Issue: March 3, 1997

Fertilize trees now?

When to fertilize trees

This is a topic which will have different answers depending on who you ask.

Some prefer to fertilize in the autumn after the tree becomes dormant and the fertilizer cannot induce renewed growth. The roots of many of our shade trees grow in the autumn and in some cases through the winter. This is especially true of trees native to areas with consistent moisture throughout the year. This includes many of the trees grown for shade in New Mexico.

Another group of people recommend waiting until after leaves have formed in the spring. They are concerned that nitrogen fertilization early in the spring will cause the production of an excess of leaves which will cause a water deficit later in the spring and early summer, especially in New Mexico's arid environment. We often see mid-summer leaf drop in many of our shade trees as the temperatures increase and water loss through transpiration increases. Some of our trees, cottonwoods for example, will drop leaves every summer. Early spring or late winter fertilization may increase this mid-summer leaf drop. Trees which may not otherwise exhibit this summer leaf drop will do so if they are fertilized excessively before the leaves form. There is some controversy as to whether or not this excess leaf production and summer leaf drop actually occur.

Why does timing make a difference? Plants need nutrients for proper growth. Our soils are often deficient in some nutrients, or because of the alkaline nature of our soils some nutrients, though present in the soil, are not available to the plants. A soil test will help determine what should be added. In any case, nutrients from the soil are not the only thing plants need to produce new leaves. Energy stored in the form of carbohydrates in the tree are also needed. When more leaves are formed, more of this stored energy is used by the tree. If these leaves are then dropped early, some of this energy is wasted and the plant may be more susceptible to damage due to drought, insects, and other stresses. Therefore, fertilization in late winter can sometimes set the tree up for later problems. This is especially true if the tree has been topped or severely pruned as this causes the same kind of energy starvation.

The discussion above has been directed at nitrogen fertilization. Nitrogen is needed for vegetative growth but can cause problems if used in excess or at the wrong time. From what I have related above, I prefer not to fertilize with nitrogen now and I even more strongly recommend not to fertilize with nitrogen from July 4 until the trees go dormant in the autumn. Fertilization with other nutrients now is not harmful. While shade trees need nitrogen primarily, fruit trees need higher levels of phosphorus. Iron and other micronutrients may also be needed by fruit trees and shade trees. These nutrients, phosphorus, iron, and other micronutrients may be safely applied now. Just be careful with nitrogen fertilization.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

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