Regional Gardening Reports :: National Gardening Association

In the Garden:
New England
June, 2010
Regional Report

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Choose a phosphorus-free lawn fertilizer unless a soil test shows your soil is deficient in this nutrient.

Healthy Lawns and Healthy Lakes

Nothing sets off landscape plantings like a lush, green lawn. But most gardeners don't want their carpet of grass to cause environmental harm. Runoff containing the nutrient phosphorus from lawn fertilizers entering lakes, streams and rivers is a big contributor to algal blooms and the depletion of oxygen they cause in these waterways. Research has shown that a pound of phosphorus can fuel the growth of 300 to 500 pounds of algae in a body of water.

Where I live in Vermont, in the Lake Champlain watershed, about half the phosphorus problem in the lake is attributed to runoff from developed land. In fact, it's estimated that one acre of urban/suburban land contributes about four times more phosphorus to the lake than four acres of farmland! As a result, some states and municipalities have begun to restrict the use of phosphorus-containing fertilizers on lawns. Minnesota, for example, now restricts their use on turf (but not agricultural or general garden use), with exceptions for soils that are shown to be deficient with a soil test, for the first year turf is being established and certain golf course uses.

Even if there are no legal restrictions in place, it's a good idea for gardeners to protect the environment by being "phosphorus-wise" as they care for their lawns. Recent research at the University of Minnesota suggests ways to protect both the quality of your lawn and nearby bodies of water. Researchers found that, on established lawns growing on soils that did not test deficient in phosphorus, adding this nutrient in fertilizer did nothing to benefit the growth of the turf. So, 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 5-0-10 or 10-0-10. (That middle number indicates the percentage of phosphate in the fertilizer.)

The research also demonstrated that, even if the soil test showed a need for additional phosphorus, adding it in late fall did not benefit the grass and increased phosphorus in the runoff. So apply any phosphorus-containing fertilizers in spring or early fall.

The research did show that phosphorus was helpful in the establishment of a new lawn the first year after sodding or seeding and did not contribute to runoff when it was applied according to rates recommended by State Extension Service Specialists. It also showed that, regardless of whether or not fertilizer was applied, there was less phosphorus in the runoff (and less runoff) from turf that was dense and healthy.

Several years ago, a number of Vermont and New York organizations active in the Lake Champlain watershed began an outreach campaign to educate home gardeners on lake-friendly gardening methods. In addition to publicizing their catchy "Don't "P" on Your Lawn" message, they provide information and links on good lawn care practices and watershed protection on their website Some of their helpful suggestions include fertilizing the cool season lawn grasses we grow only once a year, in early fall, the time of year when the grasses make best use of applied fertilizers, sweeping up fertilizer that lands on driveways and sidewalks as you spread it, and avoiding fertilizing right before heavy rain is predicted.

So keep your lawn vigorous through good lawn care practices and add phosphorus carefully only when there is a demonstrated need for it. This way your lawn and nearby waterways will both thrive.

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