Corn Gluten Meal is the new natural weed killer for lawns. The move away from synthetic chemical herbicides has left a void in turfgrass management, and researchers are scrambling to find an organic solution.
One promising product is corn gluten meal. Proponents claim this prevents weed seeds from germinating, and if the seeds don’t germinate, you’ll have a weed-free lawn. It seems like a perfect solution.
There are scientific reports both for and against the product. Anecdotal evidence for outfielders is also mixed. Does the product work? How should it be used? Are people using it correctly?
What is corn gluten meal?
In the 1990s, Dr. Nick Christians of Iowa State University was working on golf courses 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 cornstarch and corn syrup from corn. Contains 60% protein and 10% nitrogen by weight.
Corn gluten meal is not the same as what you find in supermarkets, as many social media sites claim. Cornmeal has no herbicidal properties and, as far as I know, the only thing it does in the garden is feed ants and slugs.
Not all corn gluten meals are the same
There are different grades of corn gluten meal and the herbicide one contains 60% protein. This product is always labeled as pre-emergent herbicide .
True corn gluten meal herbicide is expensive and many people have tried a cheaper product called corn gluten meal or still grain. These animal feed products may even be called corn gluten meal, but they are not labeled as pre-emergent herbicides.
One reason corn gluten meal can get a bad rap is that gardeners are trying to use the food and then reporting that it doesn’t work. They don’t work because they don’t have a high enough protein level: they are the wrong product.
Another common problem is that corn gluten meal must be applied at a high rate of 20 pounds per 1000 square feet and most home spreaders cannot achieve this level. If it is not applied enough, it will not be effective.
Does this natural herbicide prevent seed germination?
Many websites report that corn gluten meal prevents seeds from germinating, but this is a myth.
After treatment with corn gluten meal, the seeds will germinate normally, producing a radicle (also known as a root). The food for radicle growth comes from within the seed and is unaffected by its new environment. Once the radicle forms, it begins to form roots that absorb water and nutrients from the soil. Corn gluten meal inhibits root formation, it does not prevent seed germination.
The death of seedlings depends on a perfect storm of events. The developing roots need to absorb enough protein from the corn gluten meal to be effective. This is why higher application rates often result in better weed control. The roots must also be dry after germination. Too much water dilutes the protein’s effect and the roots continue to grow. As discussed below, product application is critical.
If corn gluten meal stops root growth, why doesn’t it affect mature plants?
Mature plants have much more roots and have roots deeper in the soil. They are never exposed to enough protein to have a significant effect. Corn Gluten Flour does not harm existing plants, even if they are weeds.
Corn gluten meal: does it work as a natural herbicide?
The original fieldwork of 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.
When 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, other research has identified some weeds that appear to be immune to corn gluten meal.
Most discussions are about weeds, but non-herb seeds such as grasses, perennials and vegetables are also affected.
Others have also tried corn gluten meal. An Oregon State University study says that they were unable to replicate initial field findings. Not sure if this work has already been published? The work was part of a master’s degree and is available as a thesis. It turned out that corn gluten meal did not reduce the number of weeds.
In his conclusion, he states that this could be due to the fact that the test was done on clean ground with no grass and therefore no competition, or that it could be due to an old product. The product has not been lab tested to see if it works. The job had no rain during the test and Oregon can be quite humid, so too much rain can also prevent the product from working.
The University of Guelph Turf Grass Institute investigated corn gluten meal and concluded that the product controls 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/1,000 square feet needed to be used to be effective, making the application very expensive. He also commented that it is necessary to apply it for at least two years since “first year results are disappointing”.
You can find both positive and negative reviews for this product. The key can 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.
a lot of nitrogen
Corn gluten meal contains approximately 10% nitrogen by weight in organic form, primarily protein. Nitrogen is slowly released into the soil as it breaks down over a period of 3 to 4 months.
This is an important fact, as this nitrogen makes both grass and weeds grow better. It is actually a good but expensive lawn fertilizer.
Any weed seed that is not stopped can use the extra nitrogen to grow faster than an untreated lawn. This can be a big problem if you apply it at the wrong time.
This points to one of the serious limitations of anecdotal reporting. Almost none of them count the actual weeds, so it’s quite possible that people will conclude that the product didn’t work because it produced large weeds that cover a large area of the grass. The overall appearance of weeds has not been reduced.
Is this natural herbicide safe for lawns?
Corn Gluten Meal will not harm any 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 the weed seeds begin to germinate. Most weed seeds germinate in the spring, with a second flush in the fall. To control crabgrass, application at the beginning of forsythia flowering is recommended.
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. Therefore, it is difficult to apply it at the right time. Apply too late and the nitrogen feeds the already germinated seed. If applied too early, it has limited effect.
A rate of 20 pounds per 1,000 square feet reduced crabgrass by 60%. The higher rates are supposed to be more effective, but they will also cost a lot more. You can get 90% control, but the cost doesn’t guarantee results.
The next step is critical. It must remain dry for 7 days after its application. If it rains a lot, excess water will reduce the herbicidal qualities of the product and you will not notice weed reduction. How well do you trust the meteorologist?
Corn gluten meal will remain effective in 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
change the pH. Corn gluten meal will not change soil pH by any significant amount.
Effective on all weed seeds. It is effective on most types of seeds, but not all.
It works by drying the seedlings. Many websites incorrectly claim that dry corn gluten meal changes it. This is not correct. Alaninyl-alanine and 4 other dipeptides have been shown to cause roots to stop developing.
Can be used all summer. Product advertisements and some gardening websites recommend that the product can be used all summer. True, it can be used all summer, but few weed seeds germinate in summer. Using the product in summer unless you are trying to get rid of summer germinating weeds is a waste of money.
It’s natural and safe. Since it is a natural product, everyone assumes that it can be used without risk, but this is 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 herbicide?
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, it probably won’t work.
If your lawn has a lot of existing perennial weeds, the nitrogen in the corn gluten meal will make them grow better and make your lawn worse. Deal with 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 app.
This is a good product to try if you have a lot of crabgrass (it’s an annual), live in a dry spring area, and aren’t allowed or don’t want to use synthetic herbicides.
References:
1) Corn Gluten Flour Research ; https://www.hort.iastate.edu/horticulture-research/corn-gluten-meal-research/