Spawn to bulk

Spawn to bulk — S2B — is where colonised grain meets the substrate the mushrooms will actually fruit from. It takes ten minutes and it sets up everything that follows.

The logic: grain is nutritious enough to grow mycelium fast but too nutritious to leave exposed to room air. Bulk substrate is nutritionally poor enough to tolerate open handling, but too poor to colonise from a standing start. Combining them gives you fast colonisation of a large volume that contaminants find difficult.

Spawn rate

The ratio of spawn to bulk, by volume. It is the most consequential number in this stage.

Ratio	Colonisation	Use when
1:1	Very fast, 5–7 days	Recovering from repeated contamination
1:2	Fast, 7–10 days	Default for reliability
1:3	Moderate, 10–14 days	The usual compromise
1:4	Slower, 14–18 days	Economical, experienced growers
1:5+	Slow, high risk	Commercial scale with good conditions

Higher spawn rates are almost always the right answer for home growers. Grain is the expensive input, which is what tempts people toward 1:5, but a contaminated tub costs more than the grain saved. Colonisation speed is contamination resistance — the race is only lost when it takes long enough for something else to establish.

There is a calculator if you would rather not do the arithmetic.

Preparing the bulk

Whatever you are using — CVG, straw, supplemented sawdust — it must be:

• At field capacity. Squeeze hard: a few drops, not a stream, never nothing.
• Cooled to room temperature. Warm substrate kills mycelium.
• Treated appropriately. Pasteurised for coir and straw, sterilised for anything supplemented. See sterilisation vs pasteurisation.

Get the water right before anything else. Over-hydrated bulk is the direct cause of both Rhizopus and most bacterial problems at this stage.

Doing it

Work as cleanly as you reasonably can. This does not need to be sterile — bulk substrate is open to the air by design — but it should be tidy.

1. Wipe down the work surface and the outside of the tub with 70% isopropyl.
2. Wash your hands, or wear fresh gloves wiped with alcohol.
3. Break the grain up while still in the jar. Squeeze and shake until it falls apart into individual kernels.
4. Layer, don’t dump. Alternate spawn and substrate in two or three layers.
5. Mix thoroughly with your hands, then mix again. Even distribution is the entire point — a clump of grain in one corner colonises that corner and nothing else.
6. Level the surface and press down lightly. Good contact matters; compaction does not — you want it firm, not packed.
7. Cover. Lid on, or a layer of substrate over the top.

Ten minutes. Do not rush the mixing step; it is the one that matters.

Casing, or not

A casing layer — a thin, non-nutritious layer of moist coir or a coir/peat mix over the colonised surface — is applied after full colonisation, not at spawn time.

It holds moisture exactly where pins need it and produces more even pinning. It is close to essential for some species and optional for others. If your surface tends to dry and crust, or your pins keep aborting, a casing layer will probably help.

Common mistakes

Under-mixing. The most common. Grain in clumps produces patchy colonisation and leaves un-colonised substrate for contaminants.

Over-compacting. Mycelium needs air. Packing a tub down hard creates anaerobic zones where bacteria thrive.

Spawning a jar that isn’t ready. Partially colonised grain has less established mycelium and often carries the contamination that slowed it down.

Spawning into warm substrate. Pasteurised substrate needs to cool fully. Above about 95°F you will kill what you just added.

Too much free water. If water pools when you press the surface, it is too wet. Mix in dry material now — you will not get another chance.

Next

Sealed and mixed, the tub goes into colonisation.