Mushroom substrate is the nutrient-rich material, such as sawdust, straw, or grain, that the fungal network, or mycelium, colonizes and uses as its food source. This block supports the entire mushroom life cycle, but the process of fruiting causes a significant loss of moisture. Since mushrooms are over 90% water, the first harvest, or flush, severely depletes the substrate’s hydration levels, often causing a loss of 15–30% of the block’s total water content. Rehydration is necessary between flushes to replenish this lost moisture, allowing the mycelium to recover, become metabolically active, and initiate the growth of a subsequent crop.
Preparing the Substrate Block
Before introducing any water, the substrate block requires preparation to ensure a successful and contamination-free second flush. First, thoroughly clean the surface by removing all residual mushroom tissue, including small stems and aborted pins—tiny mushrooms that stopped developing.
These remnants are prone to decay and can become vectors for mold or bacteria during the rehydration process. Use a clean tool, such as sterilized tweezers or a spoon, to gently scrape or twist off any remaining material, leaving a clean mycelial surface.
Following the cleaning, inspect the block for signs of contamination. Discoloration like green (often Trichoderma mold), pink, or dark brown patches, or any strong, unpleasant off-odors, indicate contamination and require the block to be discarded. Finally, a weight assessment confirms the block’s dryness; a significantly lighter block compared to its pre-flush weight confirms the need for rehydration.
The Substrate Dunking Method
The most effective method for rehydrating a depleted substrate block is total submersion, often referred to as “dunking.” This technique ensures the water is absorbed evenly throughout the dense block, fully re-saturating the mycelium. Select a clean container large enough to completely hold and cover the entire substrate block with water.
The water used should be potable tap water, ideally at room temperature or slightly cool. Avoid using very hot or very cold water, as extreme temperatures can shock the mycelium or inhibit its recovery. While some growers use cold water for a potential “cold shock” to trigger fruiting, room-temperature water is sufficient for rehydration.
Place the substrate block into the water; it will likely float due to air pockets created during the first flush. Weigh the block down to ensure complete submersion and uniform absorption across all surfaces. A clean, sealed plastic bag filled with water, a ceramic plate, or a sanitized brick can be used as a weight.
The soaking duration generally ranges from 4 to 24 hours, depending on the substrate’s density and how dry it feels. Denser substrates like sawdust blocks may require closer to the maximum time, while lighter substrates like coir or smaller blocks may only need 6 to 12 hours. A good rule of thumb is to allow the block to absorb water for approximately one hour per inch of substrate thickness.
Post-Rehydration Care and Subsequent Flushes
After the specified soaking time, the substrate block must be carefully removed from the dunking container to allow excess water to drain completely. This drainage is a crucial step because standing or pooled water can create anaerobic conditions, which suffocate the mycelium and attract contaminants. The block should be placed on a clean surface or rack, angled slightly, and allowed to drain for approximately 15 to 30 minutes.
Once the dripping has stopped, the rehydrated block is ready to be returned to its fruiting chamber or terrarium. It is important to re-establish the same environmental conditions that successfully produced the first flush, specifically high humidity (85–95%) and the appropriate temperature. Consistent fresh air exchange is also necessary to stimulate the formation of new mushroom primordia, or pins.
With the moisture replenished and the environment stabilized, the mycelium will begin to prepare for the next round of growth. Growers can typically expect the second flush to begin pinning approximately 7 to 14 days after the rehydration process is complete. While the number of mushrooms in the second flush may be slightly lower than the first, the individual mushrooms are often larger and denser.