Herbicide in Compost

There is nothing more disappointing when gardening than coming out to the vegetable garden to find that your plants are exhibiting the curled, twisted, or misformed leaves characteristic of herbicide damage. While herbicide drift is a common issue, especially when it is windy and hot where herbicide can drift for well over a mile, there might be another cause for the damage rather than your neighbors. The answer might be in the soil, especially if you have added compost or composted manure to your gardens.

Most commonly, herbicide damage through contaminated organic material is caused by grass clippings. Herbicides were applied to the lawn, and then the lawn was mowed, and those grass clippings were applied to the garden before the herbicide had a chance to dissipate. It usually takes three times to mow the yard for the herbicide to be out of the grass clippings, so I recommend not catching the clippings for the first month after application to allow the chemicals time to dissipate. If you are getting grass clippings from other people, ask if they have sprayed for chemicals recently and, if so, what they have sprayed. I would never recommend getting grass clippings from a city compost pile because you never know what has been sprayed, and there have been too many times people have lost their gardens for years due to chemical contamination.

The second way herbicide can be introduced into the garden is through contaminated compost or composted manure. Composted grass clippings are a common way for compost to be contaminated; however, sometimes, the pile itself is sprayed to kill weeds on the outside, which can result in contamination. Like contaminated grass clippings, composted manure can be contaminated by herbicides through the grass or hay the animals eat. The herbicides are safe for the animals to eat; however, the animal’s digestive system does not break them down, nor are they broken down quickly through the composting process. Several herbicides can be carried in compost, grass clippings, or manure. The herbicides of concern are aminopyralid, clopyralid, fluroxypyr, picloram, and triclopyr, a class of herbicides known as pyridine carboxylic acids. Before acquiring or using manure — fresh, aged, or composted — ask what the animals were fed, the origin of the hay, and what herbicides were used on the hay or pasture. Some livestock owners can tell you this, but many might not know the products used or the origin of the hay they purchased. They may suggest the manure is “safe” because their animals have not been affected.

If your garden soil is contaminated by herbicide, you don’t have to start completely over or replace the soil. These herbicides eventually break down through exposure to sunlight, soil microbes, heat, and moisture. The herbicides can be deactivated in as few as 30 days, depending on the situation. Still, some field reports indicate that breakdown can take three to four years, depending on weather conditions (dry weather slows down this process). There are a couple of ways to help encourage the chemical to break down faster, and they include:

• Till the garden several times during the growing season,
• Irrigate the area to promote microbial activity
• Plant it into a non-sensitive cover crop for a year or two to help the herbicides break down.

Conduct a pot or field bioassay before planting sensitive crops in the previously contaminated area. You can take some small pots of soil from the garden and plant peas or beans in the pots, then observe the growth of the plants to see if it’s normal; if not, you have to wait to plant tomatoes in the garden. You can also plant bean or pea seeds and see how they grow. Peas and beans are very sensitive to chemicals and will show symptoms quickly.

If you want to add purchased compost or composted manure to your garden this fall, be sure to ask some questions before you pick it up and till it in so that you don’t have issues next growing season.

Have questions? Contact our office where our Horticulture Extension Agent will assist you with questions.

Phone: (316) 321-9660

Email: callae@ksu.edu