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The Corn Gluten Meal Question
About every fourth yard we visit, the homeowner has applied corn gluten meal the previous Fall, and is completely puzzled about why he or she has so many weeds. The most common comment from is, "I think I caused a bigger weed problem, look I have more weeds than my neighbor and they didn't apply anything". The sad thing is I know exactly how they feel. Soils Alive used cgm for more than 5 years trying to get it to work. We always thought our timing was off or it was not a good product so we would change suppliers, but every year we had many client calls wondering why their weeds were so bad.
The reality is they are absolutely right; they did cause a larger problem when they applied corn gluten meal in the fall. Why you ask? To answer that we have to understand how corn gluten works. Dr. Nick Christian at Iowa State University discovered the pre-emergent qualities of cgm and later Dianna Liu as a graduate student at ISU eventually isolated the five individual dipeptides (combinations of two amino acids) that had the ability to inhibit root formation of germinating seedlings.
It is important that we understand that cgm does not inhibit germination, it inhibits root formation. For cgm applications to be successful it must be applied 2-3 weeks before seed germination, watered in sufficiently then all irrigation shut off to allow the soil to dry out. This allows seeds to germinate then given time to desiccate before the next rain/water occurrence. If the soil is moist after the point which the cgm's effectiveness is degraded then the weed seedling will produce roots and will "grow out" of the cgm. If this scenario occurs then the weed seed will have the benefit of the cgm's10 nitrogen allowing it to grow faster, taller, and stronger than if nothing was applied.
The fall/winter application of cgm in my opinion is not a good horticultural decision, and does not provide effective weed suppression. The reason it is not a good horticultural practice is because 20 lbs of cgm per 1000 sq. ft. is the equivalent of 2 lbs of nitrogen at a time when plants are naturally going dormant. That amount of nitrogen will stimulate freeze prone tender growth instead of allowing the plant to harden-off for the winter. Also high nitrogen applications increases the chance St. Augustine grass will succumb to fungal disease, such as brown patch. The weed suppression does not occur as advertised because in the DFW area weeds germinate for several months in the fall and late winter. This would require an application of cgm every 3 to 4 weeks to effectively inhibit the roots from forming. When you add on the challenge of needing a dry period to allow for the desiccation of the weeds then chances are cgm is not going to deliver as advertised.
When should Corn Gluten Meal be used? The only time I would recommend using cgm would be in the spring as the grass is greening up. This application supplies the grass a shot of nitrogen so that it greens up quickly, increasing its density and height so that it can physically out compete the weeds. This approach is the most effective weed suppressing tool there is, physical competition. Any weed suppression the cgm offers is a bonus and thus not a waste of money or labor. I still have a but to inject here. Cgm is composed mostly of nitrogen, containing little to no phosphorus, or potassium. This makes cgm a very "white bread" sort of diet for the plants, and not a well balanced product that feeds the soil and plant in a holistic manner.
I am not a fan of corn gluten meal; it is a by product of the corn syrup industry, and is sold as an animal feed commodity so the price is always higher than products that are waste products that should be used as fertilizers. I think cgm was born out of advertising hype, and a need for an organic weed control. My hope is that we will soon have effective tools that stand up to field trials and not just laboratory studies. Maybe ISU will be able to isolate the effective portions of cgm and create a stable organic weed preventative that has longevity in the field.
