Last Average Frost Date -- Once your region's last average frost date arrives, if it hasn't already, you can go ahead and plant warm-season annuals (tomatoes, peppers, basil, marigolds, petunias and the like).
- Those warm-season annuals include plants for containers, pots, windowboxes, and planters. Remove any cool-season flowers you may already have there. Work in a slow-release fertilizer.
- Plant seeds for corn, green beans, squash, cucumbers, okra, sweet potatoes and other heat-lovers once the soil has warmed to 60 degrees F. That's warm enough for you to walk on it comfortably barefoot, usually about two weeks after the last average frost date.
Planting Trees and Shrubs -- Continue to plant container-grown trees, shrubs, perennial herbs, and perennial flowers. You can give planting bare-root plants a try this month, but it's getting awfully late and they aren't as likely to thrive at this point as the more established container plants.
- Keep new plantings well-watered.
- After that frost date has passed, you can move your houseplants outdoors to a shady spot. It's a good time to repot and fertilize them to ready them for a summer growth spurt.
- After the frost date, plant tender summer bulbs outdoors, including dahlias, glads, cannas, and tuberous begonias.
Dividing Perennials -- Divide most perennials as long as they're not spring bloomers and as long as the foliage isn't more than 5 or 6 inches high. Divide them if they are getting crowded (reduced blooms, a dead spot in the middle) or you simply want more plants.
Deadheading 101 -- Deadhead spent flowerheads on spring-blooming bulbs to direct their energy back to their roots so they can build vigor for next year. Also deadhead fading flowers on other plants.
- Prune evergreens any time from now until late summer. (Don't prune later than that or you'll prompt new, tender growth that will get zapped by winter's cold.)
Mulch Matters -- By the end of the month, the soil will have warmed up enough that you can apply a layer of mulch on flower beds and around trees and shrubs. It reduces weeds, conserves moisture, and prevents disease. Great stuff!
- For mums, pinch off the last inch or so of the branches until July to assure bushy, well-flowering plants. While you're at it, cut back asters and other tall, floppy, late-summer bloomers by about one-third once they're a foot or so high. They'll be sturdier and flower better.
- Continue to fertilize roses, either with chemicals or compost. Some rose fanciers also swear by fish emulsion, an organic fertilizer.
Annual Stakes and Supports -- Stake tall plants that will need it now while they're just a foot or so high.