As a general rule of thumb the few sources of good information about applying Corn Gluten Meal say about when the Forsythia are in blossom. Considering that CGM has an effective growth suppressing life of about 6 weeks that may also provide some guideline information.

Your University of Georgia Cooperative Extension Service may have some good information about when to apply this product.

http://extension.uga.edu/garden/lawn/

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