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Could there be a killer hidden in your compost?

Social media is rife with reports of
plants suffering mysterious herbicide
damage,
Social media is rife with reports of plants suffering mysterious herbicide damage Credit: Alamy Stock Photo

Is weedkiller in compost and manure affecting your plants? Over the past few months gardeners have been taking to social media to report seedlings distorting and dying from the effects of weedkiller residues found in composts and manures.

Gardener Rebecca Headd says: “In spring we mulched our polytunnel beds with bought-in rotted farmyard manure. Into these beds we planted tomatoes, aubergines, peppers and basil. Three to four weeks later we noticed severe distortion of the new leaves and stems curling inwards. Plants started to twist themselves into spirals and knots as if in some medusa-esque horror show… I will never touch manure again. It simply can’t be trusted.”

It’s always disheartening to see newly sprouted seedlings grow poorly or die – especially for those growing organically – while for commercial growers it’s especially serious.

First to raise the alarm was organic market grower, Charles Dowding, the bestselling authority on no-dig gardening: “This spring I was gutted to see broad bean leaves curling… and traced it to compost made last summer, when I had added some horse manure to one compost heap, because of a shortage of green matter in the dry summer.”

A video on Dowding’s YouTube channel showing the damage to crops sparked comments from other people experiencing the same problems (see “Growth problems caused by aminopyralid residues in hay and horse manure”). Weedkillers can reach our gardens via composts based on green waste and/or in manure when areas grazed by animals (primarily horses) have been treated with aminopyralid or clopyralid herbicides. These weedkillers kill only dicotyledonous plants such as thistles and ragwort, leaving grass alive. But when the grass is grazed by animals the weedkiller remains active in their manure.

As yet it’s unknown how widespread the problem is this year; Guy Barter of the RHS says: “We have a handful of reports of herbicide damage every year that have originated via gardeners’ own use of a weedkiller and occasionally those suspected to have been caused by shop-bought potting compost or manure.”

Plant grown with good compost (left) versus plant grown with bad compost (right) Credit: Healthcliffe O'Malley

While it’s not illegal for farmers to use these weedkillers – although they were temporarily banned following a similar problem in 2008 – it is illegal to knowingly sell on any infected manure for garden use. Barter adds: “Any manures or composts suspected of having been infected by herbicides are reported to the Health and Safety Executive (HSE) who have responsibility for chemical use in the UK.”

A spokesman for the HSE confirmed that anyone supplying hay or manure containing weedkillers would be breaking the Plant Protection Products Regulations 2011 and Plant Protection Products (Sustainable Use) Regulations 2012. “These regulations both contain prosecutable offences which potentially carry an unlimited fine.”

Problems occur when a contaminated product is sold – perhaps unknowingly – because it is hard to detect weedkiller residues without testing the product first, by growing plants in it; a time-consuming and costly process for the compost industry. Dowding has conducted his own tests that can be easily replicated at home:

“I did some trials with a compost, with susceptible pea and bean seeds, also tomato plants and basil. All grew in a stunted way and the basil died, compared to healthy growth of the same seeds and plants in a Soil Association-certified compost.”

A plant grown in good compost (left) versus bad (right) Credit: Healthcliffe O'Malley

If your seedlings are growing poorly, don’t panic, it’s more likely your plants have been affected by fluctuating weather, insects or disease. All of these problems can look similar to weedkiller damage. Signs to look out for are whether different types of plants using the same compost or manure are showing the same problems, and if the new shoots and leaves look distorted, stunted and pale yellow.

Once plants have been infected by aminopyralid or clyptopyralid it is highly unlikely they will survive. So, dispose of the plants and the affected compost/manure and start again. “Should your plants show the symptoms, your options of disposal are few apart from sending plants and compost to landfill,” says Dowding, “If you send them to recycling, they will contaminate the green waste compost.”

If you’ve unwittingly spread or incorporated infected compost or manure into your soil, there is hope – the weedkillers do eventually break down, though it does take time. Dowding was first affected back in 2014; but on testing the soil after 12-15 months he found there was no more leaf curling.

For further information: search 
rhs.org.uk for the advice page “Weedkiller in manure”; search hse.gov.uk for the advice page “Issues associated with the use of farm yard manure containing aminopyralid residues”.
 Also visit charlesdowding.co.uk