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ORNAMENTALS

Established landscape trees do not require frequent fertilization. Some seem to grow on indefinitely without any nutrient supplementation. A moderate rate of growth and good green color is all that is desired of woody plants. Excessive vigor, which is evident by lush green leaves and long shoot growth, is undesirable. Occasionally, fertilization can benefit mature trees, but it must be done correctly and be properly timed to produce the desired results.

First, determine if fertilization is necessary. When new shoots grow more than six inches long in one season, no fertilizer is needed. When they grow between two and six inches in length, fertilization is optional, and when new growth is less than two inches in length, unless the plant is a slow growing or dwarf species, it is time to fertilize. Foliage color is not as good an indicator of fertilizer need as tree growth. Yellowing foliage can be caused by nutrient deficiencies, but it can also be caused by high pH. In addition, soil pH has a direct effect on the availability of nutrients. Ornamentals vary in their requirement, and many such as azaleas require an acid soil, which occur rarely in Arizona. If nutrient shortages remain suspect after investigating other possible causes, it may be necessary to send a soil test to a lab for testing which will disclose deficiencies in major (except for nitrogen) and minor soil elements.

Nitrogen deficiencies are determined by observing tree growth as described above, and through cultural history of the tree, such as how frequently the tree has been fertilized in the past.

If fertilization is necessary, follow amount recommendations given with the soil test results. A nitrogen-phosphorous-potassium fertilizer is frequently recommended, though the nitrogen is usually the most important nutrient. If turf grass or ground covers are growing around the tree to be fertilized, split any application so that no more than one pound of actual nitrogen per 1000 square feet is applied at any one time. This will prevent fertilizer burn of the ground cover.

Space applications six weeks apart. Apply fertilizer at the correct time of year for best results and minimum potential for groundwater contamination. Fertilizer can be applied in the fall approximately one month after the first frost. At this time, roots are still growing and will benefit from the nutrients, but tender top growth will not be stimulated. The loss of nutrients from winter fertilization is not only expensive, it can result in water pollution.

Surface or broadcast application is the most desirable method of application. Not only is it simple to perform, research indicates that it produces the best results.