Bulk Monotub Mushroom Grow Guide
Welcome to the Midwest Grow Kits bulk monotub guide. Bulk monotub growing is one of the most productive and beginner-friendly methods for cultivating gourmet mushrooms at home. By combining fully colonized grain spawn with a nutrient-rich bulk substrate inside a modified tub, you create the ideal fruiting environment for high yields of healthy mushrooms.
This guide walks you through every stage — from setup to harvest — with photos from real grows. With clean technique, the right ratios, and a little patience, a single tub can produce two to four flushes of mature mushrooms.
Supplies You'll Need
Clean Preparation Matters
Bulk monotub success starts with clean, careful preparation. Wipe down your work area with isopropyl alcohol, wear gloves, and minimize air movement during the spawn-to-bulk step. The spawning process is the highest-contamination-risk moment of the entire grow.
Have all your supplies opened and ready before you begin so you can move quickly and limit exposure.
What You Need
- A modified monotub with pre-cut holes for plugs and filter disks
- Fresh tub liner (optional but recommended)
- Fully colonized Premium 5 Grain Spawn Bag
- Hydrated Midwest's Select Bulk Substrate & Casing Mix
- Black incubation plugs (typically included with tub)
- Filter disks for the fruiting stage
- Isopropyl alcohol and clean gloves
Prepare Your Monotub
Start with a clean, modified monotub. Install a fresh liner if you're using one — this makes cleanup easier between grows and provides a sanitary surface for your substrate. Wipe down the interior with isopropyl alcohol and let it dry completely before adding anything.
- Wipe the tub interior with isopropyl alcohol.
- Install a fresh liner if using one.
- Clean your work surface and surrounding area.
- Wear gloves and minimize air movement during the next steps.
Add the Bulk Substrate
Open your bag of hydrated bulk substrate and scoop it into the tub. Spread it evenly across the bottom. The substrate should already be at field capacity — fully hydrated but not dripping water when squeezed.
- For a medium monotub, use about 7.5 lbs of bulk substrate paired with one colonized grain spawn bag.
- Scale quantities proportionally for larger tubs.
- Spread the substrate evenly across the bottom of the tub.
Break Up the Grain Spawn
Before opening your colonized grain spawn bag, gently break up the grain inside while the bag is still sealed. Squeeze and massage the bag with your hands until the grains are loose and separated. This step is critical — it ensures mycelium-coated grains distribute evenly when mixed with the substrate.
- Only use bags that are 100% colonized (fully white).
- Massage the bag until grains feel loose and separated.
- The bag should smell earthy and fresh — never sour or musty.
- Discard the bag if you see green, black, pink, or other discoloration.
Mix the Grain Spawn into the Substrate
With clean, gloved hands, open the grain spawn bag and pour the contents into the tub on top of the bulk substrate. Gently fold and mix the spawn throughout the substrate until grain particles are evenly distributed from top to bottom. This is called "spawning to bulk."
- Open the bag close to the tub to minimize exposure.
- Pour the grain on top of the substrate.
- Gently fold and mix until grain is evenly distributed.
- Lightly flatten and level the surface — do not compact.
Install Plugs for Incubation
Insert the black incubation plugs into the pre-cut holes on the sides and lid of your monotub. These seal off the holes during the colonization phase, which keeps CO₂ levels elevated. Higher CO₂ during incubation encourages mycelium to spread aggressively before fruiting begins.
- Place all black plugs into the tub's pre-cut holes.
- Store the tub in a dark, stable spot at 70–75°F (21–24°C).
- Avoid direct sunlight and temperature swings.
- Most grows fully colonize in 7–14 days.
Switch to Filter Disks
After about a week — once you see significant mycelium spread across the surface and sides of the tub — swap the black plugs for filter disks. Filter disks allow fresh air exchange (FAE) while preventing contaminants from entering. The drop in CO₂ and increase in oxygen is what triggers pinning.
- Remove all black plugs and insert filter disks in their place.
- Move the tub to a spot with indirect natural light or a low-wattage LED on a 12-hour cycle.
- Mushrooms use light as a directional cue, not for photosynthesis.
- Avoid placing the tub in direct sun or near heating vents.
Pinning and Fruiting Stages
Over the next several days, you'll watch your tub progress through the most exciting stages. First, you'll see hyphal knots — small dense patches of mycelium. These quickly develop into pins (tiny baby mushrooms), then rapidly expand into full-sized fruiting bodies.
- Fan the tub once or twice daily to encourage fresh air exchange.
- Mist the inside of the lid lightly if conditions feel dry.
- Avoid spraying directly onto the pins — they're delicate and can abort.
- Keep humidity high (90%+) and temperatures stable.
Expected Grow Timeline
Every species and spawn ratio is different, but this general timeline helps you understand what to expect from a bulk monotub grow.
Harvest Your Mushrooms
Harvest mushrooms just before the veils underneath the caps begin to tear or open. Twist and pull each mushroom gently at the base — or use clean scissors to cut at the substrate line. Harvesting at the right time produces the best texture, flavor, and shelf life.
- Twist and pull at the base, or cut with clean scissors.
- Harvest before veils tear for the best shelf life.
- Pick clusters together when most caps are ready, not one at a time.
- Refrigerate harvested mushrooms in a paper bag, not plastic.
How to Get Additional Flushes
A well-managed bulk monotub typically produces 2–4 flushes. After your first flush, you can rehydrate the substrate to support additional flushes.
- After the first harvest, dunk the cake in cool water for several hours.
- Drain thoroughly and return it to the tub.
- Continue providing humidity, fresh air exchange, and indirect light.
- Yields are usually largest on the first flush and decrease over time as nutrients deplete.
Side pins along the substrate edges are normal. Continue with the same misting and fanning routine throughout each flush cycle.
Quick Tips for Better Results
Beginner Success Tips
- Stick to a 1:1 spawn-to-bulk ratio for your first grow.
- Always grow on a clean, flat surface in a low-traffic room.
- Avoid direct sunlight at every stage.
- Keep temperatures consistent — swings stress the mycelium.
- Patience matters more than poking around. Less is more.
Pro Tip for Bigger Yields
For maximum yields, use multiple grain spawn bags with a larger monotub. A higher spawn-to-bulk ratio also colonizes faster and reduces contamination risk. New growers can streamline the whole process with our Complete Refill Kit for the 44Q Monotub.
Bulk Monotub Grow FAQ
How long does a bulk monotub grow take from start to harvest?
From spawning to bulk to the first harvest, most grows take 3–5 weeks total. Colonization typically takes 7–14 days, and fruiting takes another 10–14 days after switching to filter disks.
What's the best spawn-to-substrate ratio?
A 1:1 ratio by weight (one 5 lb spawn bag to 5 lb of bulk substrate) is ideal for beginners. Experienced growers sometimes go up to 1:2 or 1:3 to stretch their spawn, but colonization takes longer.
Do I need to pasteurize the bulk substrate?
If you're using a pre-hydrated substrate like Midwest's Select Bulk Substrate, it's already pasteurized and ready to use straight from the bag. No additional preparation is needed.
What temperature is best for a bulk monotub grow?
Most species colonize best between 70–75°F (21–24°C) and fruit best between 65–75°F. Keep your tub away from windows, vents, and heat sources to maintain consistent temperatures.
How many flushes will my monotub produce?
A well-managed bulk monotub typically produces 2–4 flushes. Yields are usually largest on the first flush and gradually decrease as the substrate's nutrients deplete.
What if I see contamination in my tub?
Small areas of green, black, or pink mold indicate contamination. Minor surface contamination can sometimes be cut out, but widespread contamination usually means the grow should be discarded to prevent spreading. Cleanliness during the spawn-to-bulk step is the #1 way to prevent issues.
Ready to Start Your Bulk Grow?
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