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Over the last 20 years, corn gluten meal has been hyped as a natural "weed and feed". It has been used by home owners, lawn care professionals and golf course superintendents with mixed results.
Weed control reports have varied from poor to excellent and it's finicky when it comes to environmental conditions - they have to be just right. On the other hand, if you want to use corn gluten meal as straight fertilizer it's a very good source of organic nitrogen.
CGM only works as a preemergent herbicide - preventing weed seedlings from growing into mature plants. It does not work on established weeds.
In the mid 1980s, Iowa State University researcher Nick Christians observed corn gluten meal to have preemergent herbicidal effects while researching pythum fungus on bentgrass greens. Further investigation found CGM releases a protein that disrupts the root development of sprouting weed seeds leaving them vulnerable to dry conditions.
To use CGM effectively as a preemergent you need to be familiar with the life cycle of the weed you are trying to control. Crabgrass, chickweed, dandelions, purslane, plantain, pigweed, foxtail and barnyard grass are some common lawn weeds that have been effectively controlled.
CGM needs to be applied at a rate of 20 lbs per 1000 ft2 just prior to weed seed germination. The useful window is short since the active ingredient vanishes in 4 to 6 weeks. A period of light rain or irrigation is required to activate the product and then a dry period is needed to kill the affected weed seedlings. If the soil remains moist, the seedlings will recover and the 2 lbs nitrogen per 1000 ft2 that was applied will probably cause them to thrive.
Studies have shown that you will generally see 60% control the first year, 80% the second and up to 90% control the third year. These results could be credited to the heavy nitrogen fertilizer causing the grass to grow vigorously and denying weed seedlings water, light and room to grow.
Tip: Only purchase CGM that is labeled "preemergent herbicide" or you might end up buying a corn gluten feed that is cheaper but does not work.
It's probably best to use corn gluten meal as a straight fertilizer and forget about the weed control. The 20 lbs per 1000 ft2 required for weed control is equal to 2 lbs nitrogen per 1000 ft2 - that's a lot of nitrogen! An overdose of nitrogen for cool-season grasses in the spring. It's really never a good idea to apply more than 1 lb of nitrogen at a time, even if it is slow release. The excess nitrogen has the potential to leach into our ground waters and will encourage fungal diseases of cool-season grasses in humid weather.
Drop the rate to 5 or 10 lbs per 1000 ft2 and you have a very nice organic fertilizer that gives lawns that nice dark green color we aim for.
New products have been developed that can be sprayed making them easier to handle and apply.
The new corn gluten hydroslate products are water soluble, contain less nitrogen by weight and might be more effective than CGM for weed control - if they can be improved. CGH has been shown to be more effective than corn gluten meal in controlled environments. However, the hydroslate form is roughly equivalent to CGM in the field and seems to be more sensitive to environmental conditions.
These hydroslate products the have potential to be effective organic herbicides if scientists can figure out how to prevent the active ingredient from dissipating quickly.
Virginia Cooperative Extension publication 452-129: http://pubs.ext.vt.edu/430/430-532/430-532.html