Once you know why vermiculite, coco coir and gypsum work together (see our Vermiculite vs Perlite guide if you haven’t yet), the next question is how to actually turn those three ingredients into a substrate that’s ready for spawn. This walks through the full process end to end – hydration, mixing, pasteurising or sterilising, cooling, bagging, and the mistakes that cause most contamination.
What you’ll need
- Coco coir block – the bulk carbon source
- Vermiculite – moisture buffering
- Gypsum – structure and pH buffering
- Distilled water – tap water carries chlorine and minerals that can work against you
- Grow bags with an injection port and filter patch – let the substrate breathe without letting contaminants in
- Sterile examination or medical grade – see our Nitrile Glove Buying Guide for which grade suits this job
Step 1: Hydrate the coco coir
Coir bricks are compressed and need to fully expand before use. Submerge the brick in freshly boiled water (or hot tap water) and let it sit until fully broken apart and cool enough to handle – usually 20–30 minutes. Break up any remaining lumps by hand.
Step 2: Squeeze to field capacity
Field capacity means damp enough to hold together when squeezed, with no more than a few drops of water released – not dripping. Squeeze the hydrated coir firmly in batches to remove excess water. This is the single most common place bulk substrate goes wrong: too wet and you starve the mycelium of oxygen; too dry and colonisation stalls.
Step 3: Mix in vermiculite and gypsum
Add vermiculite and coir at roughly a 1:1 ratio by volume, plus gypsum at 2–5% of the total dry volume (about two tablespoons per litre of coir). Mix thoroughly by hand until the vermiculite is evenly distributed rather than sitting in pockets.
Step 4: Pasteurise or sterilise
Which method you need depends on what you’re making. Bulk CVG substrate is naturally low in easily accessible nutrients, so pasteurisation is usually enough. Grain spawn is nutrient-dense and needs full sterilisation, or competing organisms will typically win the race.
| Pasteurisation | Sterilisation | |
|---|---|---|
| Temperature | 65–80°C, held for 60–90 minutes | 121°C at 15 PSI in a pressure cooker, held for 60–120 minutes |
| Kills | Most competitor moulds and pests | Everything, including bacterial spores |
| Best for | Coir/straw-based bulk substrate (CVG) | Grain spawn and nutrient-rich substrates |
| Risk if skipped | Higher contamination risk, but coir/vermiculite is naturally low-nutrient so most growers get away with it | Grain is nutrient-dense – competitor organisms will usually win without full sterilisation |
Step 5: Cool and bag
Let the substrate cool fully to room temperature before doing anything else – adding spawn to warm substrate is one of the most common causes of a failed batch, since heat-stressed mycelium is far more vulnerable to competing contaminants. Once cool, bag it into grow bags with an injection port and filter patch, working with clean or gloved hands.
Step 6: Add spawn
Mix spawn into the cooled substrate at your chosen spawn-to-bulk ratio (commonly somewhere between 1:2 and 1:5 depending on species and how quickly you want colonisation to finish), seal the bag, and move it to a warm, dark spot to colonise.
Troubleshooting common problems
- Green or black patches (mould) – usually too much moisture, not enough gas exchange, or a stalled sterilisation cycle – check your field-capacity squeeze test and filter patch first
- Sour or slimy smell – colonisation stalling, substrate looks wet and grey – the mix was over-squeezed past field capacity, or bags weren’t fully cooled before sealing
- Substrate crumbling and not holding together – usually under-hydration – go back to the coir and add water gradually until it passes the field-capacity squeeze test
Frequently asked questions
Do I need a pressure cooker to make bulk substrate?
Not necessarily – CVG bulk substrate is commonly pasteurised in a large pot of hot water rather than pressure cooked, since it’s naturally lower in nutrients than grain. A pressure cooker is more important for grain spawn.
How long does bulk substrate take to colonise?
Typically 1–3 weeks depending on species, spawn ratio and temperature – faster with a higher spawn ratio and warmer conditions, within the species’ preferred range.
Can I reuse the water from hydrating coir?
It’s generally not recommended – it can carry tannins and any contaminants picked up during hydration. Discard it and use fresh distilled water for anything that touches the substrate afterwards.
Shop the essentials
- Coco Coir Block 70L – the base of any CVG mix
- Vermiculite 100L Bag – moisture buffering for the whole colonisation and fruiting cycle
- Agricultural Gypsum (Calcium Sulphate) – structure and pH buffering
- Distilled Water 1000ml – for hydration without chlorine or mineral interference
- Mushroom Grow Bags with Injection Port and Filter Patch – for bagging your finished substrate