Can I add herbicides to the HOTBIN heap?

6 February 2014  |  HOTBIN Composting

The golden rule is ‘read the manufacturers label’ – if it says no, then no. If it says compost made from grass treated with ABC needs to left 6 months before use -leave it 6 months.

These two specific chemicals are currently not on sale in the UK. Aminopyralid and Clopyralid are both selective herbicides manufactured by Dow AgroSciences for control of broad leaf weeds on grasslands. They were supplied mainly to professional users (eg farmers etc), but were also added to some lawn care products depending on the country.

Both chemicals are reported to survive a reasonable length of time (2 years, dependent on conditions) and this long duration was an advantage as less spraying was needed. Animals eat the sprayed grass/hay and the chemicals pass through into their manure. Issues arose when several growers of sensitive broad leaf (eg tomatoes, potato, peas) used ‘immature, non-composted’ horse and grass based compost. Both chemicals break down in composting (see Clopyralid Report.pdf , the problem arises in that even at very low concentrations they affect certain sensitive plants. Here is an extract from the WRAP report

“The predominant factor with clopyralid (and similar chemicals) in compost is not its rate of decomposition. In general, clopyralid seems to decompose fairly well in the typical time frame of a composting process (Australian studies apparently suggest otherwise). The dominant factor is concentration. Clopyralid (and like chemicals) simply remain potent at very low concentrations, even after the original concentration decreases substantially during composting (e.g. 99%)."

"The second dominant factor is the ultimate use of the compost. Clopyralid remains potent for only a few families of plants; which unfortunately include important and popular food and landscape crops. High concentrations of clopyralid in compost will go unnoticed for the many uses/plants for which clopyralid is quite acceptable. Hence, clopyralid in compost is a story about concentration and coincidence: Concentration because of clopyralid potency at very low concentrations (and that of like products); coincidence because tainted compost must coincide with particular uses to become a problem.

Another important coincidence is that it takes a somewhat unlikely combination of similar feed stocks, similarly treated with clopyralid, to establish a troublesome clopyralid concentration to begin with. In most composting situations, the feed stocks are diverse and/or come from dispersed sources that do not have the same patterns of pesticide use. Important exceptions to this typical situation occur exemplified by the experience in Spokane, Washington State and by the use of 'single-source' feed stocks (e.g. manure obtained from one or two farms)."

Neither chemical is currently on sale in the UK. We go back to the basic rule – follow the manufactures guidelines.

Read more: Can bacteria break down pesticides and herbicides?