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The Myth of "Winterizer" Fertilizer

By Robert Cox,Colorado State University Cooperative Extension Agent - Horticulture

We hear a lot about using a "winterizer" fertilizer on our lawns during the fall months. Such a fertilizer will, according to claims, "enhance winter hardiness" of our grass.

But, will it? Only if the fertilizer is high in nitrogen, and you will know that by understanding how to read the numbers on a bag of fertilizer.

Most "winterizer" fertilizers marketed at this time of year, contain higher percentages of phosphorus and potassium and lower levels of nitrogen. This might be expressed on a fertilizer bag as "8-18-22" fertilizer -- 8 percent nitrogen, 18 percent phosphorous and 22 percent potassium, listed in that order.

Those who advocate using fertilizers with this type of formula say that the need is based on research. But, the research has been done with WARM-SEASON grasses, such as Bermudagrass or Zoysiagrass, neither of which is well-adapted to our climate (and should not be fertilized with nitrogen in fall anyway!) No evidence is available to suggest that extra phosphorus and potassium in fall benefit COOL-SEASON grasses used here, such as bluegrass, fescue or ryegrass.

The most important nutrient for fall fertilization, as with earlier-season applications, still is NITROGEN. Nitrogen applied in the fall is the most important lawn fertilization of the year. Use a high-nitrogen fertilizer, such as 25-5-5 or something with a similar formula. Because of the widespread acceptance of the "winterizer" myth involving phosphorus and potassium, high-nitrogen lawn fertilizer may not be readily available at this time. If you have high-nitrogen fertilizer left from earlier applications, use it. If you're out, use urea (46-0-0) at 2 lbs/1000 square feet or ammonium sulfate (18-0-0) at 5 lbs/1000 square feet.

Next year, be sure to buy additional high-nitrogen lawn fertilizer for use in fall. Store under dry conditions until use. Follow fertilizer applications with lawn watering.

Make your fall application to the lawn about October 15, while grass still is green and while 2-3 weeks remain before the ground freezes. If you have very sandy soil (uncommon in the metro area except along South Platte and eastern areas) do not fertilize later than late September, as nitrogen leaches readily through sandy soil, especially during winter months, and will contaminate ground water. On sandy soils, it's best to use "slow-release" nitrogen fertilizers such as organics, IBDU, or sulfur-coated urea, to reduce potential for groundwater contamination.

Photograph courtesy of Judy Sedbrook.

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Date last revised: 01/05/2010