Using Spent Mushroom Substrate in the Garden
Turning waste into abundance — locally.
Spent Mushroom Substrate (SMS) is what’s left after mushrooms are harvested from a growing medium. It could include a mix of straw, sawdust, manure, and fungal mycelium — rich, spongy, and full of organic matter.
Adding to hugel rows in our first season, 2020-2021
At the Grow More Good Garden, we’ve used it to help weave together the free and low-cost materials we can source right on site — city compost, green matter, aged manure, coffee grounds, and layers of leaves. It helps build structure, retain moisture, and feed the living network in the soil.
Why We Use It
Adds organic matter and improves soil texture
Still contains nutrients mushrooms didn’t use
Introduces beneficial fungi that support soil health
Stimulates root growth and helps break down complex carbons into plant-ready nutrients
In our garden (and personal home gardens), it’s become a sort of bridge material — blending the rougher inputs into something cohesive and ready to support new life.
How We’ve Applied It
As a layer in hugel rows and lasagna beds: it helps bulk them up and breaks down gradually, improving aeration and structure.
As mulch in summer: holds moisture and protects soil life through the heat.
As part of in-row composting: helps integrate green matter and manure right where plants grow.
In foliar experiments: we’ve tried small-scale teas and sprays, though I’ll admit, I could be a better scientist about it — I’m often trying multiple methods at once!
That’s yet another reason I’m excited to collaborate with area educators who are regularly engaging students in scientific inquiry. A symphony of innovation — with more trained ears than mine — helps us all grow smarter gardens.
Mary K, Garden Manager, Grow More Good Garden
A Few Tips & Cautions
If it smells strongly of ammonia, let it rest 1–2 months or mix it into an active compost pile to finish breaking down.
Always cover with mulch to prevent crusting or weed growth.
Mix with compost, manure, or leaf mold for best results.
Shredded leaves make an excellent pairing — maple, fruit trees, elm, and ash break down quickly and balance the mix.
Local Connection
The SMS we’ve used comes from Ferg’s Fabulous Fungi, a local grower in Caldwell who’s helped many of us discover how mushroom compost can play a role in regenerative soil-building.
Deliveries are being scheduled now for mid-October through mid-November.
If you’d like to try it in your own garden, you can:
Each order helps support your soil — and the work of the Treasure Valley Community Gardens Cooperative. 🌱
Ferg’s display of mushrooms at Boise Farmers Market during foraging season, early October!
If you’re gardening in a small space, you can just get one of his grow kits to try both the mushrooms and the SMS!