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When and how should I fertilize my trees and shrubs?
Most landscape trees and shrubs do not need yearly fertilization. Fertilizing is not a cure-all, nor will it solve drainage problems or insect and disease problems. Over- fertilization can also make plants more susceptible to insect, disease and breakage problems and can contribute to environmental pollution. If in doubt, consult a professional and have a soil test performed to determine available nutrients before applying fertilizers.
Why do landscape plants need fertilizing at all, since they do nicely in the woods, fields and fence rows without ever being fertilized? Conditions in landscape settings are much different than in the wild, where leaves and other plant parts are left to decay on the soil-surface, providing the nutrients back into the soil. This does not happen in most landscapes.
If fertilizing is required, the best times to fertilize are late fall and early spring. For fall fertilization, wait until the plants are dormant. For deciduous plants, (those that lose their leaves in fall), this means waiting until the leaves have turned color and begin to drop.
Do not fertilize trees and shrubs from mid-summer to early fall. This may stimulate late-season growth which is more easily winter-killed. Also, stimulating growth late in the season may interfere with the plant's dormancy process, thus increasing the risk of winter injury.
There is no best fertilizer for trees and shrubs, although most garden stores sell fertilizers made specifically for trees and shrubs. Normally a complete fertilizer, one that contains the main plant nutrients; nitrogen, phosphorous, and potassium, is fine to use. However, if a soil test indicates that your soil already has enough phosphorous and potassium, use a fertilizer that has only nitrogen. Ammonium nitrate is a good example of a nitrogen fertilizer. A soil test is always the best way to determine fertility needs. In the abscence of a soil test, use one to two lbs. of 10-10-10 or 12-12-12 granular fertilizer for every 100 sq. ft. of root zone area for deciduous trees and shrubs. For evergreen trees and shrubs, use only 1 lb. for every 100 sq. ft. Two cups of granular fertilizer is approximately equal to one lb.
Fertilize as much of the root area as possible. For large trees this area extends at least to the drip line of the branches and usually farther. In shrub beds, or where you have mulched trees and shrubs, you can simply broadcast granular fertilizer on the entire surface and water it in. For trees in lawn areas, you can use the drill hole method. Drill or punch 8 to 12-inch deep holes an inch or so wide throughout the tree's root zone. Start a few feet from the trunk and extend out beyond the drip line of the branches. Make one hole every two feet in the entire area. Divide the fertilizer evenly among the holes. Fill the holes with sand or small stones. This keeps the holes open, although you won't notice them, and it will facilitate aeration in heavy clay soils.
If you fertilize your lawn, trees within that area generally receive sufficient fertilizer. Before adding more fertilizer, run a soil test and check the trees annual growth. Mature trees should put on 4 to 6 inches of new growth each year, while young trees should put on a minimum of 9 to 12 inches.