Spawn to bulk

Grain is rich food that contaminants love, so it colonizes fast but fruits poorly on its own. Bulk substrateBulk substrateThe large, low-nutrient bed (usually coir-based) that colonized grain is mixed into so it can grow out and fruit. is the opposite: a low-nutrient, water-holding bed that most moulds find hard to live on. Mixing colonized grain into bulk lets the mycelium spread into a much bigger, wetter bed that can carry several flushes.

Spawn ratio

1:2 to 1:4 spawn to bulk by volume; 1:3 is a common default

Minimum spawn

At least about 10% of the total; around 25% colonizes in roughly a week (Bible, p. 193, 197)

Substrate depth

Even and level; 2–4 inches (5–10 cm) is typical for a monotub

Colonizing temperature

About 75–80 °F (24–27 °C), the same as jars

Days to colonize

Roughly 7–14 days, depending on spawn rate and temperature

Consolidation

A few more days after the top looks white before switching to fruiting

More spawn means faster colonization and less time for anything else to take hold. Less spawn stretches your grain further but leaves bare substrate exposed longer. Forum growers disagree on the best ratio, and both ends have produced good tubs. One controlled four-way comparison posted on Shroomery found 1:3 gave the best overall result, while the leanest mix led early but stopped pinning by the fourth flush. What everyone does agree on is that even distribution matters more than the exact number.

A spawn ratioSpawn ratioHow much colonized spawn goes in compared with bulk substrate, by volume, written spawn to bulk, such as 1:3. of 1:3 means one part grain to three parts hydrated bulk. Five quarts of spawn would go with roughly fifteen quarts of substrate.

1. 1.Wipe the tub, lid and your hands with isopropyl alcohol. Work in still air with doors and windows closed.
2. 2.Break the grain up inside the closed jar or bag first, by knocking it against something soft, so there are no clumps.
3. 3.Open the substrate and spawn only when you are ready, and work quickly.
4. 4.Either mix grain and substrate together evenly, or layer them: grain on the bottom, substrate, grain, ending with a thin layer of grain or a light dusting of substrate over the top (Bible, p. 200–203).
5. 5.Level the surface and press it gently so it is firm and flat. An even surface is what gives an even pinset later.
6. 6.Close the tub with holes covered (tape or filters) so it stays humid and dark while it colonizes.

Leave it alone. Opening the tub to look lets in spores and dries the surface. Keep it out of light, because light on a not-yet-finished surface encourages early pins along the walls (side pinsSide pinsPins that form against the container wall, down the sides of the substrate, instead of on top.). Expect white patches spreading out from each piece of grain within two or three days, joining up across the surface in one to two weeks.

Colonization keeps going inside the substrate for several days after the top turns white. Starting fruiting conditions the day the last bare spot closes is fine for fast strains, but most growers wait two to four days. Waiting much longer than that invites overlayOverlayA flat, leathery white mat that keeps thickening across the surface instead of forming pins., where the surface thickens into a mat instead of pinning.

If you use a casing layerCasing layerA thin, water-holding top layer with no food in it, spread over colonized substrate to keep the fruiting surface evenly moist., it goes on once the bulk is fully colonized, usually as a thin layer about a quarter of the substrate depth (Bible, p. 192). The casing gets its own guide under substrates and casing.