Corn gluten meal is a powdery byproduct of the corn milling process. Used for years as a supplement in hog feed, this natural protein is very effective for lawns and gardens as a plant food as well as a weed supressor. Corn gluten meal products offer a non-toxic, yet effective alternative to traditional, chemical-based weed and feed products for weed control in gardens and lawns, paths and driveways.
Corn gluten meal works by inhibiting root formation in weeds at the time of germination. It doesn't inhibit roots of mature plants or transplants unless it is used at a very high rate of 80 pounds/1000 sq. ft. or more. Weeds germinate and form a shoot, but no root, which prevents growth. A short drying period is required after germination, because too much water may allow the weed plants to recover and form roots.
Link to more info