seMissourian.com: Column: Paul Schnare: Show some love to trees and shrubs, too
Because most trees and shrubs put on one flush of growth in the spring, it is important to have fertilizer available to them to support this flush of growth. (Some species/varieties put on more than one flush of growth per season, but their major flush is in the spring.) With that in mind, I would suggest that you apply fertilizer in March or early April.
There are many methods for fertilizer applications. You can purchase fertilizer spikes and pound them into the ground around the drip line. When you use this method, you will find a tuft of grass growing faster and greener than surrounding turf. These tufts suggest to me that fertilizer is applied in areas just around the spike. The roots growing between these spots are probably not absorbing much fertilizer.