$10.00 offWilly's Blend - Willy Myco's Official Premium Imported Manure-Based Substrate - 5 lbs
$1999$2999Unit price /Unavailable
$5.00 off
$1.50 off-
$23.01 off
Midwest's Select Bulk Substrate Casing Mix - 30 Pounds (BULK 6-PACK)
$8499$10800Unit price /Unavailable 
$7.00 off-
$27.01 off
Midwest's Black Cow Manure Substrate & Casing Mix - 30 Pounds (BULK 6-PACK)
$9799$12500Unit price /Unavailable
Choosing the Right Bulk Substrate for Mushroom Growing
Bulk substrate is used after your grain spawn has fully colonized. Instead of fruiting directly from a small grain bag or jar, growers mix colonized grain spawn with bulk substrate inside a monotub to create a larger growing block with more moisture, more surface area, and better fruiting potential.
Our bulk substrate bags are designed for growers who want to expand into monotub growing, refill an existing tub, or scale up from smaller beginner kits. For best results, pair your substrate with fully colonized grain spawn and a clean monotub fruiting chamber.
How Bulk Substrate Works
The grain spawn acts as the inoculated mycelium source, while the bulk substrate provides the moisture, structure, and growing mass needed for the tub. Once mixed together, the mycelium recovers and spreads throughout the substrate until the tub is ready for fruiting conditions.
What Products Pair Best With Bulk Substrate?
Most growers pair bulk substrate with sterile grain spawn bags, monotub fruiting chambers, and monotub accessories. If you want the easiest option, choose a monotub refill kit that includes both grain spawn and substrate.
New to Bulk Growing?
We created a complete step-by-step bulk monotub grow guide with photos showing how to set up your tub, add substrate, break up grain spawn, mix everything together, add plugs, switch to filter disks, and follow the tub through colonization and pinning.
Bulk Substrate FAQ
Do I need grain spawn with bulk substrate?
Yes. Bulk substrate is normally mixed with fully colonized grain spawn. The grain spawn provides the active mycelium, while the substrate provides moisture and growing mass.
Can I use bulk substrate in a monotub?
Yes. Bulk substrate is commonly used in monotub grows. It is mixed with colonized grain spawn, leveled inside the tub, and allowed to colonize before fruiting.
What is the easiest way to start bulk growing?
The easiest way is to use a complete monotub refill kit that includes both grain spawn and bulk substrate sized for your tub.
Should I read the guide before starting?
Yes. Our bulk monotub grow guide walks through the full process with photos so you can see each step before opening your supplies.