A “flush” in mushroom cultivation refers to a distinct, synchronized wave of fruiting bodies emerging from the substrate. Since mushrooms are approximately 90% water by weight, the first harvest significantly depletes the substrate’s moisture content. Rehydration, often called “flushing,” is necessary to replenish this lost water, triggering the mycelium to initiate the next reproductive cycle. A properly managed substrate can often produce two to four productive flushes, maximizing the yield from a single block.
Preparing the Substrate for Rehydration
The first action after harvesting is to thoroughly clean the substrate surface. This involves carefully removing all biological material, including mature mushrooms, small developing pins, and aborted fruits. These leftover pieces of fungal tissue can quickly degrade and become potential entry points for contamination once the substrate is saturated.
Use a clean technique, such as gently twisting and pulling the mushrooms at the base or cutting them with sterilized scissors, to minimize damage to the mycelial mat. Inspect the surface closely for any shriveled or discolored remnants and remove them without tearing the underlying mycelium. Leaving the substrate block to air dry for 24 to 48 hours can help the mycelium recover slightly before rehydration.
The Rehydration Soak
The core of flushing involves reintroducing the water lost during the first growth cycle. The most common method for solid blocks is the “dunk,” where the entire substrate is submerged in water. A more precise approach is to weigh the block after harvest and compare it to its original weight, indicating the exact amount of water that needs replacement.
You should use non-chlorinated or distilled water, ideally at a cool or room temperature, and submerge the block completely in a clean container. To prevent the substrate from floating, use a clean weight to keep it fully underwater. The optimal soaking duration is generally between 4 and 12 hours; soaking longer than this can suffocate the mycelium by depriving it of oxygen, increasing the risk of bacterial contamination. After the soak, the block must be thoroughly drained for 30 to 60 minutes before being returned to the fruiting chamber.
Restoring Environmental Conditions
After the substrate has been rehydrated and fully drained, the environmental conditions must be immediately reset to encourage the next round of pinning. The mycelium needs a signal to transition from water absorption back to reproductive growth, achieved by restoring specific humidity and air exchange levels. Humidity should be maintained between 85% and 95% to prevent the surface of the block from drying out, which is required for new pin formation.
This freshly saturated block is now highly vulnerable to mold and bacterial growth, making fresh air exchange (FAE) a priority. FAE removes excess carbon dioxide that builds up around the block and provides the oxygen required for fruiting. The temperature should be held steady within the species’ preferred fruiting range, which, combined with high humidity and air exchange, mimics the natural conditions that trigger new mushroom development.
Recognizing Substrate Exhaustion
Each subsequent flush extracts more of the remaining nutrients from the substrate, and the yields will gradually decrease. The first flush often accounts for 60% to 80% of the total potential biomass, with later flushes producing significantly less. Visual and performance cues indicate when the substrate is no longer viable for further flushing.
You may notice severe shrinkage of the substrate block, which pulls away dramatically from the sides of its container, reflecting the consumption of organic material. More importantly, the time between flushes will become significantly longer, or the mycelium may fail to produce any pins at all after rehydration. The most definitive sign of exhaustion is the onset of widespread contamination, such as green mold or bacterial blotch, which the weakened mycelium can no longer fight off.