What is corn gluten?
Corn gluten meal is a powdery byproduct of the corn milling process. Originally used as a supplement in hog feed, it was accidentally discovered that it has properties similar to that of chemical pre-emergent herbicides. Part of a conventional lawn care program is the spring application of a pre-emergent herbicide to deal with the crabgrass seeds before they have a chance to takeover. Corn gluten is a great organic alternative to chemically derived herbicides.
Corn gluten meal is also about 10% nitrogen by weight, meaning one hundred pounds of corn gluten contains 10 lbs of nitrogen. This organic source of nitrogen is slowly released over a 3-4 month period.
How does corn gluten work?
Corn gluten inhibits root formation of germinating seeds. Timing a corn gluten application is crucial for it to work properly. Corn gluten needs to be applied before weed seed germination. The seed will germinate and form a shoot, but not a root. Prior to germination, a short drying period is needed to kill the germinated, but rootless, plant. If conditions are too wet during germination, the plant will recover and form a root.
Corn gluten is a pre-emergent herbicide only; it provides no post-emergent weed control. If seeds have already germinated, a late application of corn gluten will only serve as fertilizer for the weeds.
Applications of corn gluten should be timed with rainfall. If it doesn’t rain within 5 days of application, it needs to be watered in with 1/4 inch of water. A dry period of a day or two must follow the watering in to prevent the seedling from growing another root.
How much corn gluten is needed?
Recommended application rates vary depending on whether the gluten is in powder form, pelletized, or granulated. The standard seems to be 20 lbs/1000 sq ft. This rate provides about 2 lb of nitrogen per 1000 sq ft.
One application can effectively suppress weeds for 4 - 6 weeks. Heavy soils, extended rainy weather and hot spells may require monthly application or a repeated application in late summer.
The effects of corn gluten are cumulative, meaning results improve with repeated use over time.
What’s the downside of using corn gluten?
The high rates needed for corn gluten to work results in it being costlier than conventional pre-emergent herbicides. It also means that you could be handling hundreds or even thousands of pounds of the product, depending on the size of the yard. Sprayable, liquid forms of corn gluten can make applications easier but are still costly.
The first application of corn gluten will only suppress up to 60% of the weed seeds. The initial results may be disappointing but after several application it can achieve better than 80% effectiveness.
Timing is critical, applying to late will not have the intended results. This is true for chemical pre-emergents too, though there can be some post-emergent effect with some chemical products. It's very important to remember that, like its chemical counterparts, corn gluten suppresses all seeds, including grass seed. When using non-selective pre-emergents, most of the seeding needs to be done in the fall.
Why use a pre-emergent at all?
Some turf specialists, organic or otherwise, do not recommend any early spring application of fertilizer. They argue that the plant is growing enough on its own at this time, and fertilizing only gives weeds the advantage.
Crabgrass is a filler weed. It will thrive in areas with thin turf or bare patches. Many organic turf growers contend that a bag of seed can be as effective at suppressing crabgrass as any pre-emergent herbicide.
Dense, healthy turf will naturally crowd out crabgrass, so growing more grass and filling in those thin areas and bare patches will result in minimized crabgrass pressure.