In my recent post about the dangers of lawn fertilizers, I mentioned that organic fertilizers can, if misused, also cause environmental harm. A case in point is corn gluten meal, which has been enthusiastically adopted as a weed-suppressor by the organic lawn care industry.
Pause for a disclaimer: I am not a strict organic gardener, but I believe that minimization of the use of chemical pesticides and fertilizers should be the goal of any responsible gardener.
To return to corn gluten. This a by-product of corn processing, and though little used in human foods, it’s an important protein source in feeds for pets and livestock. Using a food source for lawn maintenance is, in my opinion, as indefensible as using corn to produce fuel for automobiles.
However that may be, the effect of corn gluten meal on lawns is not just weed suppression. This materials is also rich in nitrates and so functions as a turf fertilizer. In fact, corn gluten meal is typically 9 to 10 percent nitrate by weight. If you apply it to your lawn at the generally recommended rate of 20 lbs. per 1,000 square feet, you are applying 1.8 to 2 lbs. of nitrates. Recommendations about how often to apply the corn gluten meal to achieve good control of weeds varies, but many authorities call for two or even three applications a growing season.
It’s easy to do that math: two applications equal 3.6 to 4 lbs. of nitrates per 1,000 square feet of lawn, while three applications equal 5.4 to 6 lbs.
Either application rate supplies far more nitrates than your lawn will use. Rutgers University, which supports the premier turf research program in the eastern United States, recommends feeding a bluegrass lawn (if you don’t remove the clippings) 2-3 lbs. of nitrates annually, and just 1-2 lbs. for a lower maintenance turf of tall or fine fescues.
What happens to the excess nitrates from the corn gluten meal? They increase the susceptibility of your lawn to diseases and pests, they leach away to pollute local waterways, and they escape into the atmosphere as nitrous oxide, a potent greenhouse gas.
The best preventative for lawn weeds is to treat the turf as you would any other groundcover: select species that are adapted to your climate and soil, prepare the soil well before planting, and give the newly emerged turf what it needs to become a dense, weed-resistant cover. Then accept the fact that some weeds will find their way into your lawn. If, however, you find the sight of dandelions in your turf unbearable, plant another landscape treatment such as meadow whose biodiversity makes a scattering of weeds much less visually prominent – as well as providing more benefit to wildlife.