Should we bring our invasive (or noxious) weeds to our local compost facility? What does the science and current regulations tell us about whether noxious weeds can be composted? Many have asked this question in the last few years as there is more concern regarding the spread of noxious weeds. Evidence suggests that most invasive weed species, including seeds, rhizomes and other plant parts can be successfully composted at temperatures at or above 55 C. We recommend that our regulations, particularly for yard waste composting, should reflect this so that we can potentially control invasive weeds via composting.
In British Columbia, composting facilities processing yard waste only, do not require high enough temperatures to reduce potential weeds or plant pathogens. The emphasis of the Organic Matter Recycling Regulation appeared to address primarily potential human pathogens, rather than plant pathogens and weeds.
The British Compost Specifications, PAS100-2011 (BSI 2011) addresses the potential adverse effects on plant health due to plant pests, pathogens or toxins, as well as the introduction of or increase in weeds seeds or propagules resulting from the use of compost. To reduce the risk of plant pathogens, weed seeds or propagules, the entire mass of composting material must be composted for a minimum of 7 days at a temperature of 65 C or higher. There are specific testing requirements for the finished compost where the tolerance for weed seeds or propagules is zero.
The PAS 100-2011 compost specifications was in part based on the results of bench and commercial scale testing of 60 plant pathogens or nematodes (Noble et al. 2004). It appears that the PAS 100-2011 allows composting of all noxious weeds except the Japanese Knotweed. The Organics Recycling Group (2013) reports that the UK PAS 100-2011 compost specifications does not allow Japanese Knotweed to be composted.
The Soil Association (2003) stated that destruction of most weed propagules occurs when composting temperatures reach 55-75C. They also reported that under controlled composting conditions, Japanese Knotweed rhizome (crown and runners) did not regenerate if exposed to temperatures of 55 C or greater for one week or more (see also Xian et al., undated). Day et al. (2009) reported that composting for more than 3 days at temperatures greater than 55 C effectively killed growth of roots and crowns of Japanese knotweed.
In order to kill weed seeds, Wiese et al. (1998) reported that all of the six common weed species that they studied were killed in a 3 day composting process at temperatures of 72 C, whereas field bindweed required 12 days. During windrow composting of beef cattle feedlot manure, Larney and Blackshaw (2003) reported that more than 70 days of composting was required to kill seeds of the five weeds tested, and that temperature variation in the windrow may have been a significant factor in longer composting times required.
Washington State suggests that some city composting facilities may be hot enough to effectively kill noxious weeds, but that home composting is ineffective (Noxious Weed Control Board 2011).
In summary, it appears that an adequate composting process (all of the material being composted at temperatures greater than 55 C for a minimum of 7 days) will be suitable to control invasive or noxious weeds. It is also a more sustainable management strategy than burning or burying. For composting facilities in British Columbia that are meeting the OMRR requirements for yard waste only (lower temperature requirements), the temperatures may not be high enough to kill potentially invasive weeds.
We recommend that the Organic Matter Recycling Regulation be updated to require all organic material be composted at temperatures greater than 55 C to ensure destruction of noxious weeds, weed seeds and plant pathogens. Another option is that local governments can require more stringent and controlled composting requirement for yard waste to ensure that the composting process meets the higher temperatures consistently for all the material being composted. A germination test of the finished compost will also assist in ensuring adequate weed destruction such as required in the UK PAS 100 specifications.
BSI 2011. PAS100:2011 Specification for Composted Materials. British Standards Institute, January 2011.
Composting Association. 2003. Information Sheet 15. Composting Noxious Weeds.
Day, L., J. Rall, S. McIntyre and C. Terrance. 2009. Japanese knotweed composting feasibility study, Delaware County, NY. http://er.uwpress.org/content/27/4/377.refs
Larney, F.J. and R. E. Blackshaw. 2003. Weed seed viability in composted beef cattle feedlot manure. Journal of Environmental Quality 32: 1105-1113.
Noble, R., P.W. Jones, E. Coventry, S.R. Roberts, M. Martin and C. Alabouvette. 2004. Investigation of the Effect of the Composting Process on Particular Plant, Animal and Human Pathogens known to be of Concern for High Quality End-Uses. Warwick HRI (Wellesbourne), BBSRC Institute for Animal Health (Compton) and UMR INRA-Université de Bourgogne (France). Published by the Waste & Resources Action Programme, Banbury, December 2004, ISBN 1-84405-141-2.
Noxious Weed Control Board. 2011. Noxious weed disposal – what to do with noxious weeds. Washington State Noxious Weed Control Board pamphlet.
Organics Recycling Group. 2013. Composting Noxious Weeds Information Sheet. Issued February 20, 2013 Issue 1, Revision 1.
Wiese, A.F., J.M. Sweeten, B.W. Bean, C.D. Salisbury and E.W. Chenault. 1998. High temperature composting of cattle feedlot manure kills weed seed. Applied Engineering in Agriculture, 14: 377-380.
Xian, C., P. Bardos, S. Robinson. undated. Can composting kill Japanese Knotweed. http://www.organics-recycling.org.uk/uploads/article2149/Can%20composting%20kill%20Japanese%20Knotweed%20Version%202.pdf