Broad bean new growth showing leaf curl and shrivelling.

Keep your garden safe from killer compost

When Minette Tonoli’s lifelong dream finally came true, moving into a beautiful homestead in May 2019, she was convinced it was going to be the year of abundance’, harvest baskets overflowing with organic produce, and the pantry filled with preserved bounty.

Instead it became the year this herb lady turned into ‘herbicide lady’. 

This article was first published in Organic NZ, March/April 2020.

Broad bean new growth showing leaf curl and shrivelling.
Broad bean new growth showing leaf curl and shrivelling.

Stunted, twisted vege plants

Transplanted broad beans in my newly established potager garden soon started growing bizarrely with new leaves that were hardened, curled and twisted. Snow peas had similar misshapen leaves, as did hundreds of heirloom tomato seedlings that grew stunted, with older leaves cupping, and new leaves forming shepherd’s crook or fiddleneck distortions. 

While there are many reasons for leaf curl, including physiological damage due to watering problems (too much, too little, or inconsistent), fluctuating temperatures, insect infestation and viral infection, none of these seemed to be the right answer across the board for all the plants affected. 

Diagnosing the problem

Soil tests are expensive for home gardeners, so I compared my plants to online pictures of herbicide-damaged growth and they correlated exactly – I had found the most likely culprit: pyridine carboxylic acid herbicides. I knew these types of herbicides weren’t used on my property, and because of the pattern of plants affected, could not blame spray-drift 

Various savvy friends and experts in the organic gardening world agreed that the most likely source of contamination was the vegetable soil mix I had brought in from a landscape supply yard. 

My garden had fallen victim to killer compost. Killer compost is a term given to compost, soil and manure mixes that contain high enough levels of persistent pyridine herbicides to negatively affect non-target plants such as homegrown vegetables.  

Certified organic soil and compost mixes should be free of these residues, but only if they are pure. The vege mix I bought was made up of certified organic compost and added manure, and I think it was the manure that contained residues.  

Distorted tomato leaves showing leaf curl and shrivelling.
Distorted tomato leaves showing leaf curl and shrivelling.

How do these herbicides sneak in?

Pyridine herbicides are used to deal to a range of broadleaf weeds in pastures, grain crops, sports grounds and commercial turf, recreational parks, native forests, some fruit crops, and for roadside maintenance.  

The Environmental Protection Authority of New Zealand (Te Mana Rauhī Taiao) regulates the registration of products containing these herbicides, and there are labelling laws and restrictions on the sale and use of products containing these chemicals. Most are banned for home use (since 2008), and require commercial operators to have safe handling certification.  

With seemingly all the required legislation in place, it could only be through end-user ignorance of how these chemicals work and degrade that herbicide-contaminated hay, grass, manure etc. end up in commercially available home garden soil mixes. Horse manure and hay sold on the roadside are not regulated, and may contain these herbicides. 

Generally, pyridine herbicides are brought into the home garden via one of the following three pathways:  

  1. Contaminated mulch such as hay, straw, and grass clippings from crops on which the herbicide was used. 
  2. Contaminated manure and soiled bedding from livestock such as cows, sheep, pigs and horses, that have eaten crops treated with the herbicide. 
  3. Contaminated composts that were made using above inputs. 

Once the herbicide is taken up by susceptible plants, it moves systemically to growing tissues, deregulating metabolic pathways, causing uneven cell division and growth. Plants affected do not produce well, if at all, and may die.  

Many of these herbicides are very persistent in the environment, are stable in water, don’t degrade in anaerobic environments, and are highly mobile – one study found picloram residues in an untreated site a kilometre away from the original application site, up to two years later. How to heal contaminated soil

The herbicides degrade in soil over time, and active cultivation and soil amendments may speed up this degradation.  

  • Get the soil microbiology teeming as a first priority. The most effective pathway of degradation of persistent herbicides is its decomposition by microbes.  
  • Cultivate the soil often to expose all parts to soil life, water, and sunlight (UV). 
  • Using activated carbon to bind to the herbicide in the soil has also been put forward as a solution. 
  • Mycoremediation, using beneficial fungi, is another option. Although little research exists, this is one of the methods I am trialling. 

Don’t compost affected plants

Thai pink egg showing stunted and shrivelled growth.
Thai pink egg showing stunted and shrivelled growth.

Don’t compost anything that comes from your affected garden, including manure from animals you fed from it. The herbicide chemicals bind strongly to plant material and neither plant and animal metabolising, or home composting, will degrade it sufficiently. Best bet is to bin it – in council refuseNOT green waste.Page Break 

My affected tomatoes, which were transplanted into clean soil, have recovered somewhat and are flowering and setting fruit, although they are not nearly as large or productive as normal. Tomato plants I left in tainted soil have not recovered, growing spindly and stunted, with only a handful setting fruit which are deformed and tasteless. Can you eat the produce?

There seems to be consensus that produce grown in soils containing low levels of these herbicides are fine for human consumption, although some reports also indicate possible health concerns, following observations in laboratory animals fed with moderate to high doses.  

By seeing this experience as a learning opportunity, and educating others about killer compost, I still managed to turn it into a season of abundance – I may not have the fresh produce, but my cup of knowledge overflows. 

Plants most likely to be affected 

  • Solanaceae, e.g. tomato, potato, chillies, etc. 
  • Fabaceae, e.g. beans, broad beans, peas, etc. 
  • Asteraceae, e.g. artichoke, dahlia, Jerusalem artichoke, lettuce, etc.  
  • Vitaceae – grapes 
  • Rosaceae, e.g. roses, loganberry, raspberry, etc.  
  • Umbelliferae, e.g. carrots, celery, parsley, etc. 

 Least susceptible plants 

  • Grass crops and plants in the brassica family are unaffected and can be grown in contaminated soil. 
  • Pumpkin and squashes are only slightly susceptible, and still produce well. 

Chemical culprits

These are the three pyradine herbicides most commonly used in NZ. For highly susceptible plants, such as tomatoes, toxic levels of these herbicides are 13 parts per billion – equivalent to about half a teaspoon of herbicide product in an Olympic-sized swimming pool. 

Chemical name 

(sold under various brand names) 

Level at which residues are toxic to non-target plants (parts per billion)  Half-life in soil (days) 
Aminopyralid  1–3 ppb   32–533  
Clopyralid  1–3 ppb  60–425  
Picloram  10 ppb  30–400  

 About the author

Minette Tonoli in her garden.

Minette Tonoli is an Earth Mother who is passionate about herbs, and loves to inspire and encourage others toward soulful gardening and the use of homegrown plants for food and healing. For more images of pyridine-affected plants, see her website: – direct link to the photo gallery:  


  1. The United States Composting Council. 2015. Understanding Persistent 
  2. The United Kingdom Waste and Resource Action Program. 2010. An Investigation of clopyralid and aminopyralid in commercial composting systems: 
  3. Beyond Pesticides factsheets on: 

 Further reading 


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