Spawning to bulk (S2B) is the process of mixing your fully colonized grain spawn with a larger volume of “bulk substrate” (like coco coir). This provides the moisture and surface area needed for the mushrooms to fruit.
1. The Ingredients
- Fully Colonized Grain Spawn: The jar must be 100% white with no visible uncolonized grain or green/black spots.
- Pasteurized Bulk Substrate: The most common is Coco Coir (sometimes mixed with vermiculite and gypsum, known as CVG). It must be at “field capacity”—wet enough that a hard squeeze only produces a few drops of water.
- The Tub: A “Monotub” or a simple plastic storage bin (Shoebox).
2. The Step-by-Step Process
Step 1: Sanitize Everything
Even though colonized grain is more resistant to contamination than agar, you still need a clean environment.
- Wipe down your tub and your hands with 70% Isopropyl Alcohol.
- Wipe the outside of your grain jars before opening them.
Step 2: Break Up the Spawn
Shake or bang your grain jars against a tire or a padded surface to break the colonized mass back into individual grains. The more individual “points of growth” you have, the faster the tub will colonize.
Step 3: The Layering/Mixing Method
There are two main ways to do this:
- The Mix: Dump your grain and substrate into the tub and toss them together like a salad until the grains are evenly distributed.
- The Layer: Start with a layer of substrate, then a layer of grain, and repeat.
The Ratio: A 1:2 ratio (one part grain to two parts substrate) is the gold standard for speed and yield.
Step 4: The Pseudo-Casing Layer
After mixing, flatten the surface gently with your hand (don’t pack it down like a brick; keep it airy). Then, add a final 1/4 to 1/2 inch layer of plain pasteurized substrate over the top. This hides any exposed grain from mold spores in the air.
Step 5: Secure the Lid
Close the lid. If you are using a modified tub with holes, cover them with painters’ tape or plug them with polyfill. The goal is to keep CO2 levels high to encourage the mycelium to spread.
3. Incubation vs. Fruiting
4. Critical Tips
- Field Capacity is Key: If your substrate is too dry, the mushrooms won’t grow. If it’s too wet (pooling water), you will get “sour rot” or bacterial contamination.
- Don’t Peek: Every time you open the lid during colonization, you let out CO2 and let in potential contaminants. Wait until the surface is at least 75% white.
The Field Capacity Squeeze Test
This image demonstrates the classic “squeeze test” to determine if your bulk substrate (such as a 1:2 mix of coco coir and vermiculite) has the perfect moisture content.
- Top Left (Too Dry): The hand squeezes, but only a minimal, powdery amount of water drips. This will lead to slow, weak growth.
- Top Right (Too Wet – “Sour Rot” Risk): A hard squeeze results in a continuous stream of water pouring from the hand. This oversaturation creates an anaerobic environment where bacteria thrive, leading to the dreaded “sour rot” or bacterial contamination.
- Bottom (The Ideal “Field Capacity”): The hand applies full pressure. Only a few drops (about 2–3) of water are released before stopping. The resulting mycelium block should hold its shape when you open your hand. This balance provides the moisture needed for the mushrooms to form.
