Monotub

Cost

$25–40

Build time

30 minutes

Capacity

1–3 lb per flush

Skill level

beginner

A monotub is a plastic storage tub modified for passive fresh air exchange, used as both colonisation and fruiting chamber. It is the most common bulk setup for home growers because it costs almost nothing and works.

Most monotub problems come from building it to hold humidity perfectly and exchange air badly.

What to buy

A 54–66 quart clear storage tub with a solid lid. Clear matters — you want to see colonisation without opening it.

Avoid tubs with textured or frosted sides, and avoid any lid with built-in vents you cannot control.

Dimensions that matter

Substrate depth: 4–6 inches. This is the number people get wrong.

• Shallower than 4″ dries out and gives thin flushes
• Deeper than 6″ colonises slowly in the middle and can go anaerobic at the base

Headspace above the substrate: 6–10 inches. Enough room for fruits to develop without touching the lid, which causes deformity and wet spots.

A 54 qt tub with 5″ of substrate is a good default.

Holes

The part that decides whether the tub works.

Size: 2 inches diameter. Cut with a hole saw for clean edges.

Number and placement:

• 4–6 holes total
• Two on each long side, positioned at or slightly above substrate level
• Optionally one at each end
• Stagger the heights slightly side to side so air crosses the tub instead of sitting

Holes at substrate level matter because CO₂ is heavier than air and pools exactly there. Holes only near the top exchange the air above the CO₂ layer and leave the problem where the pins are.

Filling them: polyfill (pillow stuffing) stuffed loosely into each hole. Enough to filter spores, loose enough to pass air. Packing it tight is the same as not cutting the hole.

Some growers run open holes with no filter once fruiting begins. It works, and it raises contamination risk. Judge by how clean your space is.

Liner

Tape or black plastic on the bottom 4 inches of the tub, outside or inside.

The purpose: mycelium fruits toward light, and blocking light at the sides stops it side-pinning against the tub wall where the mushrooms deform and rot. You want pins on the top surface, where you can reach them.

Do not black out the whole tub. You need to see what is happening.

Building it

1. Mark hole positions — two per long side, at substrate level.
2. Cut with a 2″ hole saw. Deburr the edges so they do not tear bags or hands.
3. Tape or line the bottom 4″ to block light.
4. Wipe the inside with 70% isopropyl.
5. Add substrate mixed with spawn to 4–6″. See spawn to bulk.
6. Tape the holes shut for colonisation. High CO₂ during colonisation is what you want.
7. Lid on. Dark, 70–78°F.

Running it

Colonising: holes taped, lid on, dark, 70–78°F. Do not open it. See colonisation.

When fully colonised: consider a casing layer, wait 5–7 days for it to run through.

Fruiting: untape the holes, introduce light, drop temperature per species, mist the walls.

Misting: the walls, never the substrate. Two or three times a day depending on how fast it dries.

Fanning: lid off, two or three sweeps, twice daily. Crude and effective, and it is what most tubs actually need more of.

The failure mode

A tub that holds beautiful humidity and exchanges no air produces long stems and tiny caps, every time. See too much CO₂.

If you are unsure whether to add air or humidity, add air. Under-ventilation is far more common than under-humidification in monotubs, and the symptoms of the first are frequently misread as the second.

Common mistakes

Holes too high. CO₂ pools at substrate level and never leaves.

Polyfill packed tight. No exchange.

Substrate too deep. Slow colonisation, anaerobic base.

Misting the substrate. Causes cobweb mold and bacterial blotch.

Opening it constantly. Every look drops humidity to room level and costs pins.

Taping the lid for a better seal. This is the opposite of what the tub needs during fruiting.

When to move on

A monotub tops out around 1–3 lb per flush. Beyond a few tubs, the labour of misting and fanning each one individually becomes the constraint, and a Martha tent with a humidifier and a fan on timers is the next step.