In the urban landscape, deciduous shade and ornamental trees may benefit from yearly fertilization. The benefits include: more rapid growth; faster recovery from injury, pruning or pest problems; overall improved health and better foliage color.
Established trees are best fertilized when they are dormant, usually between November and April. The next best time to fertilize is in early spring and summer, from May through July. But trees should never be fertilized in late summer or fall. From July through November, trees store sugars, harden off new growth, set buds, and otherwise prepare physiologically for winter. Any fertilization during this time will send the wrong signals to the tree, and ultimately do much more harm than good.
To fertilize, use any complete fertilizer containing 6-12 percent nitrogen; high amounts of phosphorus and potash are not necessary. Examples of a correct analysis would be formulations such as 10-6-4, 10-20-0, 12-12-12 or 6-10-4. The proper rate of application is dependent on the diameter of the tree measured 4 ½ feet above the ground. Remember that the diameter equals the circumference divided by 3.14. Generally a tree 6 inches or less in diameter will require 1-2 lbs. per inch of trunk diameter; a tree over 6 inches in diameter, will require 3 pounds per inch of trunk diameter. Once the rate of application is determined, drill holes 1½ to 2½ inches wide, 10-12 inches deep, and 2 feet apart throughout the root zone, starting at the drip and working in toward the trunk, but coming no closer than 24 inches from the trunk. If you hit a root, drill to one side or the other.
Granular fertilizer should then be distributed evenly among the holes, with no more than ½ to ¼ cup of fertilizer in each hole. Water until the holes stand full of water, and then fill them with soil. Don’t be tempted to apply more fertilizer than called for by the size of the tree, and never use a fertilizer containing herbicides.
While established trees benefit from fertilization, a newly planted tree does not, until the root system has at least partially reestablished itself. So fertilizer is not recommended during the first year or two after planting. The exception is the addition of a low analysis starter fertilizer at the time of planting, which can help reduce transplant shock. Pines, junipers, spruce and fir trees should seldom be fertilized. These species are quite well adapted to our area soils. If evergreens are fertilized, only do so at half the recommended rate for deciduous trees.