Corn gluten meal is a powdery byproduct of the corn milling process. Originally used as a supplement in hog feed, corn gluten has become a common organic alternative to chemically derived herbicides. It can be effective as a pre-emergent herbicide used to control crabgrass and other lawn weeds. It also has fertilizing properties. Corn gluten meal is about 10 percent nitrogen by weight, meaning 100 pounds of corn gluten contains 10 pounds of nitrogen. This organic source of nitrogen is slowly released over a three to four month period.
How Corn Gluten Works
Corn gluten does not prevent weed seeds from germinating, but it does inhibit those seeds from forming roots after germination This means that applications must be very carefully timed. When the application of corn gluten is timed correctly, crabgrass seeds germinating will form shoots but not roots and will die, provided there is a short dry period after seed germination. If conditions are too wet immediately after seed germination, the weed can recover and establish a root.
Corn gluten is useful only as a pre-emergent herbicide; it provides no post-emergent weed control. If crabgrass and other weed seeds have already germinated and taken root, a late application of corn gluten will only serve as fertilizer for the weeds. Further, applications of corn gluten need to be timed around rainfall or watering. After application, corn gluten needs to be watered in, either by rainfall or by artificial watering, within five days of application. A rainfall of about 1/4 inch, or a comparable artificial watering, is ideal. After this, a dry period of 1 or 2 days is required to prevent weed seedlings that have germinated from growing roots.
In other words, corn gluten needs water just after application, but a dry period is then required in order for germinated weed seeds to have their root production inhibited.
The first application of corn gluten will suppress only about 60 percent of the weed seeds, and a single application may help suppress weeds for four to six weeks. Heavy soils, extended rainy weather, and hot spells may require a monthly application or a second application in late summer. The initial results may be disappointing but after several applications, corn gluten sometimes reaches 80 percent effectiveness at controlling crabgrass.
How Much Corn Gluten is Needed?
Application rates vary by form: powder pelletized or granulated. The standard application rate is 20 pounds of corn gluten per 1000 square feet of lawn. This rate also provides about 2 pounds of nitrogen per 1,000 square feet.
The effects of corn gluten are cumulative, meaning that the results improve with repeated use over time.
The Downside of Using Corn Gluten
Corn gluten is costlier than conventional pre-emergent herbicides. Because multiple applications are often required, 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 they are still costly.
Timing is critical for both organic and synthetic chemical pre-emergents. It's very important to remember that all pre-emergents, including corn gluten, will suppress all seeds, including grass and flower seeds. If you are using non-selective pre-emergents in the spring and summer, any lawn seeding should be done in the fall.
Critics sometimes point out that the nitrogen in corn gluten has drawbacks. Some turf specialists argue that extra nitrogen only gives weeds the advantage. Crabgrass is a filler weed that thrives in bare spots or areas with thin turfgrass, and organic turf specialists contend that seeding with new grass is just as effective as pre-emergent herbicides, including corn gluten. Dense, healthy turf will naturally crowd out crabgrass, so growing more grass and filling in those thin areas and bare patches will result in less crabgrass.
Corn gluten does work as a pre-emergent herbicide, through a mechanism that inhibits germinated weed seeds from establishing roots. But timing the applications correctly is tricky, and it may require repeated applications of corn gluten meal to really see the desired results. Further, corn gluten can also inhibit new turfgrass seeds from becoming established.