Purdue Extension Service

Vanderburgh County, Indiana

Garden Weed Control

By Larry Caplan, Extension Horticulture Educator, Vanderburgh County, IN

For the Evansville Courier and Press, March 28, 2008

I recently wrote about using pre-emergent herbicides for controlling crabgrass in home lawns. I had commented that nearly all of the pre-emergent products were mixed with fertilizer, which is not the best practice for lawn care. I received some corrected information from my friends at Tenbarge Seed in Haubstadt that they carry pre-emergent herbicides without fertilizer. You may want to check them, and some of our other local garden centers...I certainly will before I make such a broad statement again!

If you applied your pre-emergence herbicide before the monsoon we had during the week of March 17, I can pretty much guarantee that there is little to no active ingredient left in the soil. These pre-emergent herbicides work by forming a chemical barrier in the upper soil surface. Most of these products should be watered in with about a half-inch of water, to dissolve the chemical and move it to the top inch of soil. However, we received over 6 inches of rain in about a 24 hour period, which would have leached all of this chemical out of the surface layer. Unless you reapply your pre-emergent herbicide, expect to have very poor crabgrass control this year.

Speaking of weed control, I am often asked about using herbicides in the vegetable garden. My general advice for most small gardens is to not use any type of herbicide. Most gardeners plant a wide variety of vegetables in a small area. There is no herbicide that you can use that will be labeled for every vegetable in the garden.

It doesn't matter how carefully you apply these products in a small garden. One heavy rain will wash the herbicide off the area you treated, and spread it all over the garden. Also, the roots of one vegetable can easily extend to the next row, where the chemical is. A non-labeled plant will be injured or killed if it contacts the chemical; it will certainly make the crop inedible.

Now, there are a couple of herbicides that are labeled for home vegetable gardens. If you were to have only labeled vegetables in the garden, you could use these products. Crops that are not on the herbicide label can be planted in a separate garden at least four or five feet away.

Trifluralin, sold as Treflan, Preen, Miracle-Gro Garden Weed Preventer, and several other brands, is a pre-emergent herbicide that controls a wide variety of annual grasses and broadleaved weeds. It must be applied before the weed seeds germinate. It is labeled for use in asparagus, beans, members of the cabbage family, members of the cucumber family, tomatoes, and a few other crops. Read and follow all label directions before buying, using, and disposing of garden chemicals.

The best weed controls for the vegetable garden are mulching and careful hoeing. This can be done for any garden crop, and causes the least harm to you and the environment.

For more information on garden weed control, contact the Purdue Extension Service at (812) 435-5287.