Corn gluten meal has weed seeds

Corn Gluten Meal – Does it Work For Weeds?

Corn gluten meal is the new herbicide for lawns. The movement away from synthetic chemical herbicides has left a gap for managing lawns and researchers are scrambling to find an organic solution. One product that is showing promise is corn gluten meal. Its proponents claim that it prevents weed seed from germinating, and if 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 gardeners is also mixed. Does the product work? How should it be used? Are people using it correctly?

lawn and corn gluten meal

What is Corn Gluten Meal?

In the 1990s, Dr. Nick Christians at Iowa State University was doing some work on golf putting greens and stumbled upon 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.

Corn gluten meal is 60% protein and contains 10% nitrogen, by weight.

Corn gluten meal is not the corn meal found in grocery stores, as so many sites on social media claim. Corn meal has no herbicidal properties and as far as I can tell the only thing it will do in the garden is feed ants and slugs.

Not All Corn Gluten Meal is The Same

There are different qualities of corn gluten meal and the one that is a herbicide contains 60% protein. This product is always labeled as a pre-emergent herbicide.

The true corn gluten meal herbicide is expensive and so many people have tried a cheaper product called corn gluten feed, or distillers grain. These animal feed products may even be called corn gluten meal but they will not be labeled as a pre-emergent herbicide.

One reason corn gluten meal may be getting bad press is that gardeners are trying to use the feed products and then reporting they don’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 needs to be applied at a heavy rate of 20 pounds per 1,000 sq ft and most home spreaders can’t reach this level. If it is not applied thick enough, it won’t be effective.

Does it Prevent Seeds From Germinating?

Many sites report that corn gluten meal prevents seeds from germinating, but this is a myth.

After treatment with corn gluten meal, seeds will germinate normally, by producing a radicle (aka root). The food for growing the radicle comes from inside the seed and is unaffected by its new environment. Once the radicle is formed it starts to make roots which absorb water and nutrients from the soil. Corn gluten meal inhibits the formation of roots – it does not prevent seed germination.

The death of the seedling depends on a perfect storm of events. The developing roots need to absorb enough protein from the corn gluten meal to have an effect. This is why higher application rates generally result in higher weed control. The roots also need to be on the dry side after germination. Too much water dilutes the effect of the protein and roots keep growing. As discussed below, application of the product is critical.

If corn gluten meal stops root growth, why does it not affect mature plants?

Mature plants have many more roots and they have roots that are located deeper in the soil. They are never exposed to enough protein to have a significant effect. Corn gluten meal does not harm existing plants, even if they are weeds.

Corn Gluten Meal – Does it Work?

Dr. Nick Christians’ original field work showed that c orn gluten meal applied at 99, 198, 297, 396, 495, and 594 g/m2 reduced crab grass infestation by 50, 65, 80, 95, and 93%, respectively when applied 1 week before crab grass germination. Applying it 4 weeks before germination required higher amounts to have the same effect.

When 22 different weeds were tested they found that all were reduced, but the degree of reduction varied by both species and application rate. At low rates some weeds were unaffected. Since this work was done, other research has identified a few weeds that seem to be immune to corn gluten meal.

Most discussions refer to weeds, but non-weed seeds like grass, perennials and vegetables are also affected.

Others have also tested corn gluten meal. A study out of Oregon State University says that they could not replicate the initial field findings. I am not sure that this work was ever published? The work was part of a Masters degree and is available as a Thesis. It found that corn gluten meal did not reduce the number of weeds. In it’s conclusion it states that this could be due to the fact that the testing was done on clear soil, with no grass and therefore no competition, or that it might be due to an old product. The product was not tested in the lab to verify it worked. The work did not record rain fall during the tests and Oregon can be quite wet, so it is also possible that excess rain kept the product from working.

The University of Guelph Turf Grass Institute has researched corn gluten meal and concluded that the product does control weed seed germination but that it was not 100% effective.

Dave Gardner from Ohio State University made a video showing his results. He found that to be effective you needed to use twice the recommended amount of 20 lbs/1,000 sqft, which makes application very expensive. He also commented that you need to apply it for at least two years since “first year results are disappointing.”

You can find both positive and negative research for this product. The key might well be in using good quality product and using it correctly. Any research that does not provide rain data is not of much help since 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 an organic form, mostly protein. The nitrogen is slowly released into the soil as it decomposes over a 3-4 month period.

This is an important fact since this nitrogen makes both turf grass and existing weeds grow better. It is actually a good, but expensive, lawn fertilizer.

Any weed seed that is not stopped, will be able to use the extra nitrogen to grow faster than if would in an untreated lawn. This can be a big problem if you apply it at the wrong time.

This points out one of the serious limitations of anecdotal reports. Almost none of these count actual weeds, so it is quite possible that people conclude the product did not work because it produced nice large weeds which cover a large area of the grass. The overall appearance of weeds was not reduced.

Is It Safe for Grass?

Corn gluten meal will not harm any existing type of lawn grass 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 needs to be applied just before weed seeds start to germinate. Most weed seed germinates in spring, with a second flush happening in fall. For crab grass control, it is recommended that you apply it when the forsythias start to bloom.

There are two problems with this advice; different weed species germinate at different times and germination is affected by the environmental conditions, which change from year to year. So it is tricky to apply it at the right time. Apply too late and the nitrogen feeds the already germinated seed. Apply too early and it has limited effect.

A rate of 20 pounds per 1,000 sq ft reduced crab grass by 60%. Higher rates should be more effective, but they will also cost much more. You can get 90% control, but the cost does not warrant the results.

Water it in right after application.

The next step is critical. It must remain dry for 7 days after it is applied. If you get too much rain, the excess water will reduce the herbicidal qualities of the product and you will see no weed reduction. How well do you trust the weatherperson?

Corn gluten meal will remain effective in soil for up to 6 weeks provided that it stays dry enough to prevent microbes from decomposing it.

More Corn Gluten Meal Myths

Alters pH

Corn gluten meal will not alter the pH of the soil to any significant amount.

Effective on All Weed Seed

It is effective on most types of seed, but not all.

Works by Drying the Seedlings

Many sites make the incorrect claim that corn gluten meal dries out the seedling. This is not correct. It has been shown that Alaninyl-alanine and 4 other dipeptides cause roots to stop their development.

Can be Used All Summer

Ads for product and some gardening sites recommend that the product can be used all summer long. It is true that it can be used all summer long, 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 Is Natural and Safe

Since this is a natural product everyone assumes it can be used without risk, but that is not quite true. Corn and corn by-products such as this are known to cause allergies in some people and this condition can be serious. If you or your family suffer from respiratory or hypersensitivity due to corn, avoid exposure to corn gluten meal.

Should You Use Corn Gluten Meal?

The product works but is not 100% effective. It must be used correctly or it won’t work. If you are located in an area which is particularly wet in spring, it is probably not going to work.

If your lawn has a lot of existing perennial weeds, the nitrogen in corn gluten meal will make them grow better and make the lawn worse. Deal with the perennial weeds first.

Corn gluten meal has become very expensive and to be effective it needs to be applied thickly. If you do use it, don’t skimp on the application.

This is a good product to try if you have a lot of crab grass (it is an annual), live in an area with a dry spring and you are not allowed to use, or don’t want to use synthetic herbicides.

Corn gluten meal did not prevent weeds from germinating in OSU study

CORVALLIS, Ore. – Corn gluten meal is a natural substitute for a synthetic “pre-emergence” herbicide and has been advertised as a more environmentally friendly way to control weeds.

A pre-emergent herbicide is one that kills seedlings as they germinate. Pre-emergent herbicides generally have to be applied and watered in before weed seeds germinate. Other herbicides, such as glyphosate (e.g. Round Up) kill plants after they have emerged.

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. A former graduate student, Chris Hilgert completed his masters thesis by investigating corn gluten meal use as a pre-emergent herbicide in shrub beds and on lawns.

In their trials with corn gluten meal, Hilgert and Cook found the following:

Corn gluten meal did not control any weeds in any trials under any circumstances over a two-year period. They found no evidence of pre- or post-emergence weed control in any of their trials. Because it contains 10 percent nitrogen, corn gluten meal proved to be a very effective fertilizer, causing lush, dense growth of turfgrass and of weeds in shrub beds.

James Altland, nursery crops specialist at OSU’s North Willamette Research and Extension Center in Aurora, spoke to his observations when corn gluten was used in plant nurseries as a pre-emergent herbicide.

“I’ve seen nursery situations where the applied product caused a bad odor, as do some herbicides, and attracted rodents,” said Altland. “In nursery situations where the goal is complete weed suppression, my overall impression is that it doesn’t work that well.”

“My overall impression has been that in turfgrass it provides a lot of nitrogen,” added Altland. “Thicker, denser turf from high nitrogen rates will reduce weed numbers alone, without the help of herbicides.

“Applying 20 pounds per 1,000 square feet of corn gluten meal would be equivalent to two pounds of nitrogen per 1,000 square feet. That’s a lot of nitrogen! Applying that much nitrogen is not good for the environment. It doesn’t matter if it’s a ‘natural’ fertilizer or not. That nitrogen will ultimately be converted to nitrates, which potentially could leach into groundwater.”

It is not clear why the commercial version of corn gluten meal used in OSU trials was not effective, said Cook. One possibility is that the product as formulated for sale has a short shelf life and loses potency during manufacture, shipping and storage. Further research needs to be done to test this hypothesis, he said.

If you want to discourage weeds from germinating and growing in your garden beds over the winter, try adding mulch to soil surfaces. Use a minimum of three to six inches of composted material. Tuck mulch up to the shoulders of your perennials, but don’t cover the growing crown until freezing cold weather sets in. If you cover plant crowns too soon, they may begin to grow under the mulch and could be killed when temperatures dip.

Shredded bark, leaves, mint hay, wood chips, or yard waste all offer benefits. Large chunky material such as fresh clean wood chips and bark nuggets work best for weed control, as they are low in available nutrients so won’t fertilize germinating weeds.

Avoid mulching with hay or with ryegrass straw. Their seeds will sprout to create an unnecessary headache for you in the spring. And don’t use grass clippings from a lawn treated with a weed-and-feed preparation. The herbicide in the clippings can damage your shrubs.

A low-nutrient mulch such as well-rotted sawdust will benefit shrubs such as roses, azaleas, rhododendrons and hydrangeas. Lilies, dahlias and spring bulbs will do better with this type of mulching also. But be aware that composted sawdust or other fine organic material may contribute to weed growth.

Caneberries benefit from higher-nutrient mulches such as composted manure. Dormant vegetable beds can use a six-inch blanket of manure and leaves. Rhubarb and asparagus beds do best covered with a mix of well-composted straw and manure.

Over the winter, the composted material will mix with the soil, so a second application of mulch in March or April will keep your garden soil in better condition.