The idea that mushrooms must grow in complete darkness is a common misunderstanding rooted in their difference from plants. Fungi, including mushrooms, do not rely on light for energy production, unlike green plants that use photosynthesis. While the vegetative stage often thrives best out of sight, light is a necessary environmental signal for the reproductive stage. The role light plays is far more subtle and indirect than it is for most other forms of life.
Fungi: Heterotrophs, Not Phototrophs
Fungi are classified as heterotrophs, meaning they must obtain nutrition from external organic matter. This places them in the same category as animals, contrasting sharply with phototrophs, such as plants, which use sunlight for photosynthesis. Fungi lack chlorophyll, the pigment plants use to capture light energy, making light irrelevant for their sustenance.
The hidden part of the fungus is the mycelium, a vast network of thread-like cells called hyphae that colonizes the substrate. The mycelium operates by secreting digestive enzymes outside its body to break down complex materials like wood or straw. It then absorbs the resulting simple sugars and nutrients, fueling its growth. Since this process relies on chemical breakdown and absorption rather than light conversion, the mycelium is grown in darkness to promote rapid colonization of the substrate.
Light’s Role in Fruiting Body Development
Although light is not a food source, it serves as a trigger for the mushroom’s reproductive phase. Once the mycelium has fully colonized its substrate, a low level of light exposure signals that it has reached the surface or an open area, initiating the formation of the fruiting body. This transition is often referred to as “pinning” or “fruiting,” and without this light cue, some species may not form mushrooms at all.
Light also dictates the shape of the mushroom through phototropism, which is directional growth toward a light source. Adequate, indirect light encourages the formation of a normal cap and stem structure, helping to position the cap correctly for optimal spore dispersal. If light levels are too low or absent during the fruiting stage, the stems can become long and spindly as the fungus “searches” for light. Light, particularly in the blue spectrum (around 400-480 nanometers), is necessary for many species to produce their characteristic pigmentation, impacting the final color of the cap.
Essential Needs: Substrate, Humidity, and Air Exchange
Environmental factors like substrate, humidity, and air quality are crucial for a mushroom’s survival and growth. The substrate is the material the fungus consumes, such as wood chips, straw, or manure, providing all the energy and nutrients necessary for the organism’s life cycle. The selection and preparation of this substrate are foundational steps of successful cultivation.
Humidity is a primary factor, as mushrooms are composed of about 80 to 90% water. The air must maintain a high relative humidity, typically 85 to 95% during the fruiting stage, to prevent the developing mushrooms from drying out. Low humidity causes stunted growth, cracked caps, or the premature abortion of the fruiting bodies.
Fresh air exchange (FAE) is equally important because fungi take in oxygen and release carbon dioxide as they respire. If carbon dioxide levels build up excessively in an enclosed space, it inhibits proper growth. High carbon dioxide concentrations can result in malformed mushrooms with thick, elongated stems and small, underdeveloped caps. Additionally, temperature must be kept within a specific range, often between 65–75°F (18–24°C) for many species, to support both mycelial growth and fruiting.