Fertilizing a tree can increase growth, reduce susceptibility to certain diseases and pests, and can even help reverse declining health. However, if fertilizer is not applied wisely, it may not benefit the tree at all, and may even adversely affect the tree. Mature trees making satisfactory growth may not require fertilization. When considering supplemental fertilizer, it is important to know what nutrients are needed, and when and how it should be applied.

Soil conditions, especially pH and organic matter content, vary greatly, making the proper selection and use of fertilizer a somewhat complex process. When dealing with a mature tree that provides considerable benefit and value to your landscape, it is worth the time and investment to have the soil tested for nutrient content. Most quality garden centers can arrange to have your soil tested at a soil testing laboratory. With the test results in hand, you can consult your local garden center staff, ISA Certified Arborist or a plant care professional for advice on application rates, timing, and the best blend of fertilizer for each of your trees and other landscape plants.

Mature trees have expansive root systems that extend from two to three times the size of the leaf canopy. A major portion of actively growing roots are located outside the tree's drip line. It is important to understand this when applying fertilizer to your trees as well as your turf. Many lawn fertilizers contain weed and feed formulations that may be harmful to your trees. When you apply a broadleaf herbicide to your turf, remember, tree roots co-exist with turf roots. The same herbicide that kills broadleaf weeds in your lawn is picked up by tree roots and can harm or kill your broadleaf trees if applied incorrectly. Understanding the actual size and extent of a tree's root system, before you fertilize, is necessary to determine how much, what type, and where to best apply fertilizer.