Lawn weed spray residue in compost

Asked April 17, 2018, 10:48 AM EDT

If grass clippings from a lawn treated with weed control spray are put into my compost bin will the herbacide affect the finished compost? It usually takes about a year for the compost to be finished. Thank you.

Meade County South Dakota

1 Response

This is a difficult question to answer with a definite question because it depends on a number of factors. Ordinarily lawn herbicides will break down by microbial and chemical activity in grass clippings when placed in an active compost pile over the course of a year. However, it depends on the herbicide that was used, the rate applied, how soon grass clippings were collected after application and how active the compost pile is. Banvel, for example, has a longer residual activity in the lawn than does 2,4-D. If it was applied at a high rate, that will take more time to break down. If the lawn was mowed several times before clippings were collected and added to the compost, that will be better than if they were collected the next time after herbicide application. A cool, slowly working compost pile has less microbial activity than an active pile where temperatures can reach 150 to 160°F. It also depends on what crops might be in proximity to the compost. Tomatoes and potatoes are particularly sensitive to most broadleaf herbicides.

So, will it be OK to use? It depends. You could test the compost by mixing some with potting soil in a ratio of 1:4 compost to potting soil. Then, plant some tomato seeds in the mix and see how they do, compared to planting some seeds in straight potting soil.