City of Toronto compost – does it contain garlic mustard?


I have two bags if compost that u picked up at a city environment day last fall Recently someone e told me that city compost is contaminated with garlic mustard. Do you have any knowledge and advice about this? Hesitant to use it . Thanks!


I contacted the City’s Solid Waste Management Services (SWMS), specifically to ask if seeds of invasive plants like garlic mustard would be eliminated by the City’s composting process.

The bottom line is that, although the SWMS representative I spoke with is not aware of complaints concerning contamination of their compost with seeds of garlic mustard or other invasive plants, the City (understandably) cannot guarantee that these seeds or other weed seeds are not present.

If you do find invasive species growing in the composted soil, consider too that there are several ways that seeds could have arrived in your garden, after the composting process. They could be transferred into the soil from parent plants by wind, birds, squirrels or other means. As is the case for all our gardens, continue to monitor for invasive species and dispose of them appropriately (see below).

Background information

No invasive species allowed in yard waste:

The City of Toronto does not accept invasive plants as yard waste – these are required to be placed inside a tied plastic bag and set out as garbage. Examples of invasive plants include:

  • Giant hogweed
  • Dog-strangling vine
  • Garlic mustard
  • Manitoba maple
  • Norway maple
  • Buckthorn
  • Japanese knotweed

See City of Toronto. Yard waste.

City Composting

A temperature of 60 degrees C (140 deg F) is needed to kill garlic mustard seeds, so backyard composting clearly won’t get rid of this invasive species.   See Northwest Michigan invasive species network. Garlic mustard disposal options. (This resource does not advise how long the material needs to remain at this high temperature). See Texas A&M Agrilife Extension. Composting to kill weed seeds.  This article discusses the time it takes for some seeds (alas, not garlic mustard) to be destroyed at certain temperatures – however, the examples are helpful.

The City collects garden waste (e.g., in brown paper bags) and brings it to a transfer station, where it is put in a dedicated yard waste bunker.  The City contracts with third party processors, who grind the material and use “windrow composting” or “in-vessel composting” systems to process the waste.  The SWMS indicated that for the most part, the waste should reach temperatures of at least 60 degrees C during processing, and remain at that temperature for multiple days.  The yard waste processors are required to monitor temperatures regularly, although they are not required to test for seeds (e.g., of invasive species) in the finished compost product.

As noted above, it is not possible for the City to guarantee that the compost they distribute is free of the seeds of invasive species or other weeds.  However, the compost is tested against Ontario Compost Quality Standards. And the good news is that the City’s SWMS representative is not aware of any complaints concerning City compost contaminated with garlic mustard or other invasive plant seeds.

For additional information, see: Invasive Species Centre, Ontario Invasive Plant Council, Early Detection & Rapid Response Network Ontario’s The disposal of invasive plants in the Greater Toronto Area.

If you are still hesitant to use the City compost, consider applying it in a small area of your garden, to see if any weeds or other undesirables are growing more than in other spots in the garden. You may also want to ask your friend where they heard about the City compost being contaminated with garlic mustard – it would be helpful to know the source of this information.

Finally, if you would like further information, please get back to us and we can provide you with the contact information for the SWMS team member who looked into this matter for me.