A mushroom flush is a term used in cultivation to describe a distinct burst of fruiting bodies emerging from a single substrate block. After the initial harvest, the mycelium—the vegetative body of the fungus—can produce subsequent crops. This repeated cycle is possible because the substrate is not fully exhausted of nutrients. The mycelium needs to recover and be re-triggered to transition from its resting state back into a reproductive phase.
Preparing the Substrate for the Next Harvest
The primary obstacle to a second flush is the significant loss of water that occurs during the development of the first crop. Mushrooms are mostly water, meaning the initial harvest substantially dehydrates the substrate block. To encourage the mycelium to fruit again, the moisture content must be restored, a process often accomplished through rehydration.
This rehydration typically involves “dunking” or “soaking” the substrate block, sometimes referred to as a cake, in clean, cool water. The block is fully submerged and held underwater for a period, with common recommendations ranging from 4 to 12 hours. This period allows the mycelium to absorb the necessary moisture, restoring the internal water pressure required for new mushroom formation.
Before soaking, clean the surface of the substrate thoroughly to remove any old mushroom tissue. This includes carefully picking off small, undeveloped mushrooms called aborts, and any stem stumps left behind from the first harvest. Removing this organic matter prevents contamination from mold or bacteria, which can compete with the recovering mycelium. Once the block has been submerged and drained of excess water, it is returned to the fruiting chamber.
The Expected Timeframe for Pinning
Once the substrate has been rehydrated and returned to its proper fruiting environment, the mycelium needs a period of rest and recovery before the second flush begins. The first sign of the second flush is the appearance of new “pins,” which are the tiny mushroom primordia that form on the substrate surface. Under optimal conditions, these pins typically begin to emerge between 5 and 10 days after the rehydration process is complete.
While 5 to 10 days is a common range for many popular species, the waiting period can sometimes extend up to 14 days. The exact timing is highly dependent on the specific mushroom species being grown, as some varieties are inherently faster or slower in their reproductive cycle. A notable difference in this second round is that the mycelium often directs its remaining energy into fewer, but larger, individual mushrooms compared to the dense canopy of the first flush.
This timeline represents the point at which the mycelium has successfully healed and responded to environmental triggers. The subsequent growth of these pins into mature, harvestable mushrooms takes an additional few days, following the typical development rate for that species.
Environmental Factors Affecting Speed
The speed at which the second flush develops is largely governed by maintaining precise environmental conditions. Controlling three main variables—temperature, humidity, and air exchange—will accelerate or delay the pinning and fruiting timeline. A stable temperature within the species-specific range is needed, but sometimes a small drop in temperature can shock the mycelium and help initiate the pinning process more quickly.
High relative humidity (RH) is necessary, typically between 90–95%, because it prevents the substrate surface from drying out. If the surface moisture evaporates too quickly, the mycelium will not form primordia, which will significantly delay the onset of the second flush. Insufficient humidity forces the mycelium to focus on retaining water rather than committing resources to reproductive growth.
The frequency of air exchange is a third factor that directly influences timing, particularly by managing carbon dioxide (CO2) levels. The mycelium produces CO2 as it metabolizes nutrients, and a buildup of this gas signals to the fungus that it is still underground, which inhibits fruiting. Introducing fresh air multiple times a day keeps CO2 concentrations low, signaling the mycelium to initiate the formation of pins and shorten the time to the second harvest.