Your Native Grey Oyster Cultivation Guide

This is the code on your bag label. It points here — adisurbanmushrooms.co.uk/cultivation-guides — so scan it any time to find the guide again, or let a friend scan it to share.

Your mushroom: a native British strain

The mushroom in your bag is Pleurotus ostreatus, the grey or blue oyster mushroom — one of Britain’s most common wild fungi, found on dead and dying hardwood, especially beech, ash and willow.

The particular strain you are growing was isolated from a wild British fruiting body, not bred from an imported commercial cultivar. That matters. Most mushrooms grown in the UK come from a very small number of heavily selected commercial strains, chosen for shelf life and uniformity. By growing a native isolate you are keeping a piece of local fungal diversity alive, in circulation, and in people’s kitchens.

It is a vigorous, forgiving strain that colonises quickly and fruits reliably in clusters of grey-brown, fan-shaped caps. Expect a mild, savoury flavour and a firm texture.

How mushroom cultivation works

There are three broad steps. You did the third one at the workshop.

1. Create and look after mycelium cultures. A tiny piece of living tissue is grown on sterile nutrient agar or in liquid culture.
2. Feed the mycelium with grain to make grain spawn. The culture is used to inoculate sterilised grain, which the mycelium colonises completely. This becomes the ‘seed’.
3. Mix the spawn into a bulk substrate and give it the right conditions. The mycelium spreads through the substrate, then light, humidity and fresh air trigger it to fruit.

The first two steps need a clean lab set-up to keep contamination out. If you would like to learn those techniques, or use a lab rather than build one, get in touch about Adi’s Urban Mushrooms’ Community Lab and mycology courses.

What is in your bag

• 1 kg of pasteurised straw pellets. Compressed wheat straw, rehydrated with hot water. The pellets swell to roughly three times their volume, and the heat of the water pasteurises the substrate as it expands — which is why the mix went into the bag hot.
• 150 g of grain spawn. Rye or wheat grain fully colonised with the native oyster mycelium. Roughly 15% spawn rate — generous, which means fast colonisation and less chance of anything else getting a foothold.
• A filter patch bag. The white square is a micron filter. It lets the mycelium breathe while keeping mould spores and flies out. Do not cut it, cover it or block it.

Growing your bag

Stage 1 — Incubation (weeks 1 to 3)

Find a dark, warm, still place: an airing cupboard, a shelf in a warm room, the top of a wardrobe. The mycelium does not need light at this stage and does not want a draught. Leave the bag sealed and leave it alone.

Over the first few days you will see white, fuzzy growth spreading out from each grain, then the whole bag gradually going white as the strands join up. This normally takes twelve to eighteen days. Warmer is faster, but avoid anything above about 26°C.

The bag will feel warm to the touch while it is working — that is the mycelium respiring, and it is a good sign.

Stage 2 — Opening up and pinning (weeks 3 to 5)

When the substrate is completely white and has knitted into a solid block, it is ready. Give it a further two or three days after it looks finished — that consolidation makes for a stronger first flush.

1. Wipe a sharp knife and the surface of the bag with 70% alcohol (hand sanitiser or an alcohol wipe).
2. Cut a single cross into one face of the bag, about 7.5 cm (3 inches) across. One opening gives you a better cluster than several small ones and prevents the substrate from drying out.
3. Tightly fold the bag down so that there are as few air pockets as possible. This prevents fruiting in the bag.
4. Move the bag somewhere cooler and lighter — a kitchen worktop or a windowsill out of direct sun is ideal.
5. Mist the opening with water three to four times a day. Filtered or rainwater gives the best results. Do not soak the substrate.

After five to ten days you should see pins — tiny grey mushroom primordia — forming in the cut. From this point, mist around the mushrooms rather than directly onto them; water sitting on developing caps causes yellowing and bacterial spotting.

Stage 3 — What they need to fruit well

• Humidity. High and steady, ideally 85–95%. Misting several times a day does the job. A loose clear bag propped over the block, not sealed, holds humidity between mistings.
• Fresh air. Oyster mushrooms need a lot of oxygen. Too much CO₂ gives long stems and small, deformed caps. Fan the block a few times a day, or leave a window ajar.
• Light. Normal indirect daylight or an LED lamp on a normal day cycle. Mushrooms use light as a signal to fruit and to orient themselves. Direct sun will dry the block out.
• A stable, cool temperature. This strain fruits best on the cooler side — see the table below.

Temperatures at a glance

This native strain is noticeably happier fruiting cool. Above roughly 24°C the mushrooms go pale and floppy and spoil quickly. If you are growing in a warm flat in late summer, the coolest room in the house or a shaded outdoor spot will serve you better than the kitchen. Do not worry about hitting these numbers exactly — oyster mushrooms are forgiving, it just takes a little longer.

Harvesting and eating

Pins grow into fully formed mushrooms in five to seven days, and the last two days are the fastest — check daily.

Harvest when the largest mushroom in the cluster is just starting to flatten out and the edge of its cap is beginning to lift or turn up. Left a day longer they will start dropping spores, which is harmless but makes a fine white dust on the surfaces below.

Take the whole cluster at once rather than picking individual mushrooms: hold the base and twist it away from the block. Twisting leaves a cleaner wound than cutting, and cutting leaves stumps behind that can rot.

Getting a second and third flush

There is plenty of food left in the block after the first harvest.

1. Scrape the old mycelium and any remaining stumps out of the cut with a clean spoon (wiped with alcohol hand sanitiser).
2. Keep misting the opening as before. The block will rest for one to two weeks — this is normal, not failure.
3. If the block feels light and dry, rehydrate it: submerge it in cold water for 4 to 12 hours, then drain it thoroughly.

A second flush is usually about half the size of the first, and a third smaller again. Three flushes from a 1 kg block is a good result.

If something goes wrong

• Long stems, tiny caps. Not enough fresh air. Increase ventilation and fanning.
• Pins forming then shrivelling and going brown. Too dry, or a draught of dry air. Mist more often and shelter the block.
• Yellow or orange slimy patches on the mushrooms. Bacterial blotch, from water sitting on the caps. Mist around the mushrooms, not onto them.
• Green or blue-green patches in the substrate. Trichoderma mould. It competes with the mycelium. Ignore if it is only a few patches. If it is a large area it will not be beaten. Do not open the bag indoors — bag it up and put it in the general waste, and wash your hands.
• Nothing at all after three weeks. Check the temperature — it is most often too warm or too cold. Move somewhere warmer or cooler and give it another week before giving up.
• A sour, boozy or eggy smell. Bacterial contamination, usually from too much moisture. The block will not recover. Dispose of it as above.

A healthy block smells sweet, faintly of aniseed and fresh mushrooms. Trust your nose.

When it is finished

Once the block stops producing, cut the plastic away and put the spent substrate on your compost heap, dig it into a border, or use it as a mulch. It is a genuinely good soil improver, and the mycelium sometimes fruits once more outdoors in autumn if you bury it against a piece of damp cardboard or untreated wood.

Recycle the bag with soft plastics. That closes the loop.

Why grow mushrooms? A circular economy for food

Changing our food system is one of the most impactful things we can do to address climate change, create healthy cities and rebuild biodiversity.

A circular economy for food mimics natural systems of regeneration, so that waste does not exist — it is simply feedstock for the next cycle. Fungi are the original circular organisms. They take a low-value by-product, in this case compressed straw left over from arable farming, and turn it into protein-rich food. What is left afterwards is clean, spent substrate that goes straight onto the compost heap or the garden as a soil improver. Nothing in the process needs to become rubbish.

Keep going

If you enjoyed this, there is a lot further to take it — growing on logs outdoors, working with different species, or learning the sterile lab techniques behind steps one and two. Adi runs workshops, longer courses and a community lab in London, and sells grain spawn, grow kits and fresh mushrooms.

Get in touch: adisurbanmushrooms.co.uk · Instagram @adisurbanmushrooms · Facebook UrbanMushroomsLondon