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.
Before applying, mix high-nitrogen fertilizer, such as 10-6-4, with peat moss or humus to ensure even distribution. Make concentric circles of small, 12-inch-deep holes around the tree and fill them with the mixture. Start 3 to 4 feet from the trunk and make a new circle every 2 feet, spacing the holes 2 feet apart. Make the last circle 2 feet past the drip line.
Mix liquid fertilizer according to the directions on the package; space applications as above. Solid spikes of time-release fertilizer are also available for long-lasting nutrition; drive them into the ground according to the manufacturer’s directions.
If a tree looks sickly with no apparent cause, test the soil before feeding to be sure that nutrient deficiencies are the problem. Do not feed a tree that has a fungus disease until the disease is cured; the fertilizer helps the fungus more than the tree.