Should You Dunk a Bulk Monotub Grow?

Can you dunk a monotub or bulk substrate grow? The short answer is yes, but treat it as a recovery tool, not a routine step. A well-prepared bulk substrate should carry you through multiple flushes without any soaking. Dunking earns its place when a tub has clearly dried out.

This guide combines everything we've learned over the years about dunking, from classic PF Tek cakes to full monotubs: when it helps, when it isn't worth the risk, and the gentlest way to do it.

What Is Dunking?

Dunking is the process of soaking mushroom substrate in water to rehydrate the mycelium after moisture has been depleted by fruiting. Mushrooms are mostly water, and every flush pulls a large amount of it out of the substrate. Each time you dunk, you're mimicking a heavy rainstorm that saturates the ground, which is exactly why you see mushrooms pop up across a lawn a few days after a summer downpour.

Dunking PF Tek Cakes: Yes, Every Time

With BRF (PF Tek) jars, dunking is standard practice and we recommend it. The cakes are small, compact, and easy to handle without damaging the mycelium.

• After birthing: Growers disagree on whether to dunk right after birthing the cakes. Our advice is to dunk right away. A freshly birthed cake is starving for water, and we've consistently seen the best results doing so. If a few nice pins formed in the jar, pick them off and save them.
• Between flushes: Dunk your cakes after each flush to restore moisture before the next one.

New to the method? Read our history of PF Tek and how half-pint jars compare to grain jars.

Can You Dunk a Bulk Mushroom Grow?

Bulk grows, where colonized grain spawn is broken up and mixed into a bulk substrate like coco coir, manure, or casing mix, are a different story. The result is a much larger, heavier, and more fragile "cake," and it doesn't handle like a PF Tek cake.

Here's what our own trials have shown:

• The gains are modest. Across multiple trials, dunking a healthy bulk casing only increased yield by about 10–15% on average.
• The mycelial network is fragile. Lifting or moving the substrate can tear the mycelium and set growth back 5–7 days, or stall it completely.
• Contamination risk goes up. Exposing a large grow to open water and handling increases the chance of bacteria and mold, especially if the dunk container isn't clean.
• Good substrate already holds enough water. Our casing mixes are designed to hold enough moisture for at least three solid flushes without any dunking.

So for a healthy tub, skip the dunk and focus on proper misting, fresh air exchange, and temperature control. Where rehydration does make sense is when the substrate has genuinely dried out, which can happen during hot summer months, in low humidity, with excessive fresh air exchange, or after one or more heavy flushes.

When Rehydrating a Monotub May Help

• Substrate pulling away from the tub walls: If your substrate shrinks significantly and gaps form around the edges, it has lost a lot of moisture.
• Dry-looking surface: A surface that looks dull, cracked, or overly dry may benefit from additional hydration.
• Small or stalled mushrooms: If pins form but stop developing or stay undersized, low substrate moisture can be the cause. (If the tub hasn't pinned at all, see why your monotub isn't pinning first.)
• After a very heavy flush: Large flushes consume a tremendous amount of water from the substrate.
• Excessive airflow or warm temperatures: High FAE and elevated temperatures can dry out a monotub faster than expected. Our summer growing tips cover how to manage this.

Try Gentler Methods First

Before a full soak, these less aggressive techniques often solve the problem on their own:

• Heavy surface misting
• Bottom watering
• Injecting water near dry edges
• Perimeter soaking
• Temporarily reducing fresh air exchange

In many cases, improving humidity and dialing back airflow fixes moisture problems before a dunk becomes necessary. A hygrometer takes the guesswork out of it.

A Safer Bulk Rehydration Method

If the substrate is clearly dry, the method we've personally had the most success with rehydrates it directly inside the monotub instead of removing and handling the cake:

1. Slowly add clean water into the monotub until the substrate begins to float slightly.
2. Place a second monotub or tray of similar size on top of the substrate.
3. Add a small amount of weight to keep the substrate submerged.
4. Let it soak for approximately 1–2 hours.
5. Carefully drain the excess water from the tub.

This restores internal moisture without excessively disturbing the mycelial network. Be gentle and avoid breaking the substrate apart.

Risks of Dunking Bulk Substrate

• Contamination: Standing water and extra handling can introduce bacteria or mold spores. Know what to look for with our contamination identification playbook.
• Damaging the substrate: Bulk substrate cakes are heavier and more fragile than PF Tek cakes.
• Over-saturation: Too much water can create anaerobic conditions that slow mycelium recovery.
• Delayed recovery: The substrate may need several days to recover before pinning resumes.

Final Thoughts

Dunking PF Tek cakes is a proven, low-risk way to get more flushes, and we recommend it every time. For bulk grows and monotubs, it's possible but rarely necessary: a well-hydrated substrate in a dialed-in environment will usually produce multiple flushes on its own. If a tub has noticeably dried out, try the gentle methods above first, and use a controlled soak before you retire it.

For more ways to get the most from every tub, see our top 10 ways to increase your mushroom yield.

Happy growing!
— The Midwest Grow Kits Team