Unless an established tree shows symptoms of distress, such as die back at the tips of branches, discolored leaves, sparse foliage or maybe just slow growth, they generally don’t need additional fertilizer.
Sometimes in an urban environment a tree will need help. Mechanical or physical problems can be the culprit … such things like being planted in a very constricted area, receiving contaminated runoff from a parking lot, or where traffic has compacted the soil over the tree’s root zone.
A soil test, representing the entire planting area, would be helpful to check acidity and availability of major nutrients‑ phosphorus and potassium. If fertilization is needed, it’s best to apply it… either on the ground’s surface or through holes drilled or punched in the soil, while the upper part of the tree is dormant from late fall to early spring. Fertilizing during the summer may cause new growth which wouldn’t have time to harden off before cold weather.
How much fertilizer to apply? The phosphorus and potassium should be governed by the soil test. For the nitrogen apply 10 to 20 pounds of 10 percent nitrogen per 1,000 square feet based on the area within the tree’s drip line. Reduce the amount of fertilizer for any area covered by sidewalks or such. Spread the fertilizer over the area. A slow‑release product will help the tree get the most from the feeding.
Those same symptoms (leaf dieback, discolored leaves, sparse foliage, or slow growth) can also indicate stress from heat and drought conditions. Generally speaking most trees, young or old, urban or rural, will benefit much more from water during dry spells than they will from fertilizer.
Originally printed in Asheville Citizen-Times in March, 2002. Written by Glenn Palmer, Extension Master Gardener Volunteer.