Weedkiller in manure

by Jane Moore

I'm as keen on improving my soil with manure as any other allotmenteer. I love it all - chicken, cow and best of all, horse manure.

effects of weedkiller on potato haulmI'm as keen on improving my soil with manure as any other allotmenteer. I love it all - chicken, cow and best of all, horse manure.

A good dollop of well-rotted manure does wonders for your soil, enriching it with nitrogen, phosphates and potassium, while improving its structure and water-retaining ability.

But this year there's a problem. The RHS Members' Advisory Service has received a high number of calls from gardeners reporting abnormal growth of various vegetable crops, including potatoes, tomatoes, beans and peas. Ornamental plants, especially roses and delphiniums, are also affected.

This abnormal growth has been attributed to a weedkiller, which is widely used by farmers on grassland to kill broad-leafed weeds, such as chickweeed, fat hen and groundsel. The weedkiller, aminopyralid, has been ingested by farm animals and hasn't broken down. It's still present in the manure, which, when laid as a mulch or incorporated into soil, can affect garden crops.

Symptoms to look out for include distorted foliage, with cupping of leaves and fern-like growth. There's nothing you can do if your crops are already affected and there's no assurance that affected produce will be safe to eat. However the RHS does point out that, as this weedkiller can be grazed by livestock soon after application there is no reason to believe that children, pets, gardeners or wildlife are at risk.

If your crops have been affected, contact the farmer that supplies your manure to return it. I assumed you could probably stack up your muck for a year to rot it down further and break down the weedkiller residue. Apparently not. The RHS says this weedkiller is even slower to break down when the manure is piled up, so it really has to go back to the farmer to be spread on grass fields. It's also worth getting in touch with the manufacturers of the weedkiller, who are working with the Pesticides Safety Directorate to help them gather information on the issue.

So what should we do? The best bet is to switch manures for a season or two - horse manure may be similarly affected but well-rotted chicken muck should be fine. Beware though, it's very strong and can burn your plants if not rotted down properly, and it's very smelly!

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