In general, trees and shrubs that need a nutrient boost get their annual fertilizer application in early spring, right before active growth begins for the year. This timing has been the generally accepted practice by gardeners and experts everywhere for years. And although early spring is a good time, new research indicates there is an even better time.
Many experts now consider late fall, or about a month after the first killing frost, to be the ideal time for applying fertilizers. We now know plants utilize nutrients throughout the year in different ways.
In the past, the most common reason against fertilizing in the fall was the fear that plants and trees would put on new growth if unseasonably warm weather returned, only to be burned or damaged by imminently colder temperatures.
The key is to understand the difference between early fall and late fall timing. If you fertilize in late summer or early fall, when temperatures are still warm and plants are still actively growing, it is likely new growth could occur and damage to tender new foliage could be the likely result.
The rationale for late fall fertilization makes sense when you understand why. At this time, deciduous trees and shrubs have lost their foliage for the year and active growth of plants and trees has slowed. Rather than put on new foliage growth, the roots of established trees or shrubs take the nutrients from the soil and apply them to important health-promoting functions, such as disease resistance and root development. The excess nutrients are stored in the roots and become immediately available when needed for new growth in spring.
However, keep in mind, not all established plants and trees are candidates for a regular fertilization program. I always suggest a soil test be obtained through your local county extension office. Simply gather up a representative soil sample around the area where your trees and shrubs are growing. Be sure to inform the extension service you would like to have the soil tested for this.
The report will let you know what nutrients may be lacking in your soil for optimum growth. The report will also suggest the proper type and amount of nutrients to add.
A common mistake, and not just with trees and shrubs, is to assume fertilizer can and should always be added, and if a little is good, more is better. Nothing could be further from the truth. Excess nutrients are wasted and can end up contaminating the soil, and the environment beyond.
Plants and trees are far more sophisticated then we give them credit!