Last Average Frost Dates -- Plant seeds for corn, green beans, squash, cucumbers, 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. If you've already planted corn, plant another crop or two every two weeks to help spread out the harvest.
Planting Trees and Shrubs -- Continue to plant container-grown trees, shrubs, perennial herbs, and perennial flowers. Finish up planting warm-season annual flowers, vegetables, and herbs, such as marigolds, coleus, fuchsias, tomatoes, and basil.
Last Average Frost Dates -- A couple of weeks after your last average frost date, the soil has warmed enough to plant tender summer bulbs outdoors, including glads, cannas, and tuberous begonias. For a continuous supply of glads, keep planting a fresh crop every two weeks.
Dividing Perennials -- You can still divide most perennials as long as they're not spring or early summer 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 -- Keep deadheading! For the most flowers and tidiest garden, deadhead daily. Some gardeners take a few minutes each morning, making it part of their daily routine. And since roses are going full-tilt, it's especially important to deadhead roses to keep the blooms going longer.
Smart Pruning -- Prune spring-blooming shrubs and trees, such as lilacs, forsythia, and crabapples, as soon as possible after bloom.
Mulch Matters -- If you haven't already, apply a layer of mulch on flower beds and around trees and shrubs once the soil has warmed sufficiently. As a rule of thumb, this happens about the same time the tulips have faded. Mulch reduces weeds, conserves moisture, and prevents disease. Great stuff!
Feeding Roses -- Continue to fertilize roses. In cold regions, Zones 5 and colder, keep fertilizing to a minimum. Studies have shown that keeping your roses a little "hungry" helps them overwinter better. A lean diet prevents too much lush green growth, which can get badly zapped in the winter.
Annual Stakes and Supports -- Stake tall plants that will need it now while they're just a foot or so high.