Mushroom Grain Spawn
Grain spawn is colonised grain used to seed bulk substrate - the multiplication step that turns a few cc of culture into enough living mycelium to run a tub. Preparing it yourself means hydrating grain, loading filter-patch bags and sterilizing at 15 PSI for 90-120 minutes. Buying it pre-sterilized removes the pressure cooker from the equation entirely, which for most growers is the difference between doing this and not.
Grain types
| Grain | Character |
|---|---|
| Rye berries | The standard. Good moisture retention, colonises evenly, widely available |
| Red milo / sorghum | Smaller kernels, so more inoculation points per pound - fast colonisation |
| Wheat / birdseed | Cheapest, fussier about hydration |
Working with grain spawn
- Sterilization is mandatory. Grain is nutrient-dense and endospore-forming bacteria survive boiling. 15 PSI for 90-120 minutes, no shortcuts.
- Inject 3-5 cc per bag through the port, after alcohol-wiping it and flame-sterilizing the needle.
- Colonise at 75-80°F in the dark. Shake the bag gently at roughly 30% colonisation to spread growth points - it can halve total colonisation time.
- Wait for 100% before spawning to bulk, then give it a few extra days to consolidate. Rushing this is the most common cause of a failed tub.
- Spawn at 1:2 to 1:5 into bulk substrate.
What to skip
- Sterilizing grain in an atmospheric steamer or stockpot. It will not reach the temperature that kills endospores, and the bag will contaminate days later.
- Shaking a bag showing any hint of contamination - shaking spreads it through the entire bag.
- Overhydrating. Wet, glossy grain pools water at the bottom of the bag and sours.
Frequently asked questions
What is grain spawn?
Sterilized grain colonised with mushroom mycelium, used to inoculate bulk substrate. It is the multiplication step between a culture and a fruiting tub.
How long does grain spawn take to colonise?
Two to four weeks at 75-80°F, or as little as 10-14 days if you shake at around 30% colonisation to spread the growth points.
Can I make grain spawn without a pressure cooker?
Not reliably. Grain requires 15 PSI for 90-120 minutes to kill bacterial endospores. Pre-sterilized grain bags exist precisely so you can skip that.
Rye or milo?
Rye is the default and the most forgiving. Milo and sorghum have smaller kernels, giving more inoculation points per pound and faster colonisation.
