Is it too early to apply corn gluten to my lawn?
Following is the relevant section of an Ohio State University Bulletin that addresses your question:
Recent research shows that corn gluten is an effective pre-emergent herbicide that can control crabgrass, barnyardgrass, foxtails (Setaria spp.), dandelion, lambsquarter, pigweed, purslane, and smartweed. Corn gluten is a byproduct of corn syrup production. The proteins in the corn gluten act on germinating seeds to inhibit root growth. After application and a period of water stress, weed seedlings wilt and die.
Corn gluten also contains 10% nitrogen by weight, and has a slow-release fertilizing effect when applied to home lawns. If applied at the rate of 20 pounds per 1,000 square feet, this also effectively applies 2 pounds actual nitrogen (N).
Research shows that 50-60% weed control can be achieved in the first year when corn gluten is applied at 20 pounds per 1,000 square feet; 80-85% in the second year; and over 90% control by the third year. This is due to the reduction in weed seeds since weeds do not mature, and from the effect of nitrogen in the corn gluten increasing the lawn's density.
The best time to apply corn gluten is shortly after the last spring frost and again in the fall. Water following the application if there is no rain, then allow the area to dry for two or three days. A dry period following application is required for corn gluten to effectively kill emerging weed seedlings. If excessive rain occurs following application, reapplication may be necessary. A corn gluten application can inhibit grass seed germination, so avoid using this product if reseeding or overseeding an area of the lawn.
Factors to consider in the use of corn gluten include: 1) expense, 2) your knowledge of weeds and their life cycles, because it must be applied in a four- to six-week period prior to weed seed germination, and 3) possibly inconsistent results, because excessive moisture and microbial soil activity can reduce its effectiveness.
Go here to read the complete bulletin: Natural Organic Lawn Care for Ohio.
Although the bulletin was prepared for Ohio residents, most of the information also applies to Minnesota.