An image from Washington State University’s aminopyralid bioassay instructions.

Another thing to worry about! In the past two years farmers and gardeners in the UK and US have experienced the unintended effects of a powerful herbicide called aminopyralid, sold by Dow Chemical under the brand names Merit and Forefront. This herbicide is used to control weeds such as thistle, knapweed and yellow starthistle.

The problem is that aminopyralid survives the digestive systems of animals pastured on land sprayed with it, as well as compost piles made from their manure. Most other herbicides break down eventually, but this stuff sticks around. An organic farmer using compost contaminated by aminopyralid could lose crops and organic certification for years. If that isn’t enough to worry about, two other nasty herbicides, picloram and clopyralid have also contaminated compost piles around the world.

But what about us backyard gardeners? How can aminopyralid effect us? I’m fond of using a bit of horse manure in my compost pile. It’s free for the taking and helps heat up the pile. But if the horses were fed hay grown on land sprayed with aminopyralid I could lose my veggies, particularly tomatoes, lettuce and legumes which are highly susceptible to this chemical.

So what can we do? First the practical: test your compost. Washington State University has instructions for performing a simple test here (pdf). Basically, you plant three pea seeds in a 50/50 blend of compost and potting mix and compare their growth against a control group of three pea seeds grown in just potting mix. If you use manure in your compost pile and you don’t own the animal it came from, this test should be routine.

Secondly, a political solution: the Rachel Carson Council suggests writing two EPA officials to suggest banning a trio of deadly herbicides that includes aminopyralid: Kathryn Montague at [email protected], and Dan Kenny at [email protected].

For more information on aminopyralid, picloram and clopyralid see the Rachel Carson Council’s Killer Compost Q&A.

Read the articles in Mother Earth News by Barbara Pleasant that tipped me off to this problem, “Milestone Herbicide Creates Killer Compost” and “Contaminated Compost: Coming Soon to a Store Near You.”

Here’s a technical discussion of aminopyralid for those familiar with biochemistry.

From Ohio State University, a fact sheet on the equally bad clopyralid and some charts showing the persistence of other herbicides.

Lastly, beware of the recommendations of agencies tasked with the eradication of invasive weeds. The California Invasive Plant Council, in a 2006 publication on Yellow Starthistle management (availiable here as a pdf), recommends using both aminopyralid and clopyralid and fails to warn of their persistence. The USDA, Department of Defence and the Army Corp of Engineers assisted with that publication. Looks like these agencies need a little reflection on the laws of unintended consequences.