Tutorials/Spawn/Grain Spawn

Spawn is a nutritious material, usually grain, that has been fully colonized by mycelium. It's the engine of a grow: you mix it into bulk substrate, and the mycelium spreads from every kernel at once.

Grain needs to be sterilized, not just pasteurized. It's so nutritious that anything left alive in it will take over.

Choosing a grain

Grain	Pros	Cons
Whole oats	Cheap, easy to find (feed stores sell 50 lb bags), forgiving	Hulls can trap moisture
Rye berries	The classic. Holds water well, big kernels	Pricier, harder to find, easy to over-hydrate
Millet	Many small kernels mean many inoculation points and fast colonization	Small kernels clump if too wet
Wild bird seed (WBS)	Very cheap mixed seed	Quality varies; pick a mix without sunflower seeds or added oils
Brown rice / popcorn	Available at any grocery store	Rice gets mushy easily; popcorn has a hard hull

Preparation

1. Rinse

Rinse the grain in a bucket of water, stirring and dumping until the water runs mostly clear. This removes dust, chaff, and floating debris.

2. Soak (12–24 hours)

Cover with plenty of water and leave it at room temperature overnight. Besides hydrating the grain, soaking wakes up heat-resistant bacterial spores (endospores) so the pressure cooker can kill them. Skipping the soak is a common cause of bacterial contamination.

3. Simmer (10–15 minutes)

Drain, cover with fresh water, and simmer until the kernels are swollen and you can squish one between your fingers, but few are split open. Split kernels release starch and make sticky, bacteria-friendly clumps.

4. Dry the surface

Drain and spread the grain on a towel or screen for 30–60 minutes, until the outside of each kernel looks dry but the inside is plump. This is the most important step. Wet grain is the number one cause of failed grain jars.

A small amount of gypsum mixed in now helps keep kernels separate. It's optional.

5. Load the jars

Fill jars or bags about two-thirds to three-quarters full so there's room to shake. Use lids with an injection port and filter. Cover the lids with foil.

6. Pressure cook

Container	At sea level (15 psi)	Front Range (15 psi weight)
Quart jars	90 min	120–150 min
Grain bags, 3–5 lb	2.5 hr	3–3.5 hr

See pressure cooking at altitude for the reasoning. Let the cooker depressurize on its own, then let the jars cool to room temperature, ideally overnight.

Checking hydration

After cooking there should be no standing water in the bottom of the jar. Kernels should be plump and separate, not stuck together in a mass. If you see water pooled at the bottom, that jar has a much higher risk of bacteria.

Inoculating

In a still air box or flow hood:

• Liquid culture: 1–3 ml per quart jar, 5–10 ml per bag.
• Agar: 2–4 wedges per quart jar.
• Spores: 2–4 ml per quart jar. Spore-started jars are slower and riskier than LC or agar.

Incubating

• Keep jars at 70–80°F, in the dark. Darkness isn't critical, but light can encourage early pinning in the jar.
• You'll see the first white growth around the inoculation points in about 3–10 days.
• Shake when the jar is about 20–30% colonized: break up the grain and spread the colonized kernels throughout. This dramatically speeds up colonization. Some growers shake once more around 60%.
• A jar is ready when every kernel is wrapped in white. Give it a few more days after it looks finished so the mycelium consolidates.

Grain to grain (G2G)

A fully colonized jar can inoculate more sterilized grain. Roughly 1 colonized jar to 10 fresh jars works well. This stretches one good culture into a lot of spawn, but every transfer is a chance to spread contamination, so only do it from jars that are clearly clean.

What's next

Spawn goes into bulk substrate: coir and CVG or manure-based substrates, then on to fruiting.