Grain Spawn & Substrate Explained: Oak, Master Mix & Straw Pellets

If you're moving past pre-made kits and into refilling your own substrate, you'll run into three different pellet and grain products, and it's genuinely confusing which does what. Here's the plain-English breakdown.

The difference between spawn and bulk substrate

Spawn is grain (or a grain-like material) that's already colonised with mycelium, its job is to spread mycelium quickly through a larger volume of bulk substrate. Bulk substrate is the larger, cheaper material that mushrooms actually fruit from, it doesn't need to be nutrient-dense, just moisture-retentive and colonisable.

Oak Hardwood Pellets

A hardwood-based substrate pellet, popular for species that naturally grow on wood in the wild (like Lion's Mane and oyster varieties). Rehydrate before use, the pellets expand significantly once soaked and sterilised, so measure with that in mind. Good as a bulk substrate component or mixed with spawn for wood-loving species.

Master Mix Pellets

A blended substrate designed to be a well-rounded, general-purpose option across a wide range of gourmet species. If you're not sure which substrate suits your species, this is the sensible default.

Straw Pellets

Straw-based substrate, traditionally used for oyster mushrooms in particular, which colonise straw quickly and vigorously. A cost-effective option for bulk volume, especially when paired with a monotub setup where you need a larger quantity of substrate.

Rehydrating pellets

Pellets need water and heat to expand into a usable substrate, generally simmered or hydrated per the product's specific ratio, then sterilised (pressure cooking is standard for grain and wood-based substrates) before spawn is introduced. This is the step a pre-made kit like our Inject and Forget Bags skips entirely, which is why it's the easier starting point for beginners.

If you'd rather skip sterilising pellets yourself

Our sterilised CVG bulk substrate arrives pre-sterilised and hydrated to the right moisture level, a coco coir, vermiculite and gypsum blend ready to mix with spawn straight from the bag. It's the middle ground between a fully pre-made kit and sterilising your own pellets from scratch.

Using grain spawn

Once your bulk substrate is ready, mixing in colonised grain spawn (roughly one part spawn to two or three parts bulk substrate) spreads mycelium through the whole tub far faster than substrate alone would colonise on its own. This is the core technique behind the monotub method.