Managing weeds organically can truly be a challenge. After some experimentation, I have found that corn gluten meal (CGM) is showing very promising results in controlling the annual weeds that pop up in tree beds, lawns, and established garden beds.
CGM is a by-product of the wet milling process used to produce corn starch and corn syrup. This natural product contains proteins with herbicidal properties that do not prevent germination but instead hinder the development of roots after germination. With a limited root system, the weed seedlings are not able to take up enough water or nutrients to survive.
Timing and Environmental Factors Are Important
As with most organic weed control products, the timing of application and the environmental factors are important. CGM should be applied just before weed seeds start to germinate. If crabgrass is your major foe, apply CGM when the forsythias start to bloom or when soil temperatures reach 50°F. Applied at a rate of 20 pounds per 1000 square feet, it can reduce crabgrass development by 60%, and the results may be even better if the product is applied at a higher rate.
Common annual weeds that CGM can aid in controlling
- black medic
- hairy bittercress
Uses in the Vegetable Garden
In the vegetable garden, it can be applied after direct-seeded crops or transplants have become established. As a bonus, CGM contains about 10% nitrogen by volume and can therefore act as a slow-release natural fertilizer for your nitrogen-loving veggies. But as good as it is at suppressing the growth of new seedlings, CGM will not control existing perennial weeds if their root systems are fully established.
Corn Gluten Meal Application
Unless at least ¼ inch of rain falls shortly after application, it is crucial to thoroughly water the area where CGM has been spread. Water spreads the product out to create a layer at the soil surface. Any seed, including grass seed, that germinates near this layer will be affected. After the initial rain or watering, it’s ideal if the area remains dry for several days, since excess moisture will reduce CGM’s herbicidal qualities. If the dry conditions persist, CGM can remain effective for up to six weeks due to its high protein content and slow-release nature. Repeat applications are helpful and can have a cumulative effect. Consistent, even coverage is important; Small gaps in coverage allow space for weeds to germinate and survive. I have found that applying a 3- to 4-inch layer of organic mulch on top of the CGM aids in its overall effectiveness.
Tip: Beware of imposters
Corn gluten meal is not the same as corn meal. Corn meal, like many of the moonlighting products that are sold as CGM, has a high nitrogen content and will act as a fertilizer, stimulating weed growth. Be sure to check that the product you buy is labeled as a pre-emergent herbicide. Genuine CGM is typically sold in powder and granulated forms. Both forms are equally effective, but I find that the granulated form is much easier to apply.
—Adam Glas is a garden supervisor and rosarian at the Scott Arboretum of Swarthmore College in Swarthmore, Pennsylvania.
Photos: Adam Glas