As sustainable gardeners we are probably relying quite a bit on commercially sourced compost, manures and mulch to help promote healthy plant growth. But do these products reliably do that? Over the last decade or so there have been problems worldwide with contamination with substances that harm seeds and plants that we want to grow and eat.

The problem

Image: University of California Division of Agriculture and Natural Resources
Shows leaf crinkling, leaf curling, leaf distortion, leaf malformation, stunting

In the 2000s, a few reports by market and home gardeners of mysterious plant damage emerged in the UK and USA. These included poor seed germination or deformed, wilting or twisting leaves. Plants affected included roses, grapes,sunflowers, potatoes, lettuces, tomatoes, spinach, some fruit, squash, hops and legumes (beans, peas, clover, lupins, acacia). In 2009, whole crops were lost in the US and hundreds of thousands of dollars’ worth of damage was done to community gardens and organic farms.1 The damage was traced to carryover of toxic substances from straw/hay mulches, composts and manures. And, of course, if from spray drift to fields or gardens. The culprits were one or more of a group of compounds that mimic naturally occurring plant hormones, called auxins, which regulate plant growth. They were pyridine carboxylic acids (pyridines) – aminopyralid2, clopyralid3, picloram, fluroxypyr or triclopyr, the active ingredients in broadleaf herbicides which affect the division of cells causing them to became disorganized and uncontrolled, eventually destroying plant tissue.

These substances are available in agricultural products, either alone or in combination, as selective herbicides to kill broadleaf and woody weeds such as docks, thistles and nettles, blackberries, gorse, hawthorn, lantana, dock, ragwort but allow other plants to grow. They have mostly been used in pastures, but also in crops of corn, wheat, barley, oats, wheat, triticale, canola, fallow land, in forests on golf courses, parks and on grass along highways. Since pyridines are growth inhibitors, they are only effective if sprayed on actively growing plants. Problems observed in plants grown by market and home gardeners were a result of carryover of these substances from composts, manure and mulch derived from sprayed land.

In Australia a number of years back, there have been reports of problems in home gardens.  Agricultural products currently containing these pyridines include Tordon, Grazon, Stinger, Tri-Pick, ForageMax, Hotshot, Starane, Spearhead and Vigilant4.

How do these substances get into edible plants?

Under the right conditions pyridines persist in sprayed pasture and crops and in soil for a long time. The half life (i.e. time taken for half the amount to be broken down) of aminopyralid 32 – 533 days, very commonly around 103 days and it is stable in water (see Chemwatch aminopyralid.pdf). Breakdown is largely by microbial action. Clopyralid’s half life in soil is 1 – 2 months but can extend to 1 year depending on conditions3. Picloram has a half life of about 2 months in heavy clay soil but if organic material is present in the soil, its half life can be much longer. Again, breakdown is by soil microorganisms and occurs slowly. These substances are quickly taken up by plants via their roots, a little via their leaves and they easily translocate through the rest of the plant. The other two compounds, trichlopyr and fluoroxypyr have half lives of around 40 days in soil and around 1 day in water.

These substances are resistant to digestion by animals grazing on sprayed pastures so can be found in manures – and in concentrations high enough to damage plants that are sensitive to them. They are also resistant to the high temperatures of commercial composting.  And susceptible plants need only minute concentrations to be affected.

Once the problem was recognized, Dow AgroSciences, the main manufacturer, suspended production of the offending products for a while. Now, however, the substances are still in use but with labelling and warnings in information accompanying the products.

Warnings to Prevent Problems

Statements, which vary with the product, include “do not plant crops for up to 9 months after spraying”, “Do not send treated crops off-farm as fodder or forage.” For example, the statement on ForageMax (aminopyralid) is:
Do not send treated crops of farm as fodder or forage. Aminopyralid residues from treated plants may pass into animal manure. If the manure is used to make compost or spread around plants it may cause injury to sensitive plants. Do not spread manure from animals that have grazed or consumed forage or fodder from treated areas on land used for growing susceptible broadleaf crops.

Dairy and feed pad effluent
Effluent from animals grazing forage brassicas treated with ForageMax within the last 4 weeks may contain residues. Effluent from these animals may contain residues for 3 days after removal of the animals from the ForageMax treated crops. Disposal of this effluent by irrigation may cause damage to clover and other sensitive crops during this 3 day period. Do not send any effluent (or compost made from it) off-farm, from animals that have grazed on crops treated with ForageMax within the last 4 weeks, until the animals have grazed for 4 days on clean feed. This restriction is not required if 4 weeks has elapsed from treatment to grazing or the animals have been on clean feed for at least 3 days.5

Can warnings on product labels and instructions for use prevent problems?

Ideally “yes” – if users read and obey them. However, some products have brief and confusing statements – one saying that there is no withholding period for grazing, another (Tri-Pick) “check the label for the withholding period for these crops” and another states “at least 7 days”.

As the United States Composting Council wrote in 2013 (current publication date is May 2016)1 “Instructions on labels often appear complicated, they may not be read completely, or if they are, are not fully understood or not followed accurately. Though some applicators might follow instructions correctly, there are usually others downstream who receive treated residues and may be unaware of the initial labeling requirements. Others may be aware of labeling requirements but choose to ignore them”

As a result, they call for more action: “The US EPA (Environment Protection Authority) should revoke the registration of all herbicides known to persist in compost at levels that are toxic to plants and require that these products be removed from the market.” Currently, the US EPA indicates that a review started in 20146 but, as far as we are able to determine, no result has yet been forthcoming.

Pyridines in Herbicides for Garden Use

Unfortunately, pyridine compounds are also present in some garden products, so if you are using them (most sustainable gardeners don’t) watch out!

They are mostly called “blackberry and tree killer” or “blackberry killer”. Picloram and triclopyr (the less problematic chemical) are in Superway Tri-Pick. A number of other products contain triclopyr. Warnings about their use vary and include “The estimated half-life in above ground drying foliage as in a forest overstory is 2 to 3 months”, “Insufficient data to be sure of”, “Do not allow spray to get on to plants wanted”. One Tree Blackberry Killer warns not to let spray touch non-target plants.

Should we be cautious?

The warnings on agricultural products are certainly reducing the frequency of pyridine carryover in compost, mulch and manures and, therefore, subsequent plant damage. However, these compounds are not routinely tested for in such products and we don’t have information on whether they are still likely to be present.  But it would be wise when buying these products, to do one or more of the following:
– For mulches and composts, ask your supplier about the source, what sprays had been used, withholding periods etc
– For manures ask what food the animals had eaten.  If grazed on pasture, ask about sprays and withholding periods. Bear in mind the a lot of cow manure comes from feed lots and it is not easy to know what the cattle had been fed.
– Use only lucerne or pea straw as mulch since the growers of such legume crops would not use pyridine herbicides because they would kill their crop
– Buy only organic-certified products
– Make your own compost and mulch
– Get your own chickens to provide manure, but check what you feed them
– Avoid blackberry and tree killers containing pyridines.