Fungal Matters – Cultivating Mushrooms in Coffee Grounds

Sarah Courchesne on June 30th, 2019

The comings and goings of mushrooms are inscrutable. At least to the untrained eye, which mine is. There was the tuft of alcohol inkies that appeared overnight in the middle of the yard one year and then melted away and never came back; the batch of cheery yellow parasol mushrooms in the pot of a houseplant I was nursing back to health; the elfin toadstools that sprouted out of the sill of my shower in a rented apartment one time. Mushrooms make plain how humans got the idea of spontaneous generation — that rotting meat generates flies of its own accord, or that swamps produce clouds of mosquitoes out of miasma. Mushrooms appear to come out of nowhere, from no source, and suddenly. 

Being visual creatures, humans see a mushroom and equate it with the fungus itself. This is a backward view, though an understandable one given that they are conspicuous and, in the case of Pleurotus ostreatus (the species usually meant when we say oyster mushroom), delicious. But mushrooms are simply an appendage of a much larger organism, a temporary reproductive organ employed to deploy spores and then dismantled by rot. Just as you have to tend a tree to get the apples, you have to tend the whole fungus to get the mushrooms. Mushrooms seem to pop up out of nowhere because the body they belong to is usually underground or inconspicuous — a mat of tendrils nosing through the dark. For those hoping to grow their own mushrooms at home, understanding the nature of Pleurotus holistically will make the work much easier. 

If you read a bit about cultivating your own oyster mushrooms, you will see a great deal of optimistic cheerleading. “Easy to grow!” “So simple!” “Enjoy fresh mushrooms with literally zero effort!” Like most such claims, it would be better to approach them with a skeptical attitude. I have taken in many a houseplant billed as “easy care!” or “impossible to kill!” and promptly killed it. It’s true that oyster mushrooms are willing to grow on a wide range of substrates and do not require extensive daily attention, but, as with impossible to kill houseplants, there are always ways to go wrong. Learning some basics about oyster mushrooms can help you avoid the particular pitfalls involved in growing them at home. 

Photo by Kevin Harkins.

Oyster mushrooms are fairly easy to find in the woods in just about any season, and, like many wild foraged or hunted foods, they tend to have a richer taste than domestically raised stock. At least, thus sayeth the foragers I listen to; my palate is never sensitive enough to detect such differences. I am pleased when I can tell that leftovers have gone bad, and sometimes I fail at that and eat them anyway. In any case, there are other substantial benefits to home growing mushrooms: no potential to fatally confuse an edible species with a toxic one, plus the convenience of countertop freshness and accessibility, rather than an uncertain walk in the woodlands or the disappointment of a grocery store-bought carton of mushrooms found slimy and moldering in the fridge drawer on a frazzled work night. 

Oyster mushrooms can grow on many things — straw, sawdust, an old hardwood log — but it turns out that used coffee grounds are particularly well suited to the task of hosting them. The reason lies mainly in the suppression of competition. The thin filaments of the fungal body, called hyphae, and referred to as “spawn” by fungus farmers, can be outcompeted by common molds in the environment. Substrates like straw have to be pasteurized before adding the spawn. The beauty of using used coffee grounds is that the high heat of brewing effectively pasteurizes the grounds, killing off any competitor organisms and offering a clean bed for the hyphae to grow through. The setup is time sensitive, and the grounds will be swiftly colonized by undesirable molds if not inoculated with mushroom spawn within 24 hours of brewing. You can put your grounds into a purpose-built bag from a kit online, or you can make your own out of a bucket or other plastic container with holes drilled into the sides.

Oyster mushrooms have preferred moisture and light levels, optimal spawn-to-substrate ratios, ideal air circulation patterns. Their “easy to grow” nature depends on these requirements being met, but a careful reading of the instructions provided by the more reputable online mushroom spawn purveyors should keep you on the right side of things. Growing mushrooms in coffee grounds is a bit like having an ant farm. The hyphae burrow through the earthy grounds, and the mushrooms, once they appear, grow fast enough to almost watch. Less mobile than ants, true, but hovering where fungi do, somewhere between animal and vegetable, not less animate, and certainly as captivating.  

Photo by Kevin Harkins.

Elizabeth Almeida, (above) farmer and owner of Fat Moon Mushrooms, is a grower of gourmet, organic mushrooms for restaurants and boutique grocery stores in the Merrimack Valley region of Massachusetts and New Hampshire. She shared five tips for The Bean Magazine’s readers on growing oyster mushrooms. 

1. Humidity is critical. Spritz growing mushrooms with water several times a day.

2. Fresh air is also important. Keep your grow kit in a place where the mushrooms can breathe.

3. Mushrooms double in size daily. Harvest when they stop growing and the edges uncurl.

4. Double your yield! After harvesting the first batch, let your block rest for a week. Soak it in cold water for 4 to 6 hours and start the process over. 

5. It’s best to eat oyster mushrooms immediately after harvesting, but if you need to store them in the refrigerator, put them in a brown paper bag so they can get the oxygen they need to stay fresh. 

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