The best mulch ever: Spent Mushroom Substrate

Spent mushroom substrate (SMS) is waste every mushroom farm produces. It is fully colonized substrate, in our case a mix of coffee grounds and oat husks, that has given from 2-4 flushes of mushrooms and is no longer economically viable to keep in the fruiting room. After every harvest the substrate that has given their last mushrooms inside is taken out. Some growers throw it on a big pile and compost it, some leave it in their plastic bag or container and stack it up (that’s us).

If the substrate is not put into a proper composting process the mushroom grower usually does not want the spent substrate to be around their farm for long periods of time. The reason is that the mushroom substrate can attract fungus gnats and parasitic fungi like Trichoderma, which are the number 1 enemies of any mushroom grower (if they spread around the incubation room, it can contaminate and ruin the whole production).

Spent substrate makes a great fungal-dominated compost and many growers have built a community of gardeners around their farm that happily pick up this material, and sometimes even pay for it. There are also studies on other uses for spent mushroom substrate (Oei et al. 2007), like using it for animal feed (including insect feed), vermicomposting, or even extracting enzymes like chitosan, which can be used in the cosmetics industry.

Unfortunately we do not (yet) have a lab and big research budget to investigate these very promising uses for spent mushroom substrate, but we do need to get rid of it. That’s why we are now trying to sell our spent substrate as a dual purpose mulch or ”mushroom bed”. Besides giving the soil a nice living blanket to protect and feed the soil organisms, it also produces mushrooms! Multiple gardeners in the Helsinki Metropolitan Area have applied it in their gardens and around their plants and especially those who water the substrate regularly have gotten great harvests.

Some insights we gathered while testing this out are:

  • Put the spent substrate straight on the soil. Don’t put cardboard or fabric under it as this reduces the breathability between the substrate and the soil below.
  • Make sure the spent substrate is laid down at least 5 cm thick. The thicker the more mushrooms you get.
  • Make sure the spent substrate is in a shady place or add a thin layer of leaves, bark, wood chips or landscape fabric on top of the bed. This ensures more mushrooms will not dry out during the growing phase.
  • Water regularly.

We hope this inspires you to try to find some spent mushroom substrate in your local area and feed your soil and yourself with it!