Preventing Weeds

The first thing I have to tell you about weed preventers is THE BEST ONES ARE CHEMICALS! Ooh that ugly word. But there is at least one natural weed preventer which is not a chemical.

There are several chemical weed preventers available to commercial landscape contractors. However, the best one available to home gardeners to use around flowers and vegetables is Trifluralin, sometimes abbreviated to Treflan. Trifluralin is the active ingredient in Preen and several other brands of weed preventers.

The best time to add weed preventer to the soil is right after transplanting. Its effectiveness lasts about 6 to 8 weeks, and then another application can be made.

 Granules of weed preventer are scattered uniformly over the soil. If a few granules of weed preventer land on the leaves of desirable plants it will not damage them. Some of the chemical on the weed preventer granules dissolves and ends up on top of the soil every time it rains or is irrigated. As the weed seeds germinate, they absorb this chemical and are killed before they emerge from the soil (Weed preventers are referred to by professionals as pre-emergent weed killers).

Weed preventers work effectively only if they remain on top or in the top fraction of and inch of the soil. Do not hoe or cultivate around flowers or other plants after you apply weed preventers. Hoeing brings up new weed seeds from under the top of the soil and moves weed preventer granules away from the soil surface. “Oh, that explains why I have had less than the best success with weed preventers before.”

 Corn gluten which is removed from corn in processing is an excellent weed preventer. Although it is not as effective as chemical weed preventers, it is quite effective if applied uniformly at a somewhat higher rate than chemicals. Corn gluten is seldom available at big box stores, but must be purchased from a full service nursery or garden store or by mail or on line. For more information on corn gluten, go to Remember, do not cultivate after applying corn gluten.

 Corn gluten is used as a food supplement for livestock and is not toxic to children, pets or wild life. Birds will often eat some of it but will still leave plenty behind for the weeds.

 Anything which shades the soil will reduce weed seed germination. Hence, bark dust and other mulches are natural weed preventers. Black and colored mulching films which are used for vegetables are natural weed preventers.

Weed Preventer for use around woody plants, including roses, raspberries and blueberries.

 Casoron is one weed preventer for use around woody plants. It works just like Trifluralin but can be toxic to established green plants such as flowers and vegetables. It is deadly to grass, so do not spill it on your lawn.


Allen Wilson

Allen has been writing about gardening for over 30 years. He is a retired professor of Horticulture.

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