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).
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.
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.
Smart Pruning -- Finish up any pruning this month with the exception of spring-blooming trees and shrubs. You can prune them immediately after they're done flowering.
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!
Annual Stakes and Supports -- Stake tall plants that will need it now while they're just a foot or so high.