Mushrooms do not perform photosynthesis, meaning light is not a food source; they obtain nutrients from the substrate (wood or compost). This often leads to the mistaken belief that they require no light at all. However, light serves an important function in their life cycle, acting as an environmental signal rather than a metabolic one. The required hours of light depend entirely on the specific species and its stage of development.
Light’s Function in Fungal Biology
Light operates as a sophisticated environmental cue for many fungal species, triggering specific developmental phases. During the initial colonization phase, when the mycelium spreads through the substrate, light is generally unnecessary and can even be detrimental by causing premature fruiting bodies to form. Most growers keep the mycelium in complete darkness during this stage to encourage rapid colonization.
The requirement for light begins when the fungus shifts from vegetative growth to reproduction, a process called fruiting. Light exposure signals that the mycelium has reached the surface or a break in the substrate, indicating suitable conditions for forming the mushroom body. This signal initiates the formation of primordia, the tiny structures that develop into mature mushrooms. Fungi possess specialized photoreceptors that absorb light to coordinate the transition to fruit body formation.
Necessary Light Intensity and Spectrum
Mushrooms do not require the high intensity of direct sunlight, which can quickly dry out and damage the fruiting bodies. Instead, they thrive under low-intensity, diffuse light. Optimal light intensity for most cultivated species ranges between 100 and 1,000 lux during the fruiting phase.
The specific color, or spectrum, of the light is more important than its sheer brightness. The blue light spectrum (wavelengths between 400 and 500 nm) is the most effective trigger for fruiting in common species like Oyster and Shiitake mushrooms. This blue light is detected by fungal photoreceptors, stimulating the formation of the cap and stem and influencing pigmentation. Artificial sources like cool white LED lights or fluorescent bulbs (6000K to 6500K) provide a suitable balance that includes the necessary blue wavelengths.
Determining the Optimal Photoperiod
The duration of light exposure, known as the photoperiod, mimics the natural day-night cycle. For the majority of gourmet and medicinal mushrooms, a timed cycle is preferred over continuous light exposure. A common photoperiod is 12 hours of light followed by 12 hours of darkness (a 12/12 cycle), which provides a consistent signal for development. Other species may benefit from slightly longer light exposure, such as a 16 hours on and 8 hours off schedule, but consistency is a key factor in preventing stress on the organism.
Species Exceptions
Species like Button mushrooms (Agaricus bisporus) and Enoki are cultivated in near or complete darkness. Light can inhibit their growth or cause an undesirable change in morphology.
Troubleshooting Poor Lighting Conditions
When light conditions are inadequate, mushrooms often display physical symptoms. The most common indicator of insufficient light is the development of long, thin stems (stipes) with small, underdeveloped caps. This response is known as phototropism, where the fungus grows elongatedly toward the perceived light source to disperse its spores.
Another sign of poor lighting is the formation of pale fruit bodies or the abortion of pinheads, where the young mushrooms fail to develop fully. Adjustments include moving the light source closer to the growing block to increase the lux level or switching to a consistent light timer. If a strong light source causes the fruit bodies to dry out, moving the light farther away mitigates the risk of desiccation and overheating.