You need to fertilize trees and shrubs to insure their good health which prepares them to fight off pests, disease, and environmental stresses. A proper fertilization program can't solve all tree problems and over fertilization can do harm. Still, tree fertilizing at the appropriate time and with the appropriate formulation will help your tree(s).
Ideally, growing trees should be fertilized throughout the year. The greatest amounts should be applied during the early spring and summer months. Right now is an excellent time to fertilize trees.
For young trees, good times to put out fertilizer is late March through early June, and while you are at it, proper mulching will aid in proper nutrient and water uptake. When a tree reaches the desired height you may decrease the fertilizer application to only once a year.