Corn Gluten Hub-bub-i-cide

Since the 1990's, corn gluten meal (CGM) has taken the gardening world by storm.  CGM is the protein by-product of milling corn, and this high-nitrogen compound was discovered by researchers at Iowa State University to be an effective pre-emergent herbicide, reducing seed germination by inhibiting root development.  Organic gardening and farming communities have quickly adopted this natural miracle substance as a method of weed control.

Continued research, however, is showing that CGM is, in fact, far from a natural cures-all for organic gardening.  It does not adversely affect existing weeds.  To the contrary, the nitrogen in CGM (about 10%) benefits existing weeds as much as the desired plants we're trying to help.  It's not a selective product, nor is it effective as a pre-emergent against all weeds.  Most importantly, it is failing to reach an important milestone to recommend its continued use; it seems unable to make the leap from greenhouse to the field.  While greenhouse trials demonstrated efficacy, field trials in the same locations have been unsuccessful.  Washington State University and Iowa State researchers found no differences in weed control on field-grown strawberries using CGM.  Researchers in California found that the use of mulch alone in containerized plants was more effective in controlling weeds than CGM.  California and Oregon researchers found no control of turf grass weeds by CGM, although the turf itself responded to the nitrogen in the CGM. The truth is that moisture, light, and warmth all affect seed germination and these factors are all much harder to control in natural environments than in the laboratory. Since CGM inhibits seed germination primarily by desiccating soil and denying moisture to seeds, it's no wonder that it does not work well in areas of the country with abundant moisture in the spring (most of America).  Linda Chalker-Scott, in The Informed Gardener Blooms Again, concludes that corn gluten meal may act adequately as a pre-emergent herbicide in the Midwestern US, but that it's not effective for western climates or for climates with abundant pre-spring moisture.

There are many lessons in the continuing saga of CGM. I've seen the step from laboratory efficacy to field efficacy fail often in pharmaceutical trials and surgical procedure trials in my chosen profession of veterinary medicine.  There are a couple of hidden messages here to gardeners as well, though:  First, organic gardening principles are great in theory, but it's obvious that weeding is so much of a chore that even organic gardeners will seize on any chance to reduce the work involved. So just who is kidding who?  Second, just because a substance is natural does not mean it's a miracle cure or that it is economical to use. We're going to be driving the price of corn high enough with all our driving around on the ethanol we derive from corn; we shouldn't throw our gardening budgets out of whack by using up even more corn as a partially-effective pre-emergent herbicide.  And if it is not useful as a herbicide, but we use it anyway as an organic fertilizer, somebody out there had better be doing some calculation on whether we use more inorganic fertilizer on Iowa and other Midwestern fields to grow the corn for a bag of CGM than the bag of CGM provides us back as organic fertilizer.  As the grandson of Indiana crop farmers, I guarantee you that no dirt farmer is growing 200 bushel/acre field corn without artificial nitrogen and herbicides.  Is CGM really "organic" or is it organized hype?