How to Dunk and Re-flush a Monotub: Step-by-Step Guide

About 1 hour active work, plus colonization time

Every flush pulls water out of the substrate. By the second or third flush, the cake has lost so much moisture that pinning slows down or stops entirely, and the next flush is a fraction of the first. Dunking is the standard fix: pull the colonized cake out of the tub, submerge it in cold water for several hours, drain, and put it back. The water rehydrates the substrate to near-original field capacity, the cold acts as a thermal shock that triggers pinning, and the soak flushes out metabolic byproducts that have built up since the last flush.

Done right, a single dunk between the first and second flush recovers 60 to 80 percent of the first flush's yield from the second flush. A second dunk before the third flush typically recovers another 30 to 50 percent of that. After the third or fourth flush the substrate is exhausted and dunking stops helping. This tutorial covers the full procedure from picking the right moment, to weighting the cake down without crushing it, to draining and resetting fruiting conditions for the next flush.

What you'll need

Materials

Cold non-chlorinated water, ~5 to 10 L (filtered, distilled, or 24-hour-aged tap water)
Optional: 1 ice tray cube per quart of water, to hold the soak below 65°F
70% or higher isopropyl alcohol, plus a spray bottle
Filtered or pre-boiled water, for re-misting the tub after the cake goes back
Nitrile or latex gloves
Clean paper towels or lint-free wipes

Equipment

Container large enough to fully submerge the cake (a clean second monotub or a large stainless tote)
1 to 3 clean weights (gallon jugs filled with water, or sealed quart jars), for holding the cake under
Drying rack, baking rack, or angled surface, for drainage after the dunk
Reliable thermometer (water and substrate both)
Clean kitchen scale, optional, for tracking cake weight before and after
Reliable timer or phone alarm

Step 1: Decide whether dunking is the right call

Dunking is not free. Submerging the cake in open water is the single dirtiest moment in a monotub run. Every contaminant spore on the outside of the tub, on your hands, or in the water itself gets a free ride into a substrate that has lost its competitive edge. That is fine if you are dunking a healthy cake, because the mycelium is still in the lead. It is catastrophic if you are dunking a cake that already has a small Trichoderma patch, because the dunk evens the playing field and the green mold takes over in days.

Dunk if all of these are true:

The first (or second) flush is fully harvested. No partially-grown fruits remaining; pull every visible pin and tiny aborts before dunking.
The cake feels noticeably lighter than it did after the previous flush. If you weighed it before, expect to see 25 to 40 percent water loss.
The substrate surface looks visibly drier; the bright white mycelial mat may have a slight tan or dull cast.
You have not seen new pins or hyphal knots forming for the past 5 to 10 days, despite continued misting and FAE.
The cake has no green, blue, gray, or pink discoloration on its surface, and no slimy or sour-smelling patches. Healthy mycelium only.
You are dunking before flush 2, 3, or 4. After flush 4 the substrate is genuinely exhausted and dunking will not recover meaningful yield.

Do not dunk if:

Any contamination is visible. Green spots, gray cobwebbing, black mold, sour smell, slimy patches, orange rings: skip the dunk and either bag the tub for disposal or attempt isolated salvage. The dunk will not fix contamination; it will spread it.
The cake is still pinning or has live pins. Submerging live pins drowns them. Wait for the flush to finish.
The cake is genuinely exhausted (post-flush 4 or 5). Yields after the third dunk drop below 10 percent of flush 1; the time and contamination risk is not worth it. Compost the cake and start a fresh tub.

Spray vs dunk: when to skip the dunk

For mildly dehydrated cakes (loss under 15 percent), a heavy mist of the substrate surface and tub walls, repeated for two days, often pulls a second flush without the contamination risk of a full dunk. Try the spray approach first if the cake still feels heavy. Dunk when spraying alone has not produced new pins after 5 to 7 days of misting and FAE.

One more check before you start: if the cake has been sitting in fruiting conditions for more than 14 days since the last flush without dunking, the resident bacterial load has been climbing the entire time. Even healthy-looking cakes get pushed past the point where dunking helps. The sweet spot is 7 to 12 days after the previous flush finished.

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