The successful first harvest, or “flush,” marks a significant achievement in mushroom cultivation. A flush represents a single, concentrated cycle of growth where the mycelium produces a crop of fruiting bodies that mature and are harvested around the same time. The first flush significantly depletes the water and nutrients stored within the substrate block. Therefore, the block must be prepared immediately through careful steps designed to rehydrate and rejuvenate the mycelial network, setting the stage for the next round of growth.
Clearing the Growing Surface
The immediate task following the harvest is to ensure the substrate surface is completely clean of any residual organic material. Carefully remove all spent mushroom tissue, often called “stumps,” which remain attached to the substrate surface. These leftover tissues are dead organic matter that no longer contributes to the mycelial network.
Any aborted pins—tiny mushrooms that stopped developing—should also be picked off the surface. Failing to remove this decaying material creates potential entry points for airborne contaminants, particularly molds and bacteria. A clean surface prevents rot and allows the mycelium to heal and prioritize new growth sites for the next flush.
Rehydrating the Block
The single most important step for achieving a second flush is restoring the substrate’s moisture content, a process commonly called “dunking.” Since mushrooms are over 90% water, the first flush draws out a substantial amount of stored moisture. This depletion halts the mycelium’s ability to produce more fruiting bodies, requiring rehydration to restore the initial field capacity.
To perform the dunk, place the substrate block into a clean container and cover it completely with water. Use filtered, distilled, or boiled-then-cooled water to avoid introducing chlorine or other tap water contaminants. The water should be cool, often at room temperature or slightly cooler, as this temperature change can sometimes act as a subtle shock that encourages new pinning.
Because the dry substrate tends to float, use a clean, heavy object to keep the block fully submerged. The ideal soaking duration typically ranges from four to twelve hours, though thick blocks may benefit from longer soaks up to twenty-four hours. Soaking for too long can drown the mycelium and increase the risk of anaerobic bacteria growth, so it is safer to err on the side of a shorter duration. Once soaking is complete, remove the block and allow it to drain thoroughly for thirty to sixty minutes before returning it to the fruiting chamber.
Environmental Conditions for Subsequent Flushes
After the rehydration process is complete, the block must be returned to the environmental conditions that trigger fruiting. This involves re-establishing high relative humidity, between 90% and 95%, which encourages the formation of new pinheads. Maintaining this humidity often requires frequent misting of the chamber walls rather than the substrate itself, especially on blocks with a casing layer.
The necessary fresh air exchange (FAE) must also be reintroduced, ensuring that carbon dioxide levels—which naturally accumulate as the mycelium metabolizes—remain low. While the mycelium recovers from the dunk, it may take several days to a week before new pins begin to form. Subsequent flushes require patience, as the mycelium has spent a significant amount of its stored energy.
If a casing layer was used, the dunking process may have disturbed or compacted it, so a light misting or gentle resurfacing can help restore proper surface conditions. Temperature should be kept consistent with the parameters used for the initial flush, as this combination of temperature, high humidity, and low CO2 signals the mycelium to begin the next reproductive cycle.
Knowing When to Retire the Substrate
While it is common to achieve a second or even a third flush, the substrate has a finite lifespan and a diminishing return on effort. With each subsequent harvest, the yield will be lower, as the mycelium has consumed a significant portion of the available nutrients. Consider retirement when the flush yield becomes noticeably small or the time between flushes increases significantly, indicating exhausted nutrient reserves.
Contamination is a clear signal that the substrate must be retired immediately to prevent the spread of mold spores to other grows. Green molds, such as Trichoderma (bright green patches), or foul smells indicating bacterial contamination, mean the block is compromised. Once retired, the spent substrate can be safely disposed of by burying it outdoors or adding it to a compost pile. This allows the mycelium to continue decomposition while enriching your soil with residual nutrients.