Tutorials/Fruiting/Monotub

A monotub is a plastic storage tote converted into a self-contained fruiting chamber. You mix spawn and substrate right in the tub, let it colonize, then open it up just enough to trigger mushrooms. It's the most popular method for good reason: cheap, simple, and productive.

Building the tub

• Tote: a 54–66 qt clear or translucent tote with a tight lid. Clear sides let you watch colonization and let light in.
• Holes: drill or melt 4–6 holes about 2 in. wide: two on each long side a few inches above where the substrate surface will be, and one or two on each short end. Cover them with filter material (micropore tape, polyfill stuffing, or filter patches).
• Liner (optional): a clear liner or a black trash bag around the bottom section, below the substrate surface, keeps pins from forming on the sides.
• Wash the tub with soap and water and wipe it down with alcohol before use.

Spawning to bulk

1. Break up your fully colonized grain spawn while it's still in the jar or bag.
2. Mix spawn and pasteurized substrate together at 1:1 to 1:4 by volume. More spawn means faster, safer colonization.
3. Level the mix to about 3–5 in. deep. Press lightly to make the surface even, but don't pack it down.
4. Many growers save a little substrate (no spawn) for a thin top layer about ¼ in. thick. It keeps the surface even and holds moisture.

Colonization

• Close the lid. Tape over the holes, or leave them filtered; either way, keep the tub mostly sealed so humidity and CO₂ stay high.
• Keep it at 70–80°F, out of direct light.
• Don't open it to peek. The sides show you everything.
• Colonization takes about 7–14 days. You're waiting until the surface is about 70–90% covered in white.

Fruiting conditions

When the surface is mostly white, switch the tub into fruiting mode. Pins form in response to four changes:

Factor	What to do	Why
Fresh air (FAE)	Untape the holes. Fan the tub by lifting the lid briefly once or twice a day.	Mushrooms pin when CO₂ drops. High CO₂ gives long, thin, leggy stems.
Humidity	Mist the walls and lid, not the substrate directly, so the surface stays damp.	Pins dry out and abort in dry air.
Light	12 hours on, 12 off, of indirect light. Room light or a cheap LED is plenty.	Light tells the mycelium which way is up.
Temperature	Room temperature, roughly 70–75°F. A slight drop from colonization helps.

Surface conditions

The goal is a surface that is damp but not wet. Look for tiny beads of moisture on the mycelium, like dew. Puddles mean too wet; a dull, dry surface means too dry. Getting this right matters more than any single setting.

Fruiting in dry Colorado air

Indoor humidity here is often 10–25%, much drier than the places most online guides are written. Tubs lose moisture fast. Things that help:

• Start with fewer open holes and add more if you see leggy growth or fuzzy feet (signs of too much CO₂).
• Mist more often, two or three times a day, with a fine mister.
• Keep tubs away from heating vents, which blow very dry air.
• A cheap hygrometer in or near the tub takes the guesswork out.

Pins to harvest

• Pins appear as tiny white or brown knots on the surface, usually within 5–10 days of fruiting conditions.
• They grow fast: most species go from pin to harvest in about 5–7 days.
• When to pick: for most gilled mushrooms, harvest as the veil under the cap begins to tear, before the cap flattens out and drops spores. Spore drop doesn't hurt anything, but it leaves a dark mess on the substrate below.
• How to pick: twist and pull gently at the base, or cut cleanly at the substrate surface. Pull out any stem bits left behind; they rot.

Later flushes

After the first harvest, the tub can usually produce 2–4 more flushes.

1. Rehydrate by dunking: fill the tub with cold water so it just covers the substrate, weigh the cake down, and leave it 4–12 hours. Drain thoroughly.
2. Or skip the dunk and mist more heavily for a few days. This works for the second flush but not usually beyond.
3. Return to fruiting conditions. Each flush is usually somewhat smaller than the last.

Retire the tub when flushes get small or contamination appears. Spent substrate makes good garden compost.

Common problems

Problem	Likely cause
Long, thin stems with small caps	Not enough fresh air
Fuzzy white growth at stem bases ("fuzzy feet")	Not enough fresh air
Pins that turn dark and stop growing (aborts)	Surface dried out, or a sudden change in conditions
Mushrooms only on the sides or bottom	Light or air reaching the sides; add a liner
No pins after two weeks	Surface too wet or too dry, not enough fresh air, or not enough light
Colored mold or a slimy, sour patch	Contamination. See contamination ID.