Mushroom growing uses terms that sound technical but are actually simple once explained. If you have just bought a mushroom growing kit and the instructions mention mycelium, pins, substrate, or flushes, this guide explains what each one means in plain English so you know exactly what is happening inside your kit at every stage.

Mycelium

Mycelium is the white, thread-like network that grows through the substrate inside your kit. It is the living organism that eventually produces mushrooms. Think of mycelium as the root system and mushrooms as the fruit. Just as an apple tree has roots underground and produces apples above ground, mycelium spreads through the growing medium and produces mushrooms on the surface.

When you open a ready growing kit, the block will be covered in white material. This is healthy mycelium, and it is exactly what you want to see. It means the growing process is already underway and the kit is ready to start producing mushrooms.

Thin visible web of mycelium on the growing substrate

Hyphae

Hyphae are the individual thread-like strands that make up mycelium. If mycelium is the whole network, hyphae are the single fibres that build it. Each hypha is microscopic on its own, but when millions of them grow together and branch out through the substrate, they form the visible white web you see on your kit.

Hyphae are how the mushroom feeds. They release enzymes that break down the substrate into nutrients, then absorb those nutrients back through the strand walls. As they grow, they keep branching outward in search of more food, which is why a healthy kit ends up completely covered in mycelium over time.

You will not need to identify individual hyphae when growing your kit, they are far too small to see without a microscope. The term comes up in growing guides and biology articles, so it is useful to know: hyphae are the individual strands, mycelium is the network they form together.

Substrate

Substrate is the growing medium that the mycelium feeds on. It provides the nutrients and energy the mushroom needs to grow. In Merryhill Mushrooms kits, the substrate is a specially prepared organic material, sometimes called compost, that has been sterilised and inoculated with mushroom spawn at the farm.

You do not need to add anything to the substrate in most kits. It already contains everything the mushroom needs. For chestnut and white mushroom kits, there is an additional topsoil casing layer that goes on top of the substrate as part of the setup process.

Infographic showing each mushroom growing term on a block of our growing substrate

Colonisation

Colonisation is the process of mycelium spreading through the substrate. When a mushroom kit is first made, mushroom culture (called spawn) is mixed into the substrate. Over the following weeks, the mycelium grows outward from the spawn points until the entire block is colonised, meaning the mycelium has reached every part of the substrate.

With a ready growing kit, colonisation happens at the Merryhill Mushrooms farm before the kit is dispatched. That is why the block looks "mouldy" when you open it, the colonisation stage is already complete. This is the key difference between a ready growing kit and a typical kit where the customer has to manage colonisation at home.

Different species will have drastically different looking mycelium, for example yellow oyster is very thick, where as pink oyster and lion's mane is very thin, making it much harder to see.

Fruiting

Fruiting is the stage where the mycelium starts producing mushrooms. After colonisation is complete, the mycelium needs a trigger to switch from growing through the substrate to producing mushrooms on the surface. That trigger is a change in conditions: fresh air, humidity, light, and sometimes a drop in temperature.

When you open your kit, position it in the right spot, and start misting, you are providing those fruiting conditions. The mycelium responds by forming tiny bumps on the surface, which develop into full mushrooms over the following days.

Pins (Primordia)

Pins are the very first visible signs of mushroom growth. They look like tiny bumps, knobs, or dots on the surface of the substrate. The scientific name is primordia, but most growers just call them pins because of their small, pin-like shape in the earliest stage.

Seeing pins is an exciting moment because it means your kit has successfully moved from colonisation to fruiting. Once pins appear, they can develop rapidly. Some varieties double in size overnight during the pinning stage. Continue misting and keeping conditions stable, this is not the time to move the kit or change anything.

 

Left: early pins appearing as small bumps — Right: pins developing into recognisable mushroom shapes

Flush

A flush is one complete cycle of mushroom growth from a single kit. The first flush is the first batch of mushrooms the kit produces. After you harvest the first flush, the kit rests for a few days and can then produce a second flush, and sometimes a third or fourth.

The first flush is almost always the largest and most productive. Each subsequent flush tends to produce fewer or smaller mushrooms because the substrate has less nutrition left. This is completely normal. When the kit stops producing altogether, the substrate is spent and can be composted in the garden.

Between flushes, stop misting for a few days to let the block rest. Then resume your normal routine and the kit should begin pinning again for the next flush. The gap between flushes varies by variety but is typically a few days to a week.

Spawn

Spawn is the starting material used to introduce mushroom culture into the substrate. It is grain, sawdust, or another carrier that has been colonised with a specific mushroom species. When spawn is mixed into fresh substrate, the mycelium spreads outward from the spawn into the surrounding material.

You will not see or handle spawn with a ready growing kit because the spawning process happens at the farm before the kit is sent to you. It is worth knowing the term because it appears in growing guides and mushroom growing discussions online.

Casing layer (top soil)

A casing layer is a thin layer of material placed on top of the colonised substrate to help trigger fruiting. Not all mushroom kits use one. Chestnut and white mushroom kits include a bag of topsoil that you distribute over the compost as part of the setup, so does our King Oyster Growing Kit. The casing holds moisture, creates a humid microclimate on the surface, and gives the pins something to push through.

The casing layer needs to stay moist throughout the growing period. If it dries out or pulls away from the edges of the kit, the hydration guide explains how to re-wet it safely without damaging the mycelium underneath.

Spores

Spores are the reproductive cells of a mushroom, similar to seeds in a plant. Mature mushrooms release spores from the underside of their caps. You might notice a fine powder around the base of the kit or on nearby surfaces if mushrooms are left too long before harvesting.

Spore drop is a sign that the mushrooms are overripe and should have been picked earlier. Harvesting before spore drop gives better flavour, firmer texture, and longer fridge life. It also keeps the growing area clean. If you see spores, harvest immediately and wipe down the surrounding area.

Contamination

Contamination is when unwanted organisms, usually mould or bacteria, grow on or in the substrate. It is the main risk during the colonisation stage, which is why ready growing kits have a significant advantage: colonisation is done in controlled conditions at the farm, greatly reducing the chance of contamination.

Signs of contamination include green, black, or blue patches on the surface, a strong unpleasant smell, or slimy areas that do not look like healthy mycelium. White, fluffy growth is almost always healthy mycelium, not contamination. If you are unsure, take a photo and contact the support team who can help you identify what you are seeing.

Harvest

Harvesting is simply picking the mushrooms when they are ready. The right time to harvest varies by variety, but the general rule is to pick when the mushrooms have reached full size but before the caps flatten out completely or begin to curl upward. Picking slightly earlier gives firmer texture and better flavour.

The best method for most varieties is a gentle twist-and-pull motion at the base. For chestnut and white mushrooms, twist and pull the entire mushroom out including the root to avoid leaving any behind on the topsoil, which can hinder further crops. For Shiitake, a clean cut at the base protects the block for the next flush.

Freshly harvested mushrooms picked at the right time for best flavour and texture

Quick reference

Here is every term in one place for fast reference. Click any term to jump back to its full explanation.

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Frequently asked questions