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The main reason to fertilize trees and shrubs is to bolster their health so they are better prepared to fight off pests, disease, and environmental stresses. While fertilizer can't solve all of a tree's problems, it will go a long way to give it a fighting chance.
Trees growing in their natural habitat should have access to all of the minerals they need to survive and grow. Anything you can do to mimic that habitat can reduce the need for fertilizer. This may include letting leaves remain on the ground in the fall instead of raking them up. Chances are, though, that despite your best efforts, the need for fertilizer will not be entirely eliminated.
A good time to fertilize trees in most Northern temperate climates is from fall to mid-spring. At these times the tree's roots take the nutrients from the soil and apply them to important health-promoting functions such as root development and disease resistance, rather than simply putting out new growth.
During the growing season, fertilizing can help a tree overcome mineral deficiencies and fight off infections. If you are fertilizing in mid- to late summer, avoid formulations high in nitrogen as this will just promote weak, new growth that may be easily damaged in the winter.
The objective of fertilization is to put the nutrients where they will best be taken up by the tree's roots. Therefore, it is necessary to fertilize throughout the entire root system. In general, the roots extend well beyond the outer reach of a tree's branches.
The fertilizer must also be placed underneath the roots of any competing plants such as grass or other ground cover. Spreading granular fertilizer on the lawn might make your grass greener, but it will likely not help your tree.
Fertilizers are made up of macronutrients (Nitrogen, Phosphorus, and Potassium) and micronutrients (such as Iron, Magnesium and Manganese). These minerals all have different effects on the growth of a tree and different trees need different formulations. It is important to ensure that you use the right fertilizer for your tree. To find out how to obtain the correct fertilizer for your trees or shrubs, click here.