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ARBORICULTURE: FERTILIZING PLANTS
  MG Manual Reference
Ch. 13, pp. 11 - 13

Ornamental trees and shrubs planted in fertile, well-drained soil should not require annual fertilization. Trees and shrubs that are growing well don't require extra nutrients. If you have ornamentals that are not doing well, fertilization may be helpful but only after the problem causing poor growth has been corrected. Poorly growing plants will exhibit any or all of these symptoms:
  • light green or yellow leaves
  • leaves with dead spots
  • leaves smaller than normal
  • fewer leaves and/or flowers than normal
  • short annual twig growth
  • dying back of branches at the tips
  • wilting of foliage

These symptoms of poor growth may be caused by inadequate soil aeration, moisture, or nutrients; by adverse climatic conditions; by wrong pH; or by disease. You should attempt to determine the specific cause in each particular situation and apply corrective measures. Do not assume that an application of fertilizer will quickly remedy any problem encountered. Do not fertilize trees unless they need it!
The cause of poor growth may or may not be evident. Ornamentals transplanted or disturbed by construction within the previous five or ten years may be in shock, their root systems having been disturbed. Injury from insects, diseases, or air pollution can stress a tree or shrub. Fertilization of such plants should be carried out with caution since overstimulated plants may become weakened, predisposed to diseases, and attractive to insect pests. Attempts to reduce the adverse factors may be helpful in restoring plant quality. Correct fertilizer application can also help. It is important to realize that a complete growing season may be needed before much response is seen from the fertilizer.
Review the chapter on Soils and Fertilizer to determine what type of fertilizer and how much to apply. Measure the distance between the trunk and the drip line and multiply by 125%. This will determine the radius for the outer boundary for fertilizer application. Distribute the fertilizer evenly in the outer 2/3 of the circle defined by the trunk and the boundary.
For example, if the trunk is eight feet from the drip line, then the outer boundary will be ten feet (8 X 1.25 = 10). Apply fertilizer around the tree in a doughnut shaped band that is between 3.3' and 10' from trunk (10 / 3 = 3.3).
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. Such plants are more susceptible to injury by cold in winter, are more likely to be broken during wind and snow storms, and usually will have a shorter life than those making moderate growth.


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