Why you should fertilize your lawn this fall

Consumer Reports News: September 29, 2008 12:09 AM

After months of planting, fertilizing, mowing, and other chores, you might be ready to take a break from all the yard work.

But you shouldn't. Fertilizing your yard during the fall can be essential to maintaining a healthy lawn, especially if you have cool-season turfgrasses like bluegrass, fescue, and ryegrass.

"Autumn is the optimum growth period," says Martin Petrovic, Ph.D., a turf expert in the Department of Horticulture at Cornell University. "The temperatures favor a balanced growth of roots and shoots, so the nutrients in the fertilizer have the best chance of doing their job." Petrovic recommends two fall applications based on the acronym SON, for September, October, and November: Do the first feeding in late September to early October, the second in November, around your final mow of the season.

Another helpful fertilizing mnemonic is "up, down, all around." That indicates the action of the three main ingredients in lawn fertilizer. Nitrogen promotes growth and greening ("up"), phosphorous aids root development ("down"), and potassium fortifies the grass against disease, cold, insects, and the like ("all around").

The numbers on a fertilizer bag, in N-P-K order, indicate the percentage of nitrogen, phosphorous, and potassium, respectively, on a weight basis. For example, a 100-pound bag labeled 20-0-0 has 20 pounds of nitrogen but no phosphorus or potassium.

"Unfortunately, there's no magic formula for fall fertilizer," says Petrovic. You'll likely see 20-5-15 mix sold at home and garden centers, but the only sure way to determine your lawn needs is to test the soil. Private labs and cooperative extensions perform the service, including specific nutrient recommendations in their report.

Fertilizer bags should also indicate the nitrogen's release characteristic. Slow-release fertilizers contain water-insoluble nitrogen, making them suitable for early-fall applications. Later in the season, a water-soluble fast-release fertilizer is necessary, says Petrovic.

Recommendations for lawn fertilizers are usually given in actual nitrogen over a given area. As a general rule, you should apply no more than 1 pound of actual nitrogen per 1,000 square feet of lawn if it's thin and weedy. "If the turf looks dense, you don't need to put down as much nitrogen," says Petrovic.

It's always better to use too little fertilizer rather than too much, since nitrogen and phosphorous not absorbed by your lawn can get washed into storm drains and watersheds. Known as nutrient pollution, this phenomenon is particularly problematic in coastal regions with loose, sandy soil. For more information, download the Environmental Protection Agency's June 2008 "Sowing the Seeds for Healthy Waterways" report.

Use the Purdue University Turf Fertilizer Calculator to figure out how much fertilizer you'll need based on the nitrogen concentration. Also, be sure to calibrate your drop spreader to make sure you don't overfertilize. And remember that using your mower's mulch setting can reduce fertilizer needs by a third since the soil-enriching nitrogen from decomposing clippings promote turf growth naturally.

Take care of your yard this fall, and you'll have less lawn maintenance to deal with come spring.—Daniel DiClerico

Essential information: See our fall lawn-and-yard checklist for advice on trees and shrubs, flowers and gardens, and how to use herbicides safely.

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