Late summer to early fall is the prime time for fertilizing cool season lawn grasses such as bluegrass, perennial ryegrass, and fescues that are grown in the northern two-thirds of the US. These kinds of grasses grow vigorously in cooler fall weather and will reap the greatest benefit from fertilizers applied at this time of year. But the nutrients in lawn fertilizers also have the potential to harm nearby lakes, rivers, and other bodies of water.
To make sure the fertilizer you put down helps your lawn, rather than running off to cause problems in the watershed, use one that has at least 50 percent of its nitrogen in organic or slow release (water-insoluble) form. Sweep up any fertilizer that lands on driveways and sidewalks as you spread it, and avoid fertilizing right before heavy rain is predicted. Don't apply in late fall or when the ground is frozen.
Research has shown that, on established lawns growing on soils that did not test deficient in phosphorus, adding this potentially watershed-harmful nutrient in fertilizer did nothing to benefit the growth of the turf. For established lawns, test your soil every 3 to 5 years and, if the soil test does not show a phosphorus deficiency, use a fertilizer that does not include this nutrient. Look for one with a zero for the middle number in the analysis, for example 26-0-10. (That middle number indicates the percentage of phosphate in the fertilizer.)
To help protect their watersheds, many states (and some municipalities) have adopted lawn fertilizer laws that regulate the timing of lawn fertilizer applications, application rates and practices, and the use of phosphorus-containing lawn fertilizers. Protect lakes, streams, and rivers as you keep your lawn green and thriving! Check with your state Extension Service, state Agriculture Department or Department of Environmental Conservation for more information to make sure you are in compliance with these important environmental protection regulations.