In February 2019, the Facebook page “Country n garden” shared a post (archived here) borrowed from another source, which claimed that cornmeal is an effective substance in home gardens to prevent the spread of weeds:
Above an image of a metal measuring cup shaking cornmeal into a garden, text offered a purported explanation of cornmeal as weed birth control alongside a plea for engagement:
DID YOU KNOW THAT CORNMEAL IS BIRTH CONTROL FOR WEEDS? SPRINKLE IT ON YOUR GARDEN AND IT WILL KEEP WEED SEEDS FROM GERMINATING AND GROWING INTO PLANTS. REMINDER — I need your help to stay in this social network. Say something about my posts (yes, yum or 🙂 will do) or I’ll completely disappear from your news feed. Appreciate your help.
The beg for likes and shares pretty clearly marked the post as engagement bait, but that alone didn’t mean that cornmeal isn’t a weed remedy. But given the popularity of the post (with a six figure share count), it also seemed like not a lot of home gardeners were aware of the purported benefits of cornmeal as a weed-discouraging substance.
The claim about cornmeal and weeds seemed to originate with Iowa State University research on corn gluten meal (a byproduct of processing cornmeal), not cornmeal itself. A March 2005 news release from that institute reported:
Corn gluten meal (CGM) is a natural by-product from the wet milling process of corn. It contains 60 percent protein and is used as a supplement in feeds for livestock, poultry and pets. The idea of spreading CGM on lawns to control weeds came quite by accident and through close observation. In 1986, Nick Christians, professor of horticulture at Iowa State University, was using CGM as a growth media in a study of turfgrass diseases. During his research, he observed that the CGM reduced grass seed germination. Curious about the possibilities, he directed his attention to finding out if and how this was possible.
Christians’ research revealed that a naturally occurring compound in the protein faction of CGM had an inhibitory effect on the root formation of germinating seeds. In 1991, he was granted a patent on CGM as a natural, preemergence herbicide for use on all crops. As a preemergence herbicide, CGM only controls germinating seeds and has no effect on weeds that are already established. Currently, it is labeled for control of crabgrass, barnyard grass, foxtails, dandelion, lambsquarter, pigweed, purslane, smartweed and several others at the time of germination.
During the past 10 years, CGM has gained national attention as being the first effective “organic” herbicide. It is marketed and distributed under several trade names.
As a weed control product, CGM is available in two forms, powdered and granulated. The powdered form is the same as that sold at mills for animal feed. Although both forms are effective, the granulated form is easier to apply.
A separate Iowa State University background page indicated that its researchers obtained a patent for corn gluten meal as a weed inhibitor in 1991. And a 1997 webpage on the same site noted that the correct form of corn gluten meal was commercially labeled and available for retail purchase — but again, it is not cornmeal per se.
But the plot thickened in 2006, when Oregon State University researchers issued a press release titled “Corn gluten meal did not prevent weeds from germinating in OSU study,” which is exactly what it sounds like:
A by-product of commercial corn milling, corn gluten meal contains protein from the corn. It poses no health risk to people or animals when used as an herbicide. With 60 percent protein it is used as feed for livestock, fish and dogs. It contains 10 percent nitrogen, by weight, so it acts as a fertilizer as well.
The use of corn gluten meal as an herbicide was discovered by accident during turfgrass disease research at Iowa State University. Researchers noticed that it prevented grass seeds from sprouting. Further research at Iowa State showed that it also effectively prevents other seeds from sprouting, including seeds from many weeds such as crabgrass, chickweed, and even dandelions. Components in corn gluten meal called dipeptides are apparently responsible for herbicidal activity.
Researchers at Oregon State University were not able to duplicate research results reported by Iowa State researchers, said OSU turf grass specialist Tom Cook.
In 2015, research from Washington State University further disputed the findings that corn gluten meal was as effective as widely believed [PDF]:
The principal researcher and patent-holder of CGM, Dr. Nick Christians, is cautious in his recommendation of CGM for weed control. He and his students and staff have published a number of papers in the scientific and popular literature. These researchers are careful to point out that CGM does not affect existing weeds, and that the nitrogen in CGM will benefit existing weeds as well as desirable plants. Therefore, inadequate weed removal prior to treatment can actually result in an increased weed problem.
CGM is not a selective product, nor is it effective on all weed types. Several species of weeds, flowers, and vegetables are inhibited by CGM, while others are not. Effectiveness in greenhouse trials generally increases with application rate (as does the cost).
Rumors about cornmeal as a weed killer or antifungal substance have apparently spread since at least 2010, when horticultural expert Dr. Linda Chalker-Scott debunked the rumors of its efficacy as baseless.
Claims that cornmeal is an effective weed inhibitor have been circulating for at least a decade. In 1991, Iowa State University patented corn gluten meal as a weed inhibitor, but cornmeal and CGM are not the same thing [PDF]. Furthermore, the the efficacy of CGM has been disputed as well.