Natural Weed Control?

Left: A thick cover of native plants, such as the potentilla and strawberries shown here, can help to choke out weeds. However, corn gluten looks like a promising way to prevent weed seeds from germinating in early spring, when coverage may not be so lush.


A MENTION IN AN ARTICLE I READ RECENTLY sent me in search of information about corn gluten meal as a possible addition to the wild gardener's toolkit. What I learned makes me think that CGM could be just what I need for control of weeds in the area I'm trying to turn into a native garden.

CGM is a yellowish, powdery byproduct of corn milling (it is not the same as cornmeal used in baking, which is made from ground-up whole corn). It is often incorporated in animal foods, including dog food, because of its high protein content. In the late 1980s, researchers at Iowa State University accidentally found that it has the ability to stop a germinating seed from forming normal roots. As a result, the germinating seed becomes susceptible to dehydration, and will probably die.

In 1991, the first patent was awarded for use of CGM as a natural "pre-emergent" control for weeds, including dandelions, pigweed, crabgrass, plantain, lambs quarters, curly dock, creeping bentgrass, smart weed, redroot bigweed, purslane, foxtail, barnyard grass, and Bermuda grass.

Is it safe? Some people are allergic to corn and corn by-products; these people should avoid exposure to CGM. Other than that, however, CGM apparently is quite safe. The U.S. Environmental Protection Agency has classified it as "minimum risk," and it's recommended by a number of websites devoted to environmentally safe gardening practices. (The Northwest Coalition for Alternatives to Pesticides has a good fact sheet on the topic.)

I've probably been feeding CGM to wildlife for at least a month without knowing it, since it's very likely present in the stuff I feed my ducks. The duck food has been attracting squirrels, chipmunks, blue jays, and grackles, all of whom have been giving "Ducky's Diner" enthusiastic reviews. The ducks, who prefer their commercial duck food to most other foods I've offered them, seem to be thriving.

CGM attracted my attention because I've been fighting weeds in the area I plan to turn into a native garden. Right now I'm relying on aggressive native groundcovers for weed control while I wait for trees and shrubs I've planted to start creating shade. The groundcovers are providing good coverage now (see the photo above); however, they will die back over winter and provide poor coverage in early spring.

Early spring is, of course, precisely when many weed seeds are germinating. In fact, one of the (many) ways weedy plants often out-compete native rivals is that they germinate earlier, thus gaining a head start over the slug-a-bed natives. This year, in order to knock the weeds back long enough for the natives to take hold, I had to do a lot of hand weeding. An application of CGM at the right time might have saved me a lot of work!

CGM does have its limitations:

• CGM must be applied about 4-6 weeks before weeds sprout. Good luck figuring out when that is!
• Although CGM requires moisture in order to start working, dry weather is required in order to dehydrate the germinated seeds. A spell of wet weather could undo its effects.
• CGM is likely to suppress all newly germinating plants in an area: Don't use this product if you are hoping to get native plants established by seed. (I happen to be in luck on this front--most of the natives I'm favoring propagate primarily by runners.)
• CGM won't kill weeds that are already growing; those you still have to remove by hand.
• Most authorities say that CGM has to be applied for several years before you can expect to see good control of weeds.
• Finally, because of its high nitrogen content (about 10%), CGM acts as a fertilizer. The recommended application rate adds about 1 or 2 pounds per 1,000 square feet of nitrogen -- good for lawns but maybe not so good for native plantings, given that most native plants are adapted to low-nutrient environments.

Still, I think CGM is worth a try. This is what I plan to do:

• The recommended application rate is usually 12-20 pounds per 1,000 square feet; I'll go with the smallest application rate first, and see how that works out.
• Manufacturers recommend applying CGM in both spring and late summer. I'll definitely try it in spring; one recommendation is to apply it when the forsythias are blooming, so I'll be keeping a careful eye on my neighbors' shrubs. I'm not sure whether I want to try a late summer application, which is recomended "following the hot, dry stress period"(usually mid-August). If my groundcovers are still lush in mid-August, I may skip that one.
• Manufacturers recommend raking CGM into soil if possible. I probably won't be able to do much raking because of the groundcovers, but I'll scratch it in to any bare areas.
• It should be watered after application, but then the area should be allowed to dry out. I probably won't need to water because we usually have wet springs. As for that dry period ... well, I'll have to count on the spirits of the garden for help with that one!

Iowa State University maintains a list of manufacturers that are licensed to sell CGM-based herbicides.