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Cultivating oyster mushrooms on spent coffee grounds is a simple and enjoyable home activity for all ages, resulting in some good edible mushrooms to boot. If your home brewing doesn’t provide enough grounds, try asking your local coffee shop or roaster if you can leave a bucket for them to toss their grounds into, especially if they would otherwise go into the trash. If you’re not able to inoculate your grounds with spawn right away, freeze them until you’re ready to do so; otherwise molds will form within days.
Although the yields you’ll get from this method are not as high as when you use commercial oyster mushroom formulas, such as pasteurized wheat straw or cotton waste, if you factor in the production costs, the lower-yield coffee grounds method becomes as economically viable as the more sophisticated cultivation. If you simply recycle your own grounds you can expect to produce a few pounds of beautiful oyster mushrooms a week—at which point you’ll need to create an oyster mushroom dressing, sautéing your harvest in a balsamic vinaigrette and tossing it over fresh greens crumbled with feta cheese. (Please note: although I have primarily used this process for cultivating oyster mushrooms, some European growers have successfully fruited parasols from coffee grounds.)
Step-by-Step Cultivation on Coffee Grounds
To begin, you’ll need a container with a lid, a steady supply of coffee grounds (with or without paper filters), and grain- or sawdust-based oyster mushroom spawn.
- Carefully collect the cooled and spent coffee filter, grounds and all, and place it into the container faceup. If using a press or strainer just add the grounds to your container once they are drained well.
- Massage your mushroom spawn bag to separate the grain or sawdust into individual bits to maximize the spreading capability.
- Sprinkle the mushroom spawn sparingly over the surface of the coffee grounds. You only need a small amount. Crack the container lid so it can breathe. The container can be located anywhere, such as a kitchen counter, garage, or any other space where there is indirect light, never direct sun.
- Add coffee grounds and filters daily, sprinkling spawn sparingly over each layer as you add more. After just a few days, mycelium will start to be visible as white threads growing together.
- Fill the container almost to the top, leaving just a few inches of space to make room for developing mushrooms. When you stop adding filters and coffee, the mycelium will finish colonizing.
- Once the container is completely colonized, expose it to diffuse natural or fluorescent light at room temperature. (If it gets direct sunlight the mycelium and mushrooms will dry up and you won’t get a harvest!) Keep the surface misted lightly and the lid just cracked, to preserve moisture. If you have filled a 5-gallon bucket or similar large container, you can drill 1/2-inch holes around the sides, every 10 inches or so, where the mushrooms can also emerge, but you will need to either mist the holes several times a day indoors or cover the container with a large, clear bag to make a humidity tent until the primordia have safely emerged and are no longer at risk of drying out.
- Two to three weeks after the colonization is complete, mushrooms should begin to form. Remember that mushrooms only form when they run out of food or space, at which point they recharge their battery and fruit. Baby mushrooms will appear overnight, so check your buckets at least once a day and keep the surface misted, though not underwater. The mushrooms should double in size every day. Harvest them when the fruitbodies’ growth slows. You may notice a powdery spore deposit forming underneath the caps when they are ready to harvest.
- After you’ve harvested the mushrooms, allow the mycelium to rest by not watering or adding any additional growing media, and it may fruit again in a few weeks. During the rest period no light is needed if you need to move the container. Soaking the coffee grounds with a generous amount of water after a few weeks of resting can help shock the mycelium into fruiting more prolifically. Once rehydrated, the biomass will respond with additional fruitings.
- After the second flush, your coffee grounds substrate will be pretty much spent as a mushroom growing medium. However, being full of fungal life, it has now become a living compost starter and can be mixed into your outdoor compost pile to help with the decomposition, or you can use it to inoculate cardboard cultures. Worms also love this spent media, so adding the grounds to your vermicomposting bin could possibly start a worm revolution.
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The following excerpt is from Tradd Cotter’s book Organic Mushroom Farming and Mycoremediation: Simple to Advanced and Experimental Techniques for Indoor and Outdoor Cultivation (Chelsea Green Publishing, 2014) and is reprinted with permission from the publisher.