Corn Gluten Meal Kills Lawn Weeds According of U of M: “Apply CGM in late Apr/early May, then again mid-Aug. Spread evenly, at a rate of 20 lbs per 1,000 sq. ft. of lawn. Water it lightly into the soil to activate. CGM should be effective for 5 to 6 weeks. Check your wx forecast before applying CGM, it needs a few days of dry weather to go to work after the initial light watering-in.