Your Own Substrate

Beyond the ready-made block

Your first grow kit came with the food already inside the bag. That food has a name: substrate. Substrate is simply the material that mushrooms feed on and grow through. In a kit it is usually straw or sawdust that a company prepared for you.

In this course you take the next step and make your own substrate from things most homes throw away. It is a real grower's skill, and it turns waste into food.

What oyster mushrooms like to eat

Oyster mushrooms are champions. They have been grown on more than 200 different farm and kitchen waste materials, which is why they are perfect for a first homemade grow. Two of the easiest ingredients are:

• Used coffee grounds — the damp grounds left in the filter after brewing. They are packed with nutrients and, because hot water already passed through them, they start out fairly clean.

• Straw — the dried stems of grain plants like wheat. It is cheap, airy, and gives the mycelium room to spread.

Many growers mix the two. A common blend is roughly three parts used coffee grounds to one part straw. The straw stops the coffee from packing down into a dense, airless lump, and air is something mycelium really needs.

Freshness matters

Here is a rule that keeps your grow healthy: only use coffee grounds that are less than a day old. Fresh grounds are still fairly free of moulds. If grounds sit around for several days, unwanted moulds start to move in, and those moulds are exactly the competitors you will be fighting later.

So the best plan is to collect grounds the same day they are brewed. If you cannot use them right away, spread them out to dry or keep them in the fridge for a short time.

Why this is harder than a kit

A kit block was cleaned in a factory. Your homemade substrate has not been cleaned yet, so it carries tiny living things you cannot see: bacteria, yeasts, and mould spores. Some are harmless, but some would love to eat your substrate before the mushrooms can.

That is the challenge of growing beyond the kit. In the very next lesson you will learn how to reduce those competitors with nothing but hot water and an adult's help — no chemicals needed.

A reminder that never changes: we only ever grow known, safe species like oyster mushrooms from trusted spawn. We never pick or eat wild mushrooms, and we never eat any mushroom raw.

You now know what substrate is and how to build one from coffee and straw. Next, we make it clean.