Weed killer in compost?
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Issue: January 8, 2000

Weed killer in compost?

Question:

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?

Answer:

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.

Send your gardening questions to Yard and Garden, ATTN: Dr. Curtis Smith, NMSU Cooperative Extension Service, 9301 Indian School Road, NE, Suite 112, Albuquerque, NM 87112. Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist with New Mexico State University's Cooperative Extension Service. New Mexico State University is an equal opportunity/affirmative action employer and educator.