A mushroom block is a compressed, sterilised substrate — usually sawdust, straw or grain mixed with nutrients — that’s colonised by mushroom mycelium and ready to fruit indoors or outdoors. It’s the simplest entry point for both home growers and small-scale producers, requiring no specialist equipment and delivering multiple flushes of fresh mushrooms over weeks. We use blocks at Adi’s Urban Mushrooms to teach cultivation and supply local restaurants and markets across South London.
What exactly is inside a mushroom block?
A mushroom block is a dense, rectangular package of substrate — the material on which mushrooms grow. The substrate is typically made from agricultural waste: sawdust (often hardwood), straw, grain, or a blend tailored to the mushroom species you’re growing. The substrate is mixed with nutrients like bran or grain to feed the mycelium, then packed into a bag under high pressure to remove air pockets. This compressed environment allows the mycelium to colonise evenly and efficiently.
Before the block reaches you, it’s sterilised using heat — usually by steam or pressure — to kill competing moulds and bacteria. Once cool, the sterilised block is inoculated with mushroom spawn (mycelium that’s already growing on grain or sawdust). The mycelium then spreads throughout the block over 2–4 weeks, depending on the variety. When it’s fully colonised, the block turns white or cream as the mycelium knits the substrate together. This is when it’s ready to fruit.
The substrate recipe varies. Oyster mushrooms thrive on straw or sawdust. Shiitake prefers hardwood sawdust and takes longer to colonise — 6–8 weeks — but produces premium flavour. I choose substrates based on what’s available locally and what flavour profile the mushroom will develop. Sourcing matters: the quality of your substrate directly affects the quality of your crop.
How do you get mushrooms to fruit from a block?
Once the block is fully colonised, fruiting is straightforward. You make a slit or small cut in the bag, or remove the bag entirely if the block is in a netting sleeve. The exposed substrate needs fresh air, humidity and light to trigger pinning — the formation of tiny mushroom primordia (the bumps that become mushrooms). Most growers mist the block 2–3 times daily with water and position it where it receives indirect light and airflow.
Within 3–7 days of initiating fruiting conditions, pins appear. The mushrooms then expand rapidly, doubling in size every 1–2 days depending on the species. You harvest when the caps flatten or just before they open fully — timing is crucial for flavour and shelf life. At Adi’s Urban Mushrooms, we harvest oysters at their sweetest point and shiitake when the caps have just begun to flatten: that’s when the umami is most concentrated.
After your first flush, the block continues to produce. You rest it for 7–10 days, keeping it moist but not waterlogged, then initiate a second flush using the same misting routine. Most blocks give 2–4 substantial flushes before yields drop. Each flush is slightly smaller and sometimes slower, but still worth harvesting.
Why use a block instead of growing from scratch?
Building a mushroom block requires sterilisation equipment — a pressure cooker or steam chamber — and clean technique to avoid contamination. For most growers, buying a ready-colonised block removes that barrier. You skip the months of learning, the risk of mould, and the need for expensive kit. A block arrives ready to fruit in days, not weeks.
Blocks are also predictable. Because the substrate is already colonised by a single, vigorous strain, you know what you’re growing and roughly when it will fruit. There’s far less room for error. For commercial growers like us at West Norwood Feast, blocks are an efficient way to scale: we can manage dozens of blocks at different fruiting stages, harvest continuously, and supply consistent quality to local restaurants. For home growers, a block is the fastest route to fresh mushrooms on your kitchen counter.
The economics are clear too. A single block costs less than the equipment and time required to make several blocks from scratch, especially when you account for potential losses to contamination. You get multiple harvests from one block — typically 1–2 kg of fresh mushrooms per block over its lifetime, depending on the species and your fruiting technique.
What mushroom varieties work best in block format?
Oyster mushrooms are the most forgiving and fastest. Pink oysters, blue oysters and king oysters all colonise reliably within 2–3 weeks and tolerate variable conditions. They fruit prolifically and deliver tender, flavourful mushrooms. I recommend oysters for first-time growers and anyone seeking quick, rewarding results.
Shiitake blocks are slower — 6–8 weeks to colonise — but the flavour is incomparable. The umami depth and firm texture make them worth the wait, especially for chefs. We grow shiitake blocks for local restaurants who value the consistency and provenance. Shiitake also performs exceptionally well outdoors; the cool, humid conditions accelerate fruiting.
Lion’s mane, enoki and other specialist varieties also work well in blocks, though they require slightly warmer temperatures and more careful humidity control. The choice depends on your space, climate and what you want to eat or cook with. Each species teaches you something different about mycelium behaviour and fruiting triggers.
Where should you keep a fruiting block?
Blocks thrive in cool, humid, well-ventilated spaces. A kitchen windowsill works for one or two blocks. Basements, sheds or garages are ideal if they stay between 12–18°C and have some indirect light. Direct sunlight can dry blocks too quickly; shade or dappled light is better. Outdoors under a covered area — a porch, pergola or shade cloth — works beautifully if your climate is cool enough and you can mist regularly.
Humidity is non-negotiable. If the block dries out during pinning, the pins abort and you lose that flush. Misting 2–3 times daily is standard, though a humidity tent with a humidifier makes this easier for larger setups. Airflow matters too: still air invites mould and weak fruiting. A small fan on a gentle setting, or positioning the block where it catches natural air movement, keeps the microclimate healthy.
At Adi’s Urban Mushrooms, we fruit blocks in a dedicated space with controlled airflow and automatic misting systems. For home growers, even a simple DIY setup — a plastic storage box with air holes and a spray bottle — works. The key is consistency. Mushrooms respond to routine; once you establish a misting and ventilation rhythm, they reward you reliably.
How do mushroom blocks connect to learning and community?
I use blocks in our cultivation workshops and at the Community Mycology Lab in Remakery Brixton. They’re perfect teaching tools: participants watch colonisation happen in real time, learn to recognise the signs of healthy mycelium, understand fruiting triggers, and harvest their own mushrooms. Blocks make mycology tangible and demystify what growers actually do.
The South London Mushroom Club also uses blocks to explore different species and techniques. It’s a space where growers — hobby and commercial — compare notes, troubleshoot problems, and share the excitement of a successful flush. Blocks are democratic: whether you’re growing one in a flat or fifty in a commercial space, the fundamentals are the same.
If you’re interested in learning hands-on or sourcing blocks for growing, our workshops and kits are designed for exactly this. We supply ready-colonised blocks and teach the fruiting process so you can grow confidently and understand the source of your food.