According to some experts:

"Corn gluten meal prevents sprouting seeds from developing normal
roots. This does not directly kill the seedlings, but makes them
susceptible to dehydration if the soil gets dry. Established plants are
not affected."

See this entire article at Pesticide.org:
http://www.pesticide.org/pubs/alts/c...lutenmeal.html.

Apparently, several scientific studies have shown that this product
reduced the establishment of crabgrass in Kentucky bluegrass turf.
Applications of 20 pounds of corn gluten meal per 1000 square feet of
turf reduced crabgrass by about 60 percent. Perhaps it's not 100
percent, but it's safe for the environment.

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Dr. Yucca: The only good weed is your neighbor's weed.