Contaminated compost

Horse manure and hay are at a small risk of contamination by aminopyralid weedkiller, occasionally sprayed on grass for horse hay. It’s the only weedkiller I know that persists, and it’s lethal to potatoes, tomatoes and legumes, whose growing tips become curled and twisted – examples of this can be found on this website via the search bar on the homepage.

I only heard of this weedkiller after suffering its effects in 2014, a year after I had added some horse manure to bulk up a heap of homemade compost. Potatoes, beans, lettuce, chard, alliums and cucurbits grew with deformed and stunted leaves, and matched the online photographs of damage. 

I notified DOW through their website and they offered to remove the heap of compost. I declined because I wanted to find out how long the effects might last, after spreading it on some pasture.

The compost had many healthy ingredients too, and after a 12–15 months I saw no more leaf curling. If I had spread only contaminated manure, it may have taken longer to break down.

Then, in spring 2019, I was gutted to see broad bean leaves curling in the same way, and traced it to compost made in high summer 2018. I had again added some horse manure to one compost heap because of a shortage of green matter in the dry summer.

This horse manure was from the same neighbour’s heap as the one I used to fill my greenhouse hotbed in February and March. This horse manure had no aminopyralid because it went on to grow healthy legumes and solanums.

It seems that a lot of farmers increased their use of aminopyralid herbicides in 2018, including the Royal Stables and MOD. That is the origin of problems in 2019 with Country Natural’s ‘organic horse manure’, sold by Harrington & Jessups. This problem continued into 2020.

I did some trials with this 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 purchased Soil Association certified compost.

I made a video about this, to help gardeners identify whether or not this problem has affected them. There are hundreds of comments to this video and many are along the lines of, ‘So that is the problem with my tomatoes/sunflowers/beans.’

The poison has many points of entry to gardens, all invisible:

  • Gardens and allotment sites close to fields sprayed with weedkillers containing aminopyralid weedkiller.
  • Hay used by gardeners, as explained above.
  • Horse manure used in commercial bagged compost for mulching. I have heard several horror stories after purchases of Country Natural organic manure, and it’s called organic! Beware that word on bags of compost.
  • Horse manure from stables where horses have eaten hay sprayed with weedkillers such as Banish, Forefront, Grazon Pharaoh and Runway. Or if horses were bedded on rape straw sprayed with Astrokerb (see next point).
  • Some rapeseed straw used for bedding – the NFU warns farmers about the problems if rape has been sprayed with weedkiller containing aminopyralid.
  • Some cow manure.
  • Potting composts that have used contaminated horse manure or green waste composts, such as some bags of New Horizon and Jacks Magic – I saw the latter when giving a course in Cheshire. Readers have also sent photos of problems in 2020 from Bord na Mona compost, J Arthur Bowers and various Westland products. Most are fine but you can’t tell.
  • Mushroom compost if it contains contaminated horse manure.
  • Green waste compost if some of the grass mowings were from lawns sprayed with weedkiller containing clopyralid, which affects growth similarly to aminopyralid. My local composting operation do trials of all batches with pea and red clover seeds, but this is expensive because of adding time to their process.

Many gardeners suffered the same problems in 2008 and 2009, after which the pyralids were temporarily withdrawn, before being sold again but with a protocol of notification to all buyers of products such as hay, if they have been treated.

However this appears to happen rarely, so we are all left guessing. 

What can gardeners do?

Because of the lack of a paper trace, it’s buyer beware for manures and composts. Even some cow manure is contaminated, as per this blog from a Dublin allotment site. 

Should your plants show the symptoms, your options of disposal are few, apart from sending plants and compost to landfill. If you send them to recycling, they will contaminate the green waste compost.

If you have spread contaminated compost/manure on the surface, I would remove it to landfill. If you dug in the product, it will take a year or two to break down and for growth to approach normal again, although it appears there is no fixed time period. DOW/Corteva advise repeated cultivation but I disagree because this damages the very organisms that do valiant duty in dissipating the poison. If you suffer damage, please report it to Corteva here.


* See this YouTube video for more info *