Last October I updated the warning about pyridine herbicides that can damage or kill both food crops and decorative plants. Unfortunately, some readers have since had plant damage after inadvertently purchasing manures or mulch that contain one of these herbicides, despite a NSW government website stating that no damage has occurred in Australia.
As a result, I am posting a reminder.
Pyridine herbicides are only effective on broad-leaf plants, but the chemicals remain active in mulch cut from sprayed pastures and in manure from animals that have grazed on sprayed pastures until the chemicals are broken down by soil microbes. Of particular concern to home gardeners and councils that recycle waste into compost for agricultural and domestic use are the products containing aminopyralid, clopyralid and picloram because they are quite persistent, and residue from these herbicides can damage plants for up to 24 months. However, because the Australian Pesticides and Veterinary Medicines Authority (APVMA) regards the person who sprays the herbicide as the ‘end user of the product’, any warnings are limited to product labels without any regard for unsuspecting gardeners who, in good faith, purchase mulch, compost or manures contaminated with the herbicide, and who may not recognise the cause of the damage to their crops because they have not personally used any herbicides.
A recent check of their website shows that the APVMA has registered 233 herbicides that contain at least one of the pyridine group of herbicides – an impossible list to check through before purchasing mulches, manures or compost. The entire tomato family, lettuces, sunflowers, spinach, strawberries and legumes are particularly susceptible to damage from these herbicides, which can also affect a range of ornamental plants.
To protect your garden from pyridine herbicide damage: only use aerobically composted manures on gardens. Aerobic composting requires weekly turning or stirring to ensure the composting process is carried out by microbes that require oxygen. Breakdown of the herbicide will be very slow in compost heaps that are not aerated.
Mulch that carries an organic-registered label does NOT contain any herbicides. Mulches from uncertified sources are high-risk products because the drying and baling of mulch materials eliminates microbial action, and the herbicide will still be active. The only safe compost to purchase is organic-registered compost.

If you are unable to purchase certified-organic manures or mulch, test the safety of the product by sowing some seasonally suitable peas or beans in pots containing certified-organic potting mix (with the manure mixed through it) or covered with the purchased mulch. (Water this pot through the mulch). Keep the test pots well-watered to eliminate other sources of stress. You should be able to see if an input is contaminated within 21-28 days. Dispose of any affected plants and potting mix with household garbage.
Symptoms to look for are:
Poor germination or death of seedlings, twisted, cupped or elongated leaves and twisted growth, misshapen pods.

Return remaining contaminated inputs to your supplier. If this is not an option, aerobic composting is the quickest way to break down these herbicides. Test the mature compost for herbicide residue.
If you find that the mulch has been affected, use it on beds that you can leave fallow until aerobic microbes in topsoil break down the herbicide or, if space is limited, compost it aerobically. if you find that garden beds have been affected, dig organic-registered compost through the bed and keep it damp to keep soil microorganisms breaking down the herbicide as quickly as possible.
Notify your supplier of the problem as pyridine herbicide product labels state that treated crops are not to be used for hay, silage or animal bedding, and manures are not to be spread on land used for growing susceptible crops.

Please also take a minute or two to notify the APVMA of problems with these herbicides. The APVMA encourage the public to report pesticide problems through their new Adverse Experience Reporting Program (AERP) by e-mail: aerp@apvma.gov.au, phone 1800 700 583, or fax: 612 6210 4813.

Further information:
Examples of pyridine herbicide damage

You can find Australian product names of these herbicides by going to the APVMA’s Public information (PUBCRIS) page. Under product type select ‘herbicide’, then type aminopyralid, clopyralid or picloram in the active constituent panel. Click ‘Search’.

The NSW Government has been aware of the problems with these herbicides in Australia since 2005: see  Organic