Mycelium is the vegetative body of a fungus, a complex network of thread-like structures called hyphae that function as the organism’s root system. This network is responsible for breaking down organic matter in the substrate and absorbing necessary nutrients. Temperature is the primary environmental parameter that dictates the speed and success of this foundational growth process. Since fungi are heterotrophs, they cannot regulate their internal temperature, the ambient conditions directly control their metabolic functions.
The Ideal Temperature Zone for Mycelial Colonization
The process of mycelial colonization requires a consistent and warm environment to maximize the fungus’s metabolic rate. For most cultivated species, categorized as mesophilic fungi, the optimal range is between 70°F and 80°F (21°C and 27°C). Maintaining this temperature encourages rapid expansion across the substrate because the enzymes responsible for breaking down food sources operate at peak efficiency.
This accelerated growth acts as a defense mechanism against contamination. A fast-colonizing mycelium quickly establishes dominance over the substrate, reducing the opportunity for slower-growing competitors like mold and bacteria to take hold. If colonization takes too long due to suboptimal temperatures, the entire batch becomes vulnerable to these contaminants.
The mycelium’s metabolic activity generates internal heat as it breaks down the substrate. In large cultivation volumes, the internal substrate temperature can be 1 to 3 degrees Fahrenheit higher than the surrounding air temperature. Cultivators must monitor this internal temperature to prevent localized “hot spots” that push the fungus past its thermal limits.
Temperature Extremes and Mycelial Stress
Temperatures deviating significantly from the optimal range introduce stress that can halt or permanently damage the fungal organism.
Cold Stress
When conditions are too cold, typically below 60°F (15°C), the mycelium enters a state of metabolic shutdown or dormancy. Enzymes and cellular processes slow significantly, resulting in extremely slow colonization or a complete cessation of growth. The fungus does not necessarily die, but it becomes dormant until temperatures rise.
Heat Stress
Temperatures that are too high pose a serious threat to the mycelium’s survival. Any sustained temperature above 86°F (30°C) can cause heat stress leading to protein denaturation. This process permanently deactivates the enzymes required for nutrient absorption and cell maintenance, often resulting in cellular death and the collapse of the mycelial network.
High heat also favors the proliferation of thermophilic bacteria and molds, which thrive in conditions that stress the fungus. These contaminants quickly outcompete the weakened mycelium, turning the substrate into an unusable mass. The upper thermal limit is a much more rigid and dangerous threshold than the lower limit in a cultivation environment.
The Temperature Shift: From Growth to Fruiting
Once the mycelium has fully colonized its substrate, the organism requires a distinct environmental change to signal the transition from vegetative to reproductive growth. This reproductive stage, known as fruiting, involves the formation of primordia, the initial structures that develop into mature mushrooms. For many species, the most effective trigger is a deliberate drop in temperature.
This temperature reduction typically involves lowering the environment by 5 to 10 degrees Fahrenheit below the optimal colonization temperature. For example, a species that colonized at 75°F might be shocked into fruiting by dropping the temperature to 65°F (18°C). This thermal shift mimics the natural environmental cue of a seasonal change.
If the high colonization temperature is maintained during the fruiting phase, the mycelium often fails to form mushrooms. Instead, it continues its vegetative expansion, a phenomenon known as “overgrowth,” growing over the substrate without initiating the reproductive cycle. Precise temperature control is necessary to ensure the fungus receives the signal to produce fruit bodies.