Corn Gluten Meal for Weed Control? - November 29, 2017
Jeff Schalau, Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County

Many home gardeners look for horticultural products that are less toxic than synthetic products. I applaud gardeners that actively seek less toxic solutions for insect/disease management and environmentally friendly methods of weed control. Insecticidal soaps and Bacillus thurengiensis (Bt) are good examples of these products. As with many sales campaigns, there are legitimate benefits from some products while others are ineffective.

Corn gluten meal (CGM) is a by-product of the corn milling process and marketed as a least-toxic, pre-emergent herbicide. Pre-emergent herbicides are applied to prevent germination of annual weeds. CGM is one least-toxic product that has fallen short of advertised claims as a pre-emergent herbicide. It can have some benefits, but for direct weed prevention, it does not usually measure up. CGM is also a high nitrogen (9-10%) product which also qualifies it as a fertilizer.

As with many marginally effective horticultural products, CGM has shown some promise in weed prevention in greenhouse studies, but in field research, it has fallen short. To further complicate CGM’s purported efficacy, some plant species’ germination is inhibited by it while others are not affected. In addition, Research findings have been hit and miss.

Researchers in California studying weed control of containerized plantings reported that CGM had little effect on preventing germination of either broadleaf or grass weed species. In this research, use of mulch was more effective than CGM, and sub-surface drip irrigation was the best weed control strategy of all. Similarly, control of turf grass weeds was not affected by CGM, though the turf responded well because of its fertilizer effects.

Washington State University researchers found no differences in weed control on field-grown strawberries, though yield was slightly improved (potentially through a fertilization effects). The Iowa State group had similar disappointing results in their strawberry trials, with no significant differences in either weed control or strawberry yield even after multiple treatments with CGM.

The mechanism thought to be responsible for CGM’s pre-emergent effects is desiccation. First, the weed seed needs adequate moisture to germinate. Next, a lack of water in the presence of CGM causes the emerging seedling to desiccate and die. Greenhouse studies can control these factors. However, when studies are conducted in the field, there is considerably less control. The CGM research cited above followed this assumption – in order to be effective, CGM needed a short drying period between germination and root establishment. Under consistently moist conditions, desiccation will not occur and the germinating weeds survive. CGM can also be relatively expensive and shipping costs add to this expense. The recommended application rate is 20-40 lbs per 1,000 square feet and manufacturers recommend two applications per year. I feel there are more cost effective solutions for weed prevention.

A further consideration for some customers may be the presence of genetically modified (GM) materials. CGM is made from both non-GM and GM corn. If you have concerns about GM products, you should investigate the origin of the corn used to produce the CGM. If you see the word “organic” on the product label, it should be free of GM materials.

In closing, I’ll summarize: CGM can prevent weed seeds from successfully germinating under certain environmental conditions; CGM has no negative effect on established weeds; CGM is not selective and can inhibit germination of desirable plant seeds as well as weeds; there are no scientific data from field trials in the Western US to support the use of CGM in weed control; and other environmentally friendly weed-control treatments (such as sub-irrigation, mulch, or soil solarization) are cheaper and often more effective than CGM.

This edition of the Backyard Gardener was excerpted from a publication by my colleague, Dr. Linda Chalker-Scott. Dr. Chalker-Scott is an Extension Horticulture Specialist with Washington State University. She is well-known for dispelling various garden myths and investigating the science behind garden products. She has also written multiple books on garden myths and sustainable gardening practices. See the links below for additional information on CGM’s use as a pre-emergent herbicide and Dr. Chalker-Scott’s books and publications.

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Additional Resources

Horticultural Myths
Dr. Linda Chalker-Scott, Washington State University Extension

The Myth of Weed-Killing Gluten: "Corn meal gluten is an effective organic herbicide”
Dr. Linda Chalker-Scott, Washington State University Extension

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: November 20, 2017
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