Mushroom Spawn & Spawn, Bags & Yield

You pasteurise or sterilise a substrate — straw, sawdust or compost — then mix in grain or sawdust spawn at the species' inoculation rate, pack it into bags or beds, and incubate in the dark until the white mycelium colonises it. Once fully run, you trigger fruiting and harvest flushes of fresh mushrooms over the following weeks.

Spawn = substrate × spawn rate%. Oyster mushrooms run at about 4% of wet substrate, button (Agaricus) at roughly 0.75% of compost, and milky (Calocybe) at about 5%. So 50 kg of oyster substrate needs about 2 kg of spawn. A higher spawn rate colonises faster and crowds out contaminants but costs more spawn.

Yield = substrate × biological efficiency (BE). BE is the fresh mushroom weight per unit of dry substrate, but as a practical wet-substrate figure oyster runs around 70% and button around 35%. So 50 kg of oyster substrate at 70% BE gives roughly 35 kg of fresh mushrooms across its flushes — the tool also splits this per bag.

BE compares fresh mushroom harvested to the dry substrate used, expressed as a percentage — 100% BE means a kilogram of dry substrate yielded a kilogram of fresh mushrooms. It captures how well a strain and substrate convert into crop, so growers use it to compare recipes, strains and management and to forecast yield.

Bags = substrate ÷ bag size. If you fill 2 kg bags from 50 kg of substrate, that's 25 bags. Bag size affects colonisation speed and handling — smaller bags run faster and are easier to move, larger bags are more efficient on labour and plastic. The tool gives both the bag count and the yield per bag.

Follow the species and your conditions. The presets — oyster ~4%, button ~0.75%, milky ~5% — are typical starting points. Raise the rate in warm, contamination-prone conditions or for faster colonisation; you can lower it slightly with very clean, fast strains. More spawn means a quicker, safer run but a higher input cost per bag.

Spawn carries the living mycelium that colonises your substrate, so weak, old or contaminated spawn either fails to run or introduces moulds that out-compete the mushroom. Always use fresh, vigorous, contamination-free spawn from a trusted source, keep it cool until use, and inoculate quickly in clean conditions to protect the whole batch.

Yes — raw substrate is full of competing microbes. Oyster straw is usually pasteurised (heated to about 65–80 °C for an hour or so) to knock back competitors while leaving helpful ones; sawdust and grain are fully sterilised. Proper pasteurisation or sterilisation is the single biggest factor in a clean, high-yielding run.

Most species fruit in two or three flushes a week or two apart, with the first flush the largest. The BE figure here covers the total across flushes, not a single pick. Keep humidity and fresh air right between flushes; once yields drop off sharply the spent substrate is exhausted and is best composted.

They're solid planning numbers. Real spawn needs and yields shift with strain, substrate quality, moisture, temperature, humidity and hygiene, so treat the output as a target. Keep records of spawn used and mushrooms harvested per batch, work out your own BE, and feed it back in — the calculator is for planning, not a guarantee.