How does it prevent weeds?
While the root seeds will germinate, corn gluten inhibits root development which means that the weeds simply fade away. The number of weeds will decrease form 30% to 45% the first year. During the third year, these weeds should not be a factor: crab grass, foxtail, lamb's quarter, purslane and redwood pigweed, etc. It will also prevent new dandelions from developing from dandelion seed, but does not affect the perennial weekds which already have well established root systems. In addition, because corn gluten meal is an excellent fertilizer, it will thicken the turf, crowding out weeds. The best weed preventer is thick, healthy turf.
Application
As a pre-emergent weed control for the spring weeds -- crabgrass, etc -- it should be applied in the spring before crab grass and other weeds that grow from seeds germinate, when the lilacs are coming out and/or the forsythia is blooming. For pre-emergent weed and feed purposes, it is put down at a rate of 1 lb. per 50 sq. ft. (A 25 lb. bag covers 1250 sq. ft.)
If the weed problem is severe, Renaissance 10-0-0 can be applied again in the fall, after the heat stress of the summer is over but before the fall weeds sprout. This is usually some time between mid-August and mid-September.
Precautions
Do not use Renaissance 9-0-0 on turf that has not been established for at least 90 days.
For research articles on-line: http://www.gluten.iastate.edu