NC Cooperative Extension Resources

Author:

Photo of Kim Powell
Spec (Commercial Landscaping)
Horticultural Science

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

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

  1. Diameter of Trunk - Apply 3 to 5 lb of 10-6-4 (or similar analysis fertilizer) for each inch of trunk diameter measured at 4 1/2 ft above ground. For example, a 10-inch diameter tree required 30 to 50 lb of 10-6-4 fertilizer. Diameters can be found by the following formula:

    Diameter = circumference x .318

    *(This method is not recommended if the root zone area is restricted by curbs, paving, etc.)
  2. Crown Spread Area - Apply 1 to 2 lb of actual nitrogen per 1000 ft2 of soil surface area under the spread of the branches (crown spread). Crown spread can be determined by the following formula. The radius is the distance from the trunk of the tree to the edge of the branch spread.

    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.

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Last modified: Dec. 11, 2014, 1:20 p.m.