Shiitake spawn is colonised grain or sawdust that carries the mushroom’s mycelium — the living network that fruits your crop. It’s the foundation of any successful shiitake cultivation, whether you’re growing on logs, straw or blocks. Here’s how to choose spawn that will actually perform, and what questions to ask your supplier.
What exactly is spawn, and why does it matter?
Spawn is grain, sawdust or wood chips that have been colonised by mycelium — the vegetative body of the mushroom. Think of it as a living culture, ready to grow. When you introduce spawn to fresh substrate (your growing medium), the mycelium spreads through it, consuming nutrients and breaking down organic matter until it’s ready to produce fruiting bodies — the mushrooms you harvest and eat. Without quality spawn, you’re starting from a weak or contaminated base, which means slower colonisation, lower yields, and wasted time and materials.
The spawn quality determines how quickly your substrate colonises, how resilient the culture is to contamination, and ultimately how healthy your mushrooms will be. I’ve seen home growers spend weeks waiting for colonisation because they used old or poorly stored spawn, or they’ve lost entire batches to mould because the spawn itself was already compromised. It’s not the glamorous part of mushroom growing, but it’s the most critical one.
What should healthy shiitake spawn look like?
Good spawn should be white or cream throughout, with no discolouration, dark patches or visible mould. The grain or sawdust should be dry to the touch — not wet or slimy — and should hold together slightly when you squeeze it gently, showing the mycelium has bound the particles together. It should smell fresh and earthy, never sour, vinegary or musty. If it smells off, don’t use it.
The spawn should also be recent. Mycelium is living tissue and it weakens over time, particularly in storage. Spawn that’s more than a few months old will colonise more slowly and may not colonise at all. Ask your supplier when it was made and stored, and whether it’s been kept cool and dark. If they don’t know or won’t tell you, that’s a red flag.
Grain spawn versus sawdust spawn: what’s the difference?
Grain spawn — usually rye, millet or barley — colonises very quickly, typically within two to four weeks depending on temperature. The individual grains create lots of surface area for the mycelium to spread, and they’re nutrient-rich, which feeds the culture. Grain spawn is ideal if you want a fast, energetic colonisation and you’re comfortable with the higher cost. It works well for shiitake on straw or in bags.
Sawdust spawn uses hardwood sawdust mixed with bran and sometimes other nutrients. It colonises more slowly than grain — six to eight weeks is typical — but it’s cheaper and it integrates well with certain substrates, particularly if you’re fruiting on hardwood logs or woodchip-heavy blocks. Some growers prefer it because the sawdust particles more closely mimic the shiitake’s natural habitat in decaying wood. The choice depends on your substrate, your timeline and your budget.
Where should you source your spawn from?
Source spawn from a supplier who has a reputation for quality, is transparent about their methods and storage, and is based close enough that spawn arrives fresh. A reliable supplier will tell you when the spawn was made, what strain it is, and what conditions it’s been kept in. They should be happy to answer questions about their sterilisation process and their quality control. If they’re evasive, move on.
I work with suppliers who have been in the craft for years and understand that contaminated or weakened spawn damages the grower’s reputation as much as it damages theirs. I also encourage growers in South London to connect through the South London Mushroom Club and the Community Mycology Lab at Remakery Brixton — both are spaces where growers share knowledge about local sources and can recommend suppliers they trust. If you’re in the region, those communities can point you towards reliable stock.
You can also make your own spawn if you have access to a clean workspace and basic equipment, though it requires confidence with sterilisation and aseptic technique. I run cultivation workshops where we cover spawn-making from scratch. If you want to start that way, that’s absolutely valid — but if you’re new to growing, buying from a reputable supplier removes a lot of variables.
How much spawn do you need, and how do you store it?
The amount of spawn depends on your substrate volume and the substrate type. A rough rule: use ten to fifteen percent spawn by weight. So if you have ten kilograms of straw, you’d add one to one-and-a-half kilograms of spawn. Denser substrates like wood blocks may need slightly more; lighter straw-based mixes can work with slightly less. Your supplier should advise based on what you’re growing in.
Store spawn in the coolest, darkest place you have — ideally between four and ten degrees Celsius. A fridge or cool cupboard works well. Keep it in its original packaging, which is usually designed to allow gas exchange without letting in contamination. Check it regularly for signs of drying out or mould. Most spawn will remain viable for two to three months in cool storage, though fresher is always better. If you can’t use it within that window, ask your supplier about longer-term storage options or consider ordering smaller quantities more frequently.
The questions to ask before you buy
Before you commit to a spawn supplier, ask: When was this made? Has it been tested for contamination? What temperature and humidity was it stored at? What strain is it — and do you have documentation? Can they provide a sample so you can inspect it before you buy a large quantity? How quickly can they deliver, and will it arrive in an insulated package? Do they offer any guarantee if it fails to colonise?
A good supplier will welcome these questions. They’ll have answers ready and won’t mind if you’re cautious. If you’re buying in person — at West Norwood Feast, for example, where I trade — you can inspect spawn directly, ask questions face-to-face, and build a relationship with your grower. That transparency matters, particularly when you’re starting out and trying to understand what success looks like.