Spawn ratio is grain spawn to substrate by weight. At 1:2 you mix one kilo of colonised grain through two kilos of substrate; at 1:4 one kilo covers four. Both work. The ratio sets how fast the substrate colonises, how much room contaminants get in the meantime, and how much grain you have to buy. Get it right and the tub is white in a week; get it too lean and you are watching a slow race you might lose.

Why more spawn is faster and safer

Every colonised kernel is a starting point. Double the spawn and you double the starting points, so the mycelium has half the distance to cover before the substrate is fully colonised. A fully colonised substrate is close to immune to contamination; a half-colonised one is not. That is why beginners should err towards the heavy end: a 1:2 coir tub of oysters can be white in five to seven days, while a 1:5 tub might take two weeks, and those extra days are when green mould finds its footing.

Why less spawn is cheaper

Grain costs more per kilo than substrate, and every kilo of spawn you mix in is a kilo you could have expanded into five more bags. Experienced growers who trust their technique run lean because they have learned that their clean spawn will win the race anyway. Lean ratios also leave more substrate per tub, which means more mushrooms per tub at the same size.

The ratios in practice

Worked examples

• A 5kg CVA bag at 1:2 wants 2.5kg of spawn: one 2kg grain bag plus one 1kg bag, with half a kilo spare for a small tub.
• A 5kg CVA bag at 1:4 wants 1.25kg of spawn: a 2kg grain bag with 750g left to expand into fresh grain.
• A 2kg wood lovers bag at 1:5 wants 400g of spawn, so one 2kg grain bag spawns five blocks.
• A 2kg all-in-one is already mixed at roughly 1:2 inside the bag; nothing to work out.

Mixing it properly

Ratio only works if the spawn is spread evenly. Break the grain bag up from the outside before you open it, so kernels are already separate. In a still air box, open both bags, tip the substrate into a clean tub, scatter the spawn across it and fold it through with gloved hands until you cannot see clumps. Flatten the surface, lid on, into the dark. The substrate guide covers tub depth and field capacity.

What about colonised spawn?

If you buy colonised grain spawn the ratios are identical; you just skip the weeks of waiting for the grain to colonise. A 1kg bag covers 2kg of coir at 1:2 or 4kg at 1:4, and 4 to 5kg of hardwood at 1:4 to 1:5.

Written by the Aus Myco Kingdom crew. Everything here is general growing information for gourmet and medicinal species; your room, your climate and your culture will have the final say.