Herbicide in Compost

There is nothing more disappointing when you are gardening than to come out to the vegetable garden to find that your plants are exhibiting the curled, twisted or mis formed 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 actually 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 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 has had a chance to dissipate. It usually takes three times mowing 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, be sure to 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 piles because you never know what has been sprayed and there has been too many times people have lost their garden for years due to chemical contamination.

The second way that 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 the contamination. Similar to 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 they are not broken down by the animal’s digestive system nor are they broken down quickly through the composting process. There are several herbicides that can carryover in compost, grass clippings or in composted manure. The herbicides of concern are: aminopyralid, clopyralid, fluroxypyr, picloram and triclopyr are in 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, if any, 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 was 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. Depending on the situation, the herbicides can be deactivated in as few as 30 days, but some field reports indicate that breakdown can take as long as three to four years depending on weather conditions (dry weather slows down this process). There are a couple 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 encourage 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 any 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 on 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 in the garden itself and see how they grow. Peas and beans are especially sensitive to chemicals and will show symptoms very quickly.

If you are looking 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 hopefully 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