Clopyralid: The Weed Killer That Stays in Compost

The Problem at a Glance

Clopyralid belongs to a family of herbicides called pyridine carboxylic acids. It’s effective at controlling thistles, clovers and other legumes in lawns and grassland. So far, so normal.

The problem is what happens afterwards. Unlike glyphosate, which binds to soil and breaks down within weeks, clopyralid survives the composting process. If you treat your lawn with a product containing clopyralid, then compost the grass clippings, the clopyralid remains active in your finished compost. Spread that compost on your vegetable beds and you can damage or kill sensitive crops.

How the Contamination Chain Works

The contamination pathway is straightforward:

1. Lawn or pasture treated with a clopyralid-containing product
2. Grass clippings composted — clopyralid survives the heat of composting
3. Compost applied to vegetable beds, containers or borders
4. Sensitive plants damaged — curled, distorted growth, stunted development

The same pathway applies to manure. If horses or cattle eat hay or grass from treated fields, the clopyralid passes through their digestive system intact. The resulting manure is contaminated, and any compost made from it will be too.

This is how allotment holders and vegetable growers get caught out. They buy “well-rotted horse manure” in good faith, not knowing the horses were grazed on clopyralid-treated pasture. The result: an entire growing season of damaged crops.

What Does Clopyralid Damage Look Like?

The symptoms are distinctive:

• Curled, cup-shaped leaves — especially on tomatoes
• Fern-like leaf distortion — leaves become narrow and twisted
• Stunted growth — plants fail to grow or develop properly
• Twisted stems — stems grow in spirals rather than straight
• Poor fruiting — flowers drop and no fruit develops

Most sensitive plants: tomatoes, potatoes, beans, peas, lettuce, sunflowers, dahlias and other members of the legume, daisy and nightshade families.

Less affected: brassicas (cabbage, broccoli), sweetcorn, onions and most grasses.

How to Avoid the Problem

If you grow vegetables or compost grass clippings:

1. Check product labels before buying lawn weed killers. Avoid any containing clopyralid, aminopyralid or picloram
2. Don’t compost clippings from treated lawns for at least the first three mowings after treatment
3. Test manure and compost before use. Plant a few bean seeds in a pot of the compost and watch for distorted growth over 2-3 weeks
4. Ask about treatment history when buying manure. Has the grass/hay been treated with any herbicide?
5. Use alternatives — MCPA, 2,4-D and mecoprop-P all break down much faster and don’t persist in compost

If your compost is already contaminated:

• Don’t use it on sensitive crops this season
• Spread it thinly on non-food areas (borders, paths) where it can break down naturally
• Clopyralid does eventually degrade — it just takes 12-18 months in compost rather than the few weeks you’d expect from most herbicides
• UV light and microbial activity in exposed soil speed up breakdown

Clopyralid vs Aminopyralid

Aminopyralid is clopyralid’s more potent relative. It’s primarily used in agriculture for grassland weed control and is even more persistent. The contamination pathway is the same — treated hay, straw or manure carries residues into gardens.

Aminopyralid was behind the widely reported “killer compost” cases in the UK, where commercial compost and manure products damaged thousands of allotment plots. The issue led to tighter labelling requirements and greater awareness, but the risk hasn’t gone away.

If you’re buying manure, hay or straw from agricultural sources, aminopyralid contamination is actually the bigger risk than clopyralid, because it’s used more widely on farmland.

Does Clopyralid Have Any Legitimate Garden Use?

Yes. If you have a serious thistle problem in your lawn and don’t compost your clippings, clopyralid is very effective. It’s also useful for controlling clover and other legumes that resist standard lawn herbicides.

The key is knowing what you’re using and managing the aftermath. If you don’t grow vegetables and your clippings go in the green waste bin (council composting reaches higher temperatures), the risk to you personally is low. But your council compost could still contain residues — another reason to test any bought compost before use.

For most home gardeners, there are better alternatives. MCPA, 2,4-D and mecoprop-P combination products handle the same weed killer tasks without the persistence risk. You might need two applications instead of one, but you won’t be gambling with your compost.

Frequently Asked Questions

Does clopyralid break down in compost?

Very slowly. Standard home composting doesn’t generate enough sustained heat or microbial activity to break down clopyralid quickly. Expect 12-18 months before residues fall to safe levels. Hot composting (above 60°C sustained) breaks it down faster but still not as quickly as other herbicides.

How do I test if my compost is contaminated?

Plant 3-4 bean seeds in pots of your compost and 3-4 in shop-bought potting compost as a control. Keep both in the same conditions. After 2-3 weeks, compare them. If the beans in your compost show curled, distorted growth, it’s contaminated. Tomato seedlings are even more sensitive if you prefer to test with those.

Can I still use clopyralid on my lawn?

Yes, but don’t compost the grass clippings from the first three mowings after treatment. If you do compost, use them only for non-food plantings. Or better yet, choose a lawn weed killer without clopyralid — there are plenty of effective alternatives.

Is glyphosate safer for compost than clopyralid?

Yes, significantly. Glyphosate binds to soil particles and breaks down within weeks. It does not persist in compost and is not taken up by plants from the soil. However, glyphosate is non-selective, so it can’t be used on lawns — it would kill the grass too.

My tomatoes have curled leaves — is it definitely clopyralid?

Not necessarily. Leaf curl in tomatoes can also be caused by viral infection, overwatering, heat stress or aphid damage. Clopyralid damage is distinctive because it affects the newest growth first and causes fern-like distortion rather than simple rolling. If you’ve recently used compost or manure from unknown sources, contamination is likely.

Can I sue if bought compost contains clopyralid?

This is a legal question beyond our scope, but contaminated compost and manure cases have been taken to court in the UK. Keep records of where you bought the product, take photos of the damage, and contact your local Trading Standards office. The timing of application and source traceability are key factors.