when the plants are 10 inches tall.
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. Even braiding the foliage of daffodils can reduce the food production of the leaves.
Spray emerging lily shoots with antirodent 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, windy days, hanging baskets will require water every day. Always water the soil thoroughly before adding dilute quarter-strength fertilizer to containers. Terra cotta pots will dry out faster than plastic. Consider incorporating water-conserving granules into container soil.
Plant tender water lilies and lotus when the water temperature is over 65 degrees.
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.
Trees and shrubs, including balled and burlapped 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 for the first year with 1 inch of water a week, spread throughout the root zone. Mulch root zones to conserve moisture.
Prune spring-flowering shrubs and ornamental trees immediately after they bloom. These include forsythia, viburnum, lilac, small magnolias, rhododendrons, and azaleas. Prune to the ground old canes of forsythia and lilac. Alternative time to do renovation pruning is in late winter when plants are dormant. Deadhead (or lightly prune) spent lilac blossoms to increase flower production. Avoid fertilizer with excessive nitrogen; it can encourage foliage at the expense of flower production.
Lilac blossoms will last longer indoors if they are cut in the morning on a long woody stem when the flower is only half open. Cut a second time indoors before putting in a vase and make a vertical slit up the woody tissue.
Gently pull off dried flowers of azaleas and rhododendrons. New sticky shoots are located at the base of these flower trusses. Take care not to break these shoots when removing flowers. To increase flower production for the following year, pinch off one-half of this new green growth when it is at least one inch in length.
Fertilize roses with a liquid 20-20-20 solution when flower buds are set.
Monitor roses for insects and diseases. Check daily 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 roses for rose slugs (small white caterpillars with black heads) and their damage (tissuelike patches on the leaves).
Succulent new green growth is particularly susceptible to aphid attack. Monitor newly planted shrubs, small flowering trees, and juicy perennials for signs of aphids — curled, distorted tip growth. Spray a strong stream of water on damaged foliage to remove pests.
Mow lawn at 2 to 2½ inches, removing one-third or less of the leaf blade. Leave grass clippings on the lawn to return nutrients to the soil, or add them to compost heap. Rake clippings slightly if they are heavy 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 a preferable time to fertilize. Monitor for weeds and hand pull or spot treat accordingly.
Plant corn, snap beans, summer squash, and New Zealand spinach in mid-May.
Thin carrots, beets, and late lettuce.
Harvest green onions, lettuce, and radishes. Any of the mesclun mix or cut-and-come-again lettuces can be harvested to a few inches three separate times before the plants have exhausted themselves.
Harvest mature asparagus and rhubarb.
Spread several inches of aged compost on vegetable and herb beds, if not done yet.
Remove flowers of June-bearing strawberries as soon as they appear. This is necessary just for the first growing season. The plants will now develop a stronger root system.
Remove flowers for everbearing and day-neutral strawberries as soon as they appear. Flowers that develop after July 1 can be left on the plants to set fruit for later in the season.