Most trees exist in nature without much care, but transplanting trees into urban areas or man-made conditions can create problems. Often these trees are planted in restricted root zone areas, such as along paved streets or in compacted soil. You should keep in mind that the root system is just as important (and delicate) at the top of the plant. Fertilizer or pruning will not help the growth of a tree if it stressed by one of these environmental conditions. Fertilizer is only one factor that contributes to the growth of plants and health of plants.
If you pay close attention to trees, you can detect possible problems that fertilizer may be the cure for. For example, a nutrient-deficient tree will have a slow and low amount of annual growth on twigs and the 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. The key is to make sure that nutrition is the problem. Other common tree problems to be aware of in urban situations would be poor planting techniques, moisture problems, construction damage, or girdling roots. Soil testing is highly recommended in these questionable situations.
Before selecting a fertilizer, it is always good to do a soil test. A soil test will reveal what the soil’s nutrient capability is and give a recommendation for adjusting nutrient levels. The soil test report will also tell you the pH of the soil and the report will give you a lime recommendation. A pH range of 5.8 to 6.2 is desirable.
When you select a fertilizer, choose a complete fertilizer that has nitrogen, phosphorus, and potassium. For example, 10-10-10 is a complete fertilizer. Nitrogen is a key plant nutrient and plays an important role in production and maintenance of foliage color. Using the proper amount of fertilizer and applying it evenly under the drip-line will prevent root injury.
Normally, trees should be fertilized only enough to keep them healthy. Usually, January through March is the best time to fertilize because soil moisture is best during this time. At this time, it will also ensure that the fertilizer will be in place at the beginning of the growing season.
The method of application will depend on the plant, soil conditions, and the amount of time and labor to fertilizer the tree. You can drill holes about 8 to 12 inches in a grid pattern under the canopy of the tree and out to the drip line. Then measure out the desired amount of fertilizer, pour it in the hole, and backfill with mulch. This method allows you to aerate the soil and to get the fertilizer to the roots. Another type of application method is surface apply. This method is the easiest and fastest. If your trees are located in your lawn then every time you fertilize your grass you are also fertilizing the tree. Unfortunately, surface-fed trees develop shallow (sometimes above ground) root systems, which interfere with mowing, adversely affect grass growth, and makes the tree more drought susceptible.
A standard rule of thumb is to apply 3 to 5 pounds of fertilizer for each inch of trunk diameter measured at 4 ½ feet above the ground. Another way to determine fertilizer amount is by the square footage of the canopy spread. Apply 1 to 2 pounds of actual nitrogen per 1000 square feet of soil surface under the canopy. For trees less than 8 inches in trunk diameter, use one-half the above recommended rates. Also, keep in mind that newly transplanted trees need plenty of water during their establishment and less fertilizer. You can fertilize them the next year after establishment.
If you have further questions about fertilizing trees in the landscape do not hesitate to call your local N.C. Cooperative Extension, Franklin County Center at 919-496-3344.We provide our residents easy access to the resources and expertise of NC State University and NC A&T State University.
