How to Grow Mushrooms in a Monotub: A Step-by-Step Guide

A monotub is a simple, forgiving way to grow mushrooms on bulk substrate. It's essentially a large plastic tub that holds your substrate, keeps humidity in and lets in just enough fresh air. Get the basics right and it does most of the work for you.

What you need

  • A monotub, typically a 60–70 quart tub with fresh-air holes covered by filter patches or micropore tape. The Gro Myco Systems 68 Quart Monotub is built for this.
  • Fully colonized grain spawn, about 3–5 lb for a tub this size.
  • Bulk substrate, most commonly a pasteurized mix of coco coir, vermiculite and gypsum (CVG).
  • A liner (optional), 70% isopropyl alcohol, gloves and a mask.

Step 1: Prepare the bulk substrate

Pasteurize your bulk, usually by pouring boiling water over coir, vermiculite and gypsum in a lidded bucket and letting it cool overnight. Bring it to field capacity: a hard squeeze should give a few drops of water, not a stream. Too wet is one of the most common causes of failed tubs.

Step 2: Spawn to bulk

Work somewhere clean and still. A HEPA flow hood or still air box helps a lot here. Wipe down the tub and your gloves with alcohol.

  • Break up your colonized grain so the kernels are loose.
  • Mix spawn and bulk at a ratio of roughly 1:2 to 1:3. More spawn means faster colonization and less chance for mold.
  • Level the surface gently. Don't pack it down hard. A depth of 3 to 5 inches works well.
  • Optionally, add a thin top layer of plain bulk to keep the surface even.

Step 3: Colonization

Close the tub. During colonization, mycelium prefers higher CO2, so many growers tape over the holes for the first week or so.

  • Keep it around 72–78°F (22–26°C), out of direct light.
  • Leave it alone. Opening the tub to check costs moisture and invites contaminants.
  • Expect full surface colonization in about 10–14 days, depending on spawn rate and temperature.

Step 4: Fruiting conditions

Once the surface is covered in white mycelium, switch to fruiting:

  • Fresh air: uncover the holes. Mushrooms need fresh air exchange to form properly. See Fresh Air Exchange in Monotubs.
  • Light: indirect daylight or a simple light on a 12-hour cycle. It doesn't need to be strong.
  • Humidity and surface moisture: aim for small beads of moisture on the surface and tub walls. Mist lightly if the surface dries out, but avoid puddles.
  • Temperature: slightly cooler than colonization is fine for most species.

Pins (tiny mushroom starts) usually appear within 1–2 weeks of fruiting conditions.

Step 5: Harvest

Harvest each mushroom just before or as the veil under the cap starts to tear. Twist and pull gently, or cut at the base. Remove any aborted pins so they don't rot.

Step 6: Further flushes

After harvest, rehydrate the substrate by soaking or adding clean water, drain any excess, then return to fruiting conditions. Most tubs give two to three good flushes, each a bit smaller than the last.

Common problems

  • Green patches: likely Trichoderma. Read our guide to Trichoderma.
  • Long thin stems, small caps: not enough fresh air.
  • No pins: usually a surface that's too dry or too wet, or not enough fresh air.

Questions? Email us at hello@gro-myco.com.

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