A midsummer feeding is largely wasted on a tree; the best time is early spring, just as growth begins. A fall feeding, after the leaves are down but before the ground freezes, is also good. Newly planted trees need phosphorus for root growth. Later on, the main need is nitrogen for leaves.
Feed an established tree every other year if it is not making normal growth. If its leaves are severely damaged by insects during the summer, however, feed it that fall to aid recovery. For deciduous trees more than 6 inches in diameter at chest height, use 3 to 5 pounds of granulated fertilizer per inch of diameter; for smaller trees, use 2 to 4 pounds per inch. Evergreens need half those amounts.