Grow on used coffee grounds — zero-waste substrate

Fresh spent coffee grounds are already pasteurized by the brewing process and provide a nutrient-rich substrate for oyster mushrooms. Most coffee shops give away spent grounds for free.

Steps

1. Collect fresh grounds: Ask a local coffee shop for that day's spent grounds. Use within 24 hours — older grounds grow mold. You need about 5 lbs of grounds per grow bag.
2. Mix with spawn: In a clean bucket, break up the grounds and mix in 10-20% grain spawn by weight (0.5-1 lb spawn per 5 lbs grounds). Mix thoroughly with clean hands.
3. Pack into a grow bag or container: Use a filter-patch grow bag, or punch small holes in a zip-lock freezer bag. Pack firmly, removing air pockets.
4. Incubate 14-21 days in a dark spot at 65-75°F. The mycelium will turn the entire block white.
5. Fruit: Cut a 2-inch X in the bag. Mist 3-4 times daily. Harvest when caps flatten, before spores drop.

Why It Works

Coffee grounds have the right carbon-to-nitrogen ratio for oyster mushrooms and are pre-pasteurized by hot water extraction. The caffeine does not inhibit mycelial growth in oyster species.

Tips

• Coffee-only substrate is prone to contamination — mixing 50/50 with pasteurized straw dramatically improves success rates
• Grounds from drip coffee are better than espresso pucks (less compacted, better airflow)
• This method produces smaller yields than straw-based methods but the substrate is free