Issue: January 8, 2000

Weed killer in compost?

I collected grass clippings from my lawn all summer and composted them. As I was talking to a friend about this, he warned me that I shouldn't use the compost in my garden because I used a fertilizer with weed killer last spring to kill dandelions. He said that the herbicides are still there and will kill my vegetables and flowers. Is there any way I can use my compost?

There should be no problem using your compost if you used the herbicides according to directions and if the ones you used are those commonly used in combination with fertilizer to kill broadleaf weeds in lawns. There are some herbicides which could be cause for concern, but I doubt that you used any of the worrisome herbicides. To be sure of this, contact your local Cooperative Extension Service to ask them if the herbicide you used is a problem. There is a County Extension Service office in every county in New Mexico and, for that matter, almost every county in the United States. If you remember which product you used, have the bag it came in, or if you know the active ingredients listed on the bag, it will be much easier for the Extension agent to help you.

The important facts are that herbicides are on the surface of the plant only briefly, are absorbed in small amounts into plants, and are broken down in the environment to harmless materials.

If you mowed shortly after applying the herbicide, before watering the lawn, you may have collected a greater quantity of herbicide than otherwise. Even that should not be a problem as will be discussed when I address the breakdown of herbicides. You should have mowed before applying the herbicide as that allows the product to reach the soil at the base of the weeds more effectively. Subsequent mowing, after irrigation, will collect very little herbicide as you are collecting new growth that doesn't have the herbicide on the leaf surface. There may be a little herbicide in the leaf tissue that is collected, especially that of the weeds which are being killed, but as the herbicide works, the weeds will be less and less a component of the clippings collected. In time, the collected clippings will have no herbicide, thus they will dilute that already collected.

If you mowed shortly after applying the herbicide, before watering the lawn, you may have collected a greater quantity of herbicide than otherwise. Even that should not be a problem as will be discussed when I address the breakdown of herbicides. You should have mowed before applying the herbicide as that allows the product to reach the soil at the base of the weeds more effectively. Subsequent mowing, after irrigation, will collect very little herbicide as you are collecting new growth that doesn't have the herbicide on the leaf surface. There may be a little herbicide in the leaf tissue that is collected, especially that of the weeds which are being killed, but as the herbicide works, the weeds will be less and less a component of the clippings collected. In time, the collected clippings will have no herbicide, thus they will dilute that already collected.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!