Some Preen is toxic and can contaminate waterways.

Preen is not always Corn Gluten
I got a 5# container of Preen Vegetable Garden Weed Preventer and it was 100% Corn Gluten Meal. The only precautionary statement was: "Possible allergic reaction to inhalation of dust in affected individuals" [Exempt from registration under federal pesticide act.]
Then today, I wanted to get a larger size bag to refill the container. I bought a bag of Preen which looked identical to the original container, but when I got home I found I had:
Preen Garden Weed Preventer and it was not Corn Gluten, but was 1.47% Trifluralin. The extensive precautionary statement included: Do not let product contact persons or pets. Causes moderate eye irritation. Avoid contact with eyes, skin or clothing. Keep away from food, feedstuffs, and domestic water supplies. This prduct is extremely toxic to fish and aquatic invertibrates. Do not expose dust to canals, lakes, streams, ponds, marshes, or estuaries.
Wow. What a difference. And the only difference in the name is one word, Vegetable.
For large quantities, you can get corn gluten meal for about $0.60 to $1 per pound from:
Espoma Corn Gluten Organic Weed Preventer - 25LB ($25)
Woodstream Concern Weed Prevention Plus - 25LB ($16)
SAFE N SIMPLE WEED CONTROL - 50LB ($32)
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Stephen Henning said:

We were at a stop light leaving the neighborhood when my daughter and I spotted a man sprinkling Preen around his foundation plantings. First he tried using the 'shaker' top, but that seemed to be giving him problems, so he opened the container and started pouring it into HIS BARE HANDS. My daughter and I were slack-jawed just looking at him.

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