A natural/organic alternative to spring weed'n'feeds

If you have looked for an alternative to synthetic preemergent lawn herbicides that prevent annuals like crabgrass or foxtail, you may have noticed products containing corn gluten meal (CGM). An added benefit of CGM is that it also provides a healthy amount of nitrogen, allowing you to both “feed” and “weed” your lawn at the same time. Does this sound too good to be true? 

CGM does inhibit root formation of seedlings and fertilizes the lawn, but certain steps are needed for it to work effectively. Let’s explore how CGM works and what you should consider if you decide to try it.

What is Corn Gluten Meal?

The corn wet-milling process separates corn into starch, oil, and other products; however, a byproduct is the spent corn kernel. This kernel, lacking starch and germ, still contains a significant amount of protein and is typically called corn gluten meal. CGM is used in livestock and pet foods and, in recent decades, has been marketed as a preemergent herbicide for lawns.

How does corn gluten meal kill weeds?

Iowa State University researcher Dr. Nick Christians discovered the herbicidal qualities of CGM. It turns out that the high-protein kernel contains specific amino acids called bioactive peptides that inhibit root formation in germinating seeds. So while the seeds still germinate and form leaves, there are no roots to support the leaf growth.

As CGM slowly breaks down, these peptides are water-soluble, which is beneficial because water applied after CGM helps carry the peptides to weed seeds in the soil. Unfortunately, prolonged rainfall can cause these peptides to leach deeper into the soil, away from the weed seeds. To effectively combat weed seedlings, the material must be spread, watered in, and then left in fairly dry soil for a few weeks so the peptides can prevent root development in germinating seeds. Spreading CGM happens in spring, a time that tends to be quite rainy in Illinois.

Quantity and cost can be a barrier

For CGM to serve as an effective preemergent herbicide, the bioactive peptides need to be at a higher concentration. That’s why most CGM products recommend applying at a rate of 20 pounds per 1,000 square feet. One drawback of this high application rate is that CGM also contains about 10 percent nitrogen by weight. Applying the recommended amount for herbicidal action results in doubling the amount of nitrogen the grass can absorb. For some areas located in sensitive watersheds, it may be illegal to apply nitrogen at the product's recommended rate. Illinois Extension recommends applying only 1 pound of nitrogen per 1,000 square feet during a single application.

Applying CGM at 20 pounds per 1,000 square feet is also quite high, and most spreaders can’t dispense that much in a single pass, often requiring multiple applications. One Ohio State University trial showed highly successful results in 40 pounds per 1,000 square feet. That is a significant amount of material to spread and further overapplies the amount of recommended nitrogen. 

Compared to traditional synthetic preemergent herbicides, CGM is quite expensive. This higher cost makes cheaper knock-off products labeled as “corn gluten” more attractive. However, these products do not contain enough protein to act as herbicides. The EPA requires all products claiming to control weeds to be labeled as herbicides, so look for CGM products that have an herbicide label. If you only want an organic fertilizer without the herbicide effect, corn gluten is an option.

As I've looked at a few different CGM products for this article, I've noticed a range of application recommendations on the product labels from 10 lbs/1,000 sq ft down to 3 lbs/1,000 sq ft. However, each label seems to have an asterisk, parenthesis, or footnote that states 20 lbs/1,000 sq ft is suggested for effective preemergent use of the product. Read those labels thoroughly!

Corn gluten meal is a decent, albeit expensive, fertilizer

Additionally, long-term use of CGM offers weed control benefits that many people overlook. While we focus on weed prevention, proper fertilization over many years can strengthen your lawn’s resistance to weeds, leading to a more resilient yard. While there is a valid concern about the amount of nitrogen when following the recommended rate of 20 pounds of CGM per 1,000 square feet, CGM does not rapidly break down like a quick-release fertilizer. Instead, CGM breaks down over time, prolonging the release of nitrogen.

The timing of corn gluten meal application is important

Because we're using CGM as a preemergent, timing is critical. Apply too early, and the effects may not last long enough into the spring to prevent later germinating weeds. Apply too late, and you've missed the window to impact weed seedlings, and now you are fertilizing them. As the Good Growing blog has referenced in the past, early spring weed'n'feed products compromise the timing of two individual products. Nitrogen triggers vegetative growth. Early in the season, grasses are focused on root growth and development. The excessive amounts of nitrogen too early sacrifice roots over shoots. Ideally, spring fertilizer applications occur after the initial flush of growth has begun to slow.

To prevent weeds like crabgrass in lawns, it is often suggested to apply preemergents when the forsythia blooms. Another metric is when shallow soil temperatures begin to reach 55 degrees for several consecutive days.

Read more about The Dilemma of Weed'n'Feed Lawn Products

Corn gluten meal impacts desirable lawn seedlings

Studies show the CGM will impact the seedlings of desired lawn species. Therefore, if you are applying CGM, it is suggested to avoid overseeding the lawn. CGM will not kill any established plants, weeds included. Additionally, there are some plants considered weeds that are not impacted by CGM and others that are more susceptible. Read the label carefully before applying any pesticide. 

Summary

CGM can potentially serve as a preemergent herbicide, but certain conditions may limit its effectiveness. University studies often show limited immediate control of weed seeds, but consistent use has demonstrated positive results in preventing crabgrass and other annual weeds. Furthermore, significant value may come from the nitrogen content in CGM, which nourishes your soil and helps your lawn resist weed pressure.

Key points:

• CGM becomes less effective during prolonged rain events. The increased soil moisture dilutes the bioactive peptides that inhibit root formation of seedlings.
• Areas with dry spring climates may see better results than areas with rainy spring climates.
• Most spreaders can't apply the high rate recommended for preemergent control. It will require multiple passes over the lawn.
• You wind up applying more nitrogen than the lawn can use.
• There is some confusing marketing and product names. Select CGM products with an herbicide label if you wish to control germinating weeds in the spring.
• CGM is a good organic fertilizer that releases more slowly than conventional synthetic fertilizers.
• Apply CGM when the forsythia blooms or when soil temperatures reliably hit 55 degrees Fahrenheit or more for several consecutive days. (5 to 10 days)
• CGM will impact desirable lawn seedlings. Avoid seeding/overseeding when applying CGM.

Good Growing Fact of the Week: Contrary to its name, there is no “gluten” in corn gluten meal or corn for that matter.

Additional Resources/Reference for this blog post

Iowa State University has a webpage dedicated to corn gluten meal research

OSU's Dr. Dave Gardner's presentation on "Organic" Lawncare, 2020

Dr. Gardner's explanation of a CGM application trial in the video Organic Weed Control: Corn Cluten Meal