How to use a mushroom grow kit: from box to harvest

What's in the box?

A grow kit is a ready-to-fruit block of food (straw, sawdust or similar) that has been fully colonised by mycelium, the mushroom's root-like network. You don't need to feed it. Your job is to give it the conditions it needs to fruit: the right temperature, high humidity, light and fresh air.

This guide is written for oyster mushroom (Pleurotus) kits, the easiest kind. Species such as shiitake work differently and take longer, so the instructions that came with your kit always come first.

Pick the spot

• Temperature: the University of Florida's home-growing guide (UF/IFAS) suggests about 20 °C for oyster mushrooms to fruit; University of Georgia Extension says they do best between roughly 13 and 24 °C.
• Light: bright, but no direct sun. The mushrooms need light, not sunshine. A north-facing windowsill or a corner of the kitchen counter works well.
• Avoid: radiators, heaters, air-con vents and draughts. They dry out the block and the surface very quickly.
• Air: don't shut the kit in a cupboard or a bag. Mushrooms breathe and need fresh air.

Step by step

1. When the kit arrives, check the date and instructions on it. Most kits are made to be started soon, not stored for long.
2. Open the window in the box, or cut a few slits in the plastic where the instructions show. That's where the mushrooms will come out.
3. Mist the opening and the area around it two or three times a day with a fine spray. Aim to keep the surface damp, never dripping. UGA suggests bottled water or tap water left to stand overnight.
4. After a while (days to a few weeks, depending on the species, the block and the room) small bumps appear at the opening: the first mushroom pins. Be extra careful with humidity now, as dry air can shrivel them.
5. Once the pins become a cluster, growth speeds up; UGA notes they roughly double in size each day. Check daily.
6. Harvest when the caps are about 5 cm across, before the edges flip up and thin out: cut the cluster flush with the block with a sharp knife, or grip it and twist it off.
7. Clear away any bits left stuck in the opening and let the block rest; see the section on second flushes below.

What your kit is telling you

• Long, thin stems and small caps: usually not enough light or fresh air. Move it somewhere brighter (still out of the sun) and air the room. The UF/IFAS guide stresses good air exchange to stop carbon dioxide building up.
• Cracked, dry caps: the air is too dry. Mist more often and move the kit away from heat.
• Yellowing, mushy pins: often too much water. Mist like a fine fog, not big drops.
• Green, blue, black or yellow patches: according to UF/IFAS, a sign of unwanted moulds or bacteria. Trichoderma, a fast-spreading green mould, is the common one. Don't eat mushrooms from the mouldy part, and let the seller know.

Second (and third) flushes

The block isn't finished after the first harvest. UF/IFAS says oyster mushroom blocks can give 2–4 more flushes depending on the strain and conditions, each usually smaller than the last. UGA suggests resting the block somewhere dark for 2–3 weeks and then starting again with the same steps.

When the block is light and dry, it has done its work. Don't bin it: dig it into the garden or add it to the compost.

After harvest

Mushrooms straight from a kit are far fresher than shop-bought ones, but they still don't keep long. Store them unwashed in a paper bag in the fridge; see how to store mushrooms. If nothing came up, or growth stopped halfway, read why isn't my mushroom kit growing.