Corn gluten meal is the new natural herbicide for lawns. The move away from synthetic chemical herbicides has left a void in turf management, and researchers are scrambling to find an organic solution.
A promising product is corn gluten meal. Proponents claim it prevents weed seeds from germinating, and if the seeds don’t germinate, you have a weed-free lawn. Sounds like a perfect solution.
There are scientific reports, both for and against the product. Anecdotal evidence from outfield players is also mixed. Does the product work? How should it be used? Are people using it correctly?
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What is Corn Gluten Meal?
In the 1990s, Dr. Nick Christians of Iowa State University was working on golf greens and discovered the herbicidal qualities of a product called corn gluten meal. This natural material is a by-product of the wet milling process used to produce corn starch and corn syrup from corn. It contains 60% protein and 10% nitrogen, by weight.
Corn gluten meal is not the same as what you find in grocery stores, as many social media sites claim. Cornmeal has no herbicidal properties and as far as I know the only thing it will do in the garden is feed ants and slugs.
Not All Corn Gluten Flours Are The Same
There are different grades of corn gluten meal and the herbicidal one is 60% protein. This product is still labeled as pre-emergence herbicide .
True corn gluten herbicide is expensive and many people have tried a cheaper product called corn gluten or distillers grains. These animal feed products may even be called corn gluten meal, but they are not labeled as pre-emergent herbicides.
One of the reasons corn gluten meal can get a bad press is because gardeners try to use the food and then report that it doesn’t work. They don’t work because they don’t have a high enough protein level: it’s the wrong product.
Another common problem is that corn gluten meal must be applied at a high rate of 20 pounds per 1,000 square feet and most home spreaders cannot reach this level. If not enough is applied, it will not be effective.
Does this natural herbicide prevent seed germination?
Many sites report that corn gluten meal prevents seeds from sprouting, but this is a myth.
After treatment with corn gluten meal, the seeds will germinate normally, producing a radicle (also called a root). Food for radicle growth comes from within the seed and is unaffected by its new environment. Once the radicle is formed, it begins to form roots which absorb water and nutrients from the soil. Corn gluten meal inhibits root formation, it does not prevent seed germination.
Seedling death depends on a perfect storm of events. The developing roots must absorb enough protein from the corn gluten meal to have an effect. Therefore, higher application rates generally result in better weed control. The roots should also be dry after germination. Too much water dilutes the effect of the protein and the roots continue to grow. As shown below, the application of the product is essential.
If corn gluten meal stops root growth, why doesn’t it affect mature plants?
Mature plants have many more roots and have deeper roots in the ground. They are never exposed to enough protein to have a significant effect. Corn gluten meal will not harm existing plants, even if they are weeds.
Corn gluten meal: does it work as a natural herbicide?
Original fieldwork by Dr. Nick Christians showed that corn gluten meal applied at 99, 198, 297, 396, 495 and 594 g/m2 reduced crabgrass infestation by 50, 65, 80, 95 and 93%, respectively when applied 1 week before crabgrass germination. Applying it 4 weeks before germination required larger amounts to have the same effect.
crabgrassWhen 22 different weeds were tested, all were reduced, but the degree of reduction varied by species and application rate. At low doses, some weeds were not affected. Since this work was done, further research has identified certain weeds that appear to be immune to corn gluten meal.
Most discussions focus on weeds, but non-adventitious seeds such as grasses, perennials and vegetables are also included.
Others have also tried corn gluten meal. A study from Oregon State University indicates that they were unable to replicate the initial results in the field. I’m not sure this work has ever been published? The work was part of a master’s degree and is available as a thesis. He found that corn gluten meal did not reduce weed counts.
In his conclusion, he specifies that this may be due to the fact that the test was carried out on clean ground, without grass and therefore without competition, or that it may be due to an old product. The product has not been lab tested to verify that it works. Didn’t rain during testing and Oregon can be quite humid, so too much rain can also prevent the product from working.
The Turf Grass Institute at the University of Guelph studied corn gluten meal and found that the product controlled weed seed germination, but was not 100% effective.
Dave Gardner of Ohio State University made a video showing his results. He found that twice the recommended amount of 20 pounds/1000 square feet had to be used to be effective, making it very expensive to apply. He also pointed out that it is necessary to apply it for at least two years since “the results of the first year are disappointing”.
You can find both positive and negative searches for this product. The key may well be to use a good quality product and use it correctly. Any research that does not provide rainfall data is not helpful as a dry period after application is required for the product to work.
too much nitrogen
Corn gluten meal contains about 10% nitrogen by weight in organic form, mostly protein. Nitrogen is slowly released into the soil as it decomposes over a 3-4 month period.
This is an important fact because this nitrogen improves the growth of grass and weeds. It’s actually a good lawn fertilizer, but expensive.
Any weed seed that is not stopped will be able to use the extra nitrogen to grow faster than it would on an untreated lawn. It can be a big problem if you apply it at the wrong time.
This highlights one of the serious limitations of anecdotal reporting. Almost none of these count the actual weeds, so it’s entirely possible that people will conclude that the product didn’t work because it produced large weeds that cover a lot of the grass. The general appearance of the weeds has not been reduced.
Is this natural herbicide safe for lawns?
Corn gluten meal will not harm existing lawns or other perennials. It should not be used at the same time as seeding a new lawn.
When and how should it be applied?
Corn gluten meal should be applied just before weed seeds begin to germinate. Most weed seeds germinate in the spring, with a second sprout in the fall. For crabgrass control, it is recommended to apply when the forsythia begins to flower.
There are two problems with this advice; different weed species germinate at different times, and germination is affected by environmental conditions, which change from year to year. It is therefore difficult to apply it at the right time. Apply too late and nitrogen feeds already germinated seeds. If applied too early, it has limited effect.
A rate of 20 pounds per 1,000 square feet reduced crabgrass by 60%. Higher rates should be more efficient, but they will also cost significantly more. You can get 90% control, but cost doesn’t guarantee results.
The next step is critical. It must remain dry for 7 days after application. If it rains too much, the excess water will reduce the herbicidal qualities of the product and you will not see any reduction in weeds. Do you trust the meteorologist?
Corn gluten meal will remain effective in the soil for up to 6 weeks as long as it remains dry enough to prevent microbes from breaking it down.
More Myths About Corn Gluten Meal
changes the pH. Corn gluten meal will not change soil pH significantly.
Effective on all weed seeds. It is effective on most types of seeds, but not all.
It works by drying the seedlings. Many sites incorrectly claim that corn gluten meal dries out seedlings. It’s not correct. Alaninyl-alanine and 4 other dipeptides have been shown to cause root development to stop.
Can be used all summer. Advertisements for the product and some gardening sites recommend that the product can be used all summer. It is true that it can be used all summer, but few weed seeds germinate in the summer. Using the product in the summer, unless you are trying to get rid of summer germinating weeds, is a waste of money.
It’s natural and safe. Since it’s a natural product, everyone assumes it’s safe to use, but that’s not entirely true. Corn and corn derivatives like it are known to cause allergies in some people and this condition can be serious. If you or your family suffer from corn or respiratory hypersensitivity, avoid exposure to corn gluten meal.
Should I use this natural corn gluten meal weed killer?
The product works but it is not 100% effective. It must be used correctly or it will not work. If you’re in a particularly wet area in the spring, this probably won’t work.
If your lawn has a lot of existing perennial weeds, the nitrogen in corn gluten meal will make them grow better and aggravate your lawn. Treat perennial weeds first.
Corn gluten meal has become very expensive and must be applied thickly to be effective. If you use it, don’t skimp on the application.
This is a good product to try if you have a lot of crabgrass (it’s an annual), live in an area with a dry spring, and aren’t allowed to use or don’t want to use synthetic herbicides.
References:
1) Survey on corn gluten meal; https://www.hort.iastate.edu/horticulture-research/corn-gluten-meal-research/