What is grain spawn?

Grain spawn is made by hydrating grain, sterilizing it, and inoculating it with a culture once it cools. GroCycle's method (GroCycle):

1. Soak the grain for 12 to 24 hours, then simmer it for 10 to 20 minutes.
2. Drain it and spread it on a tray for an hour or two, so the kernels dry on the outside.
3. Fill jars, leaving 1 to 2 inches of space at the top.
4. Sterilize for 90 minutes at 15 PSI, timing from when the cooker reaches pressure. Let the jars cool in the cooker for 8 hours or overnight.
5. In a still air box or in front of a flow hood, wipe the lid with alcohol, let it dry, and inject 1–2 mL of liquid culture per jar.
6. Keep the jars at room temperature, out of direct sunlight. Some growers break and shake them once they're a third colonized.

Full colonization takes 2 to 3 weeks, depending on the species, grain and conditions. For oysters, UF/IFAS Extension incubates at 23°C (73°F) in darkness and expects around 10 to 20 days (UF/IFAS). Our liquid culture syringes guide walks through each step.

No single grain is best for grain spawn. UF/IFAS lists millet, wheat, sorghum and rye, and says the choice "depends on availability, cost, and preference". It calls millet "an excellent substrate for producing oyster mushroom spawn" (UF/IFAS). GroCycle says you can use almost any grain, and each has pros and cons (GroCycle).

Grain spawn is ready when the grain is "completely covered with mycelium and uniformly white" (GroCycle). Some species fool the eye: North Spore's help center says lion's mane grows finer mycelium than oyster spawn (North Spore help center). Our pink oyster grain spawn guide judges faint species by bare kernels and a firm bag instead. A sour smell or green patches mean contamination; see our contamination ID guide.

One bag of grain spawn can seed about ten more. Fungi Perfecti says you can expand grain 10 times, then expand each second-generation bag 10 times, so "one bag can then make 100 bags of spawn" (Fungi Perfecti). Each transfer adds risk, and our pink oyster guide treats the third generation as the ceiling.

• Wet grain. Over-hydrated grain slows colonization and raises the chance of contamination (GroCycle).
• Short sterilization. Start the 90 minutes only once the cooker reaches 15 PSI (GroCycle). See sterilization vs pasteurization.
• Too much liquid. GroCycle suggests 1–2 mL of liquid culture per pound of grain (GroCycle). Extra liquid sits at the bottom of a bag, where bacteria thrive, warns Rootlab, which sells grain bags (Rootlab).
• Keeping it too long. North Spore's help center gives grain spawn three months in a refrigerator, and says pink oyster can't be refrigerated and is best used within a month (North Spore help center).

In Hypha, grain spawn gets break & shake reminders from typical timings, and you can adjust them for each block.