Do Not Routinely Fertilize Woody Ornamentals - August 20, 2003
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County
I often field questions about fertilization of trees and shrubs. Some people are especially keen to fertilize ornamental (native or non-native) trees that are threatened or already colonized by plant pests such as bark beetles. My advice is: "just say no". Ornamental trees and shrubs planted in relatively fertile, well-drained soil should not require annual fertilization. Ornamental plants that are growing well and in balance with their environment don't require extra nutrients.
Conversely, if you have ornamental trees and shrubs 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 the following symptoms:
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. Also look for signs of poor soil drainage such as standing water. Injury from insects, diseases, wind, improper staking, or air pollution can stress a tree or shrub. Attempts to reduce the adverse factors may be helpful in restoring plant quality.
If and when the cause of poor growth is corrected, a single fertilizer application can help the weaken plant recover from stress. Choose a fertilizer that is appropriate for the situation (usually a balanced fertilizer not too high in nitrogen). To calculate the application area, 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). Irrigate the plant making sure the fertilizer goes down into the soil. A complete growing season may be needed before any response is seen from the fertilizer.
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. Over stimulated plants may become weakened, predisposed to diseases, and attractive to insect pests. 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.
So, unless you are trying to correct a specific problem, just say no to fertilizers for ornamental trees and shrubs. This message was brought to you by F.A.R.E. (Fertilizer Abuse Resistance Education).
The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at email@example.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://ag.arizona.edu/yavapai/anr/hort/byg/.
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Last Updated: August 12, 2003
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