How Herbicides Contaminate Compost

by Robin Bachtler-Cushman

The Pea Test reveals the presence of Clopyralid

Of Horsetail and Herbicides

A troubled reader writes: “Horse tails have been in the front portion of my property for three years now, but they are now popping up in my side yard which is currently just lawn. I do have the dream to get rid of most of my lawn and replace it with native ground cover and shrubs. Should I accelerate this plan in order to remove the horse tails? What is your opinion on the product Crossbow? I heard that it potentially kills horse tails.”

Sadly, there is no simple solution to getting rid of horsetail, but it can be done (read on to learn how). It’s important to know that no herbicide, however toxic and persistent, will completely kill long-established horsetail, which has deep roots that penetrate at least 6 feet of soil (and can go far deeper in some circumstances). It’s most vulnerable in situations where it arrived recently with plants or topsoil, but even there, I’d never use Crossbow. It’s a soil-persistent herbicide, able to damage or kill seedlings and young plants for at least a year after application. It’s non-selective, meaning it kills everything susceptible in its path. Its main targets are woody and broad leaved evergreen trees and shrubs. Though the label has very few warnings, Crossbow combines 2,4-D ( a potent hormone disruptor banned in numerous countries) and Triclopyr, a skin-absorbent toxin that harms animals (including humans) and kills aquatic critters from frogs to fish even when diluted in rain and runoff. So no. Just no.

Compost Contamination

It’s hugely important to recognize that many supposedly safe herbicides and pesticides are both toxic and persistent. That they often carry few label warnings reflects successful industry lobbying over concern for public safety (surprise!). About 20 years ago, West Coast states had outbreaks of “killer compost” that wreaked havoc in gardens. The main culprit was Clopyralid, another persistent herbicide that remains effective indefinitely and at concentrations as low as 2 parts per million (no joke). Compost facilities began testing for it with bioassays (aka pea tests) and once it was banned for homeowner use, Clopyralid contamination hadn’t been a problem. In 2020, however, the Oregon Department of Agriculture reported compost issues traced to Clopyralid again and last year, at least one Washington facility produced compost that stunted tomato plants, probably due to Clopyralid presence in horse bedding straw and fodder. Like other herbicides, Clopyralid contamination causes foliage distortion and often death for many plants and beneficial insects; International Organization for Biological Control testing showed that Clopyralid exposure killed between 30-80 percent of control groups of ladybugs, lacewings, and many others.

Before that killer compost fiasco, it wasn’t known (or acknowledged) that even tiny amounts of this persistent herbicide could kill mature ornamental plants, because all research was focused on agricultural production. As gardeners’ reports poured in, research began: damage from Clopyralid-contaminated compost resembles moderate to severe herbicide exposure, with yellowing, twisting, and malformation of foliage. Initial growth on affected seedlings might look fine, but the second set of true leaves (not including the baby or seed leaves) displays damage. However, not all plants would be affected to the same degree, even in the same family.

Pea Test To Recognize Contamination

The main families affected by Clopyralid are the composites (artichokes and cardoons, asters to zinnias), the legumes (peas, beans), and the nightshades (tomatoes, peppers, potatoes, petunias). Damage may be variable: Cardoons and zinnias planted with contaminated compost usually die, while asters may persist but weakly. Of ten kinds of beans I planted with contaminated compost, only two sets looked normal but flowered sparsely and didn’t produce any beans. Tomato seedlings died after a few weeks, while healthy tomato starts were too damaged to flower or set fruit.

If you fear your compost is unsafe, try this simple pea test: mix the compost with potting soil and plant several kinds of peas and beans. If they produce at least three heathy sets of true leaves, there’s no residual Clopyralid. If the plants wither and look sickly, use the compost on lawns, shrubbery, and woody landscaping that doesn’t include annuals and perennials. Also, let the supplier know what you discovered, so they can follow up with the source facility.

Why Is Horsetail So Stubborn?

Horsetail (Equisetum) has been around since the dinosaurs’ day, with fossil records dating back over 300 million years. It’s a prehistoric survivor and not surprisingly ranks among the most challenging weeds to manage. The best initial strategy is CUT DON’T PULL: pulling the stems triggers a flush of new sprouts from deep root tubers, while clean cutting doesn’t. Repeated cutting slowly depletes the storage roots, but where horsetail is long established, that can take years. Even so, it’s a good practice and makes a useful moving meditation technique as well. Breathe, cut horsetail, breathe….

Our second strategy is to improve drainage and soil quality by scattering granulated humic acid, adding several inches of (clean) compost, and topping it off with wood chips (not bark chops). If you don’t like the look, cover the chip layer with more compost, and the resulting sandwich will help heal the soil. If improving soil quality seems counterintuitive, remember that heavy, moist, acidic soils that are low in oxygen favor horsetail while open, rich, pH neutral and well drained garden soils do not. Compost is a natural pH buffer, moderating soil acidity slowly but steadily. That’s how lasting change works. Onward, right?