If you have trees in the middle of your lawn, you really don't need to fertilize. Time to put out bulbs for summer. Also, time to fertilize your bulbs that have just finished blooming. Perennials will soon be picked over and gone, so do your perennial planting now. Time to divide your existing perennials, as well.
Control aphids early in the season by applying malathion or insecticidal soap.
Encourage ladybugs - they dine on aphids. If you are encouraging ladybugs, don't kill them with insecticide, however. Don't forget to leave room for all the annuals which fill in your garden between perennial blooms. Roses need feeding now. Any pruning from winter can be done now but don't forget they bloom on old wood.