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Fall is the Time to Fertilize Lawns

By Carl Wilson, Colorado State University Cooperative Extension Agent, Horticulture, Denver

If you fertilize your lawn in the spring or summer but not in fall, you're missing out. The benefits of fall fertilization extend throughout the year.

In Colorado's bright fall sunlight, photosynthetic activity remains high. Unlike summer, when food produced from photosynthesis fuels top growth, the food produced by grass in fall goes to fill storage sites in roots and shoots and to fuel new root growth.

This stored food translates into better winter survival and early spring green-up without the excess top growth that comes with spring fertilization. The new root growth produces a healthier plant through the following summer.

The argument for fall fertilization can be summarized as a gain in benefits without the extra mowing chores that accompany spring fertilization.

Fertilize lawns through September with the same slow-release products used in summer. Dehydrated poultry waste is a good example. For late season fertilization in October or later, avoid products that depend on soil microbes to release the nitrogen. Cooling soil temperatures slow microbial activity and limit the availability of such fertilizers to the grass.

Cool-temperature active fertilizers such as urea, IBDU, sulfur coated urea and ammonium sulfate are good sources of nitrogen for late-season fertilizer applications. Late season is defined as October or later when the grass still is green. Applying fertilizer on a brown lawn does no good.

The benefits of fall fertilization can be stretched even further into the following season by returning grass clippings to the lawn. Grass clippings break down quickly returning nitrogen in a natural, slow release form that avoids fast top growth and heavy mowing chores.

Grass clippings recycle about one-third of the nitrogen back to the lawn. The rest is lost to the atmosphere, soil animal life and other locations where it is unavailable to the grass.

An additional application of a slow-release fertilizer in the late spring to early summer will carry medium quality lawns receiving fall fertilization and returned clippings through the year. High quality-high maintenance lawns require three or even four fertilizations per year.

For more information about state-of-the-art lawn care, call your Colorado State University Cooperative Extension office.

Photograph courtesy of Judy Sedbrook.

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Date last revised: 01/05/2010