Fertilizing Deciduous Shade Trees in the Landscape

Revised 6/97 -- Author Reviewed 6/97 HIL-618
M. A. (Kim) Powell
Extension Horticultural Specialist

Shade trees, like any other landscape plants, will respond to fertilization. Most shade trees exist in nature without much care, but transplanting trees into urban areas or man-made conditions can create problems. Often these trees will be growing in restricted root zone areas, be surrounded by pavement or compacted soil or be physically damaged by construction activities. One should realize that the root system is just as important (and delicate) as the above-ground parts. Fertilizer will not improve the health of a tree stressed by one of these environmental conditions. Fertilizer is only one factor in the complex formula of plant requirements. The following are general recommendations for the timing, methods and rates of applying fertilizer to shade trees.

One should be able to detect when a tree needs fertilizing. Symptoms of a nutrient-deficient tree include a slow rate and low amount of annual growth on twigs and trunk, smaller than normal foliage, off-color foliage, increased amounts of dead branches, tip die-back in branches, and increased rates of disease and insect problems. Trees that possess these symptoms generally would respond to a fertilization treatment. One should make sure that nutrients (or lack of) are the problem before fertilizing. Other common tree disorders to be aware of in urban areas would include poor planting techniques, moisture problems, construction damage, girdling roots, or utility leaks from a natural gas line or sewer line. Soil testing is highly recommended in questionable situations.

Soil Testing - Before selecting a fertilizer, take a soil test. Take several soil samples from the area, 6 to 8 inches deep, using a soil sampling tube. Cores should be collected in a clean pail and mixed thoroughly. A soil test will reveal what situations exist and give a recommendation for adjusting nutrient levels. Soil pH should be considered also. Generally a pH range of 5.2 to 6.2 is the most desirable. More acid soils should be limed to raise the pH while more basic soils should be treated with sulfur to adjust the pH downward. Maintaining a soil pH range of 5.2 to 6.2 for shade trees generally insures that the availability of essential plant nutrients will be available to the tree.

Fertilizer Types - When selecting a fertilizer, purchase complete ratio fertilizer. Nitrogen is a principal plant nutrient, and is important in production and maintenance of color in the foliage. Using the proper amount is important since overdoses can result in root injury while lack of nitrogen may result in poor vegetative growth.

The ratio of nitrogen to the other two major nutrients, phosphorus and potassium, should be approximately 2 or 3 times higher. Fertilizers which can be used include 10-5-5, 12-6-6, and 18-6-12. Several of these fertilizers contain both the urea formaldehyde and nitrate form of nitrogen, which allows for smaller amounts of nitrogen to be released over a longer period.

Root disorders caused by soil compaction, construction damage, or drought can be corrected by using fertilizers high in phosphorus which will promote new root growth. Superphosphate (0-20-0) or triple superphosphate (0-46-0) are recommended. High-nitrogen fertilizers, particularly fast-release forms, should be avoided because the resulting increased top growth will strain the already inadequate root system.

When to Fertilize - Normally, shade trees should be fertilized only enough to keep them healthy. In heavy soils or soils underlain by a hardpan, too frequent or too heavy fertilization may cause nutrients to build up to toxic levels. This may be avoided by periodic soil testing which will show abnormally high or low nutrient levels.

Usually, October through March are the best times to fertilize. Soil moisture and temperature conditions are best during these periods, and this will insure that the fertilizer will be in place at the beginning of the growing season when maximum benefit will be realized. Root growth is best during this time also. Late summer fertilization may promote late growth which is undesirable before winter.

Distressed trees should be fertilized at once. Stress problems will normally show up during the active growing season. Root damaged trees should be fertilized during periods of maximum root growth which occur during spring, early summer and in early to mid autumn.

Where to Apply Fertilizer - Fertilizer must be placed near the tree's absorbing roots. The absorbing roots begin several ft from the trunk and can extend beyond the spread of outermost branches by as much as 40 to 50%. Shallow-rooted species such as elm or maple typically have roots that extend well beyond the spread of the branches. In such cases, extend the area fertilized to match the estimated root spread.

Application Methods - The method selected will depend on the plant cover under the tree, soil conditions, and amount of time and labor available to fertilize the tree.The typical methods are discussed below.

  1. Drill Hole - Holes are drilled or punched in the soil in the area to be fertilized. An auger, punch bar, or crowbar is used to make slightly slanted holes 12 to 15 inches deep spaced about 3 ft apart. The recommended amount of fertilizer should then be distributed evenly among the holes. It is recommended to mix the fertilizer with peat moss, pine bark, or gravel to backfill the holes. A funnel and a small can as a measuring device can be used to place the fertilizer/backfill into the holes. This technique has the advantage of aerating the soil as well as placing the fertilizer in close contact with the absorbing roots.
  2. Feeding Needles - Several types of "needles" are available which inject liquid fertilizer solutions into soil. The addition of fertilizer in solution adds moisture to the soil, provides nutrients almost immediately since nutrients must be in solution before roots can absorb them, and provides more even distribution. Several types of liquid feeding needles are available commercially. In most types, a plant food cartridge is placed in a chamber and water is supplied by a garden hose in the ground at the proper depth. Needles do not work well in heavy soils at normal home water pressures. Avoid high nitrogen, water soluble plant food cartridges for root damaged trees.
  3. Surface - The simplest and fastest way to apply fertilizer is to spread it on the lawn or soil surface under the branch spread. (Many horticulturists believe this method to be just as effective.) To prevent lawn or groundcover damage, the fertilizer should be applied in split applications with thorough watering after each application. One application of the recommended amount followed by thorough watering is sufficient on bare soil areas. Phosphorus, and to a lesser extent potassium, do not move well through soil; therefore, the fertilizer should be tilled or watered in thoroughly. Frequently, surface-fed trees develop shallow (sometimes above ground) root systems which interfere with mowing, adversely affect grass growth, and make the tree more drought susceptible.

Application Rates for Deciduous Trees - The rate of application will vary with the size of the tree. Two methods of determining fertilizer rates are commonly used for deciduous trees.

Diameter = circumference x .318

*(This method is not recommended if the root zone area is restricted by curbs, paving, etc.)

Crown Spread = (Radius) 2 x 3.14

Example: If a tree has a crown radius of 18 ft, crown spread = 18 x 18 x 3.14 = 1017 ft2. The recommended rate would be 10-20 lb of 10-6-4 fertilizer (10% nitrogen, 6% phosphorus, 4% potassium).

For trees less than 8 inches in trunk diameter, use one half the above recommended rates. If the surface area beneath the crown spread of tree is obstructed (as by sidewalk, driveway, or street), reduce application rate by the approximate percent that the obstruction covers the soil surface area under the spread of the branches.

The most important thing to remember for newly transplanted trees is to provide plenty of water during the establishment period. Lack of adequate moisture will kill a tree much faster then lack of nutrients.

Published by

North Carolina Cooperative Extension Service

Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.