Do Mushrooms Grow in the Dark?

Mushrooms can absolutely grow in the dark, and many do so naturally. Fungi are not plants; they do not photosynthesize and have no need for light as an energy source. But light is far from irrelevant to their biology. Many mushroom species use light as a developmental signal, particularly for triggering the formation of the fruiting bodies we recognize as mushrooms, and the relationship between fungi and light turns out to be surprisingly rich once you look past the simple yes-or-no question.

Why Fungi Do Not Need Light the Way Plants Do

The fundamental reason mushrooms can grow without light is that fungi feed on organic matter rather than making their own food from sunlight. All true fungi are heterotrophs, meaning they obtain both energy and carbon from existing organic material like dead wood, leaf litter, dung, or living hosts.¹ A mushroom growing on a rotting log is digesting that log externally, secreting enzymes and absorbing nutrients. This works just as well in pitch darkness as it does in daylight.

Researchers have found that some fungi do possess light-harvesting molecular machinery, including proton pumps fueled by carotenoid pigments, which means they can extract a small amount of energy from visible light. But they lack the carbon-fixing apparatus that would make them self-sufficient the way a plant is. At best, they are “photoheterotrophs,” organisms that can use light for a bit of extra energy while still requiring organic carbon to survive.¹ In practical terms, this means light is a bonus, not a requirement. Fungi can grow in the presence or absence of both oxygen and visible light.

How Light Shapes Mushroom Development

Even though fungi do not need light for energy, many species use it as an environmental cue. The distinction matters: the threadlike network of mycelium that makes up the main body of a fungus spreads happily through dark soil or wood, but when conditions are right for reproduction, light often serves as the trigger that tells the organism to produce a fruiting body, the cap-and-stem structure most people call a mushroom.

This has been studied in detail with oyster mushrooms. Researchers demonstrated that fruiting induction in the oyster mushroom is solely dependent on the illumination regime, and that it occurs well before the available nutrients in the growing medium are used up.² In other words, it is not starvation that triggers mushroom formation in this species; it is light exposure. Without light, the mycelium keeps growing vegetatively but does not form mushrooms.

Blue light appears to be the most effective wavelength for stimulating fruiting body development in edible mushrooms. Work on shiitake cultivation showed that blue light irradiation produced the highest yields, and studies on enoki mushrooms identified specific photoreceptor proteins, called the white collar complex, that detect blue light and regulate both mycelial growth and fruiting.³⁴ These blue-light receptors were first identified in the bread mold Neurospora crassa and have since been found across a wide range of fungal species.⁵

This light-sensing ability is ancient. Phylogenetic analysis of early-diverging fungal lineages has shown that a light-perception pathway involving specialized cellular structures was present in the common ancestor of all fungi.⁶ Fungi have been detecting and responding to light for hundreds of millions of years, which suggests that even though they do not need light for food, light provides information valuable enough that evolution has maintained the sensory hardware across the entire kingdom.

Fungi That Thrive in Total Darkness

If you want proof that fungi can flourish without a single photon, look underground. Caves and abandoned mines host diverse fungal communities growing in permanent darkness. A comprehensive analysis of fungi isolated from cave environments worldwide documented 445 species across 394 genera, with the majority belonging to Ascomycota. The most species-rich groups were Penicillium and Aspergillus, both common molds that clearly have no trouble colonizing lightless habitats.⁷

Abandoned mines offer a particularly vivid picture. In a former iron ore mine in Minnesota, researchers found abundant fungal growth in complete darkness at cool temperatures, with water seeping through the tunnels. Black rope-like structures called rhizomorphs, produced by a honey mushroom species, grew on old wooden timbers left behind from mining operations and extended across bare mine floors for distances of more than twenty feet between wood sources.⁸ The fungus was perfectly capable of navigating and feeding in an environment that had not seen daylight in decades. These conditions are extreme by human standards but routine for fungi, which evolved to exploit organic matter wherever it accumulates.

Truffles and the Fully Underground Life Cycle

Some fungi have gone a step further than merely tolerating darkness: they have adapted to complete their entire reproductive cycle underground. Truffles, the prized culinary fungi, produce their fruiting bodies entirely below the soil surface. Unlike a typical mushroom that pushes up into the air and releases spores on the wind, truffles have lost the ability to shoot spores into the air altogether.⁹

This creates a dispersal problem. If your spores are sealed inside a fleshy lump buried in soil, wind is useless. Truffles solved this by evolving strong aromas that attract animals. Squirrels, voles, wild boar, and even certain birds dig up truffles, eat them, and later deposit the spores in new locations through their droppings. The fungus essentially outsources its dispersal to the animal kingdom.¹⁰ Light plays no role at any stage of this process. The truffle grows in darkness, matures in darkness, and relies on scent rather than visual signals to complete its life cycle.

What Happens When You Grow Mushrooms in the Dark Commercially

The common white button mushroom, the kind you find in most grocery stores, is typically grown in dark or dimly lit rooms. This is not an accident or a myth. Button mushrooms belong to the species Agaricus bisporus, which is relatively indifferent to light when it comes to fruiting. Commercial growers cultivate them on composted substrate in climate-controlled rooms where temperature, humidity, and carbon dioxide levels are carefully managed, and light is largely a non-factor.

But other cultivated species tell a different story. For shiitake mushrooms, blue light exposure significantly boosted fruiting body production, with yields reaching roughly 555 grams per batch for one cultivar and about 1,090 grams for another under blue light, well above what darker conditions produced.³ Oyster mushrooms are similarly light-responsive; growers typically provide several hours of light per day during the fruiting phase to get good yields and normal mushroom shapes. Without it, oyster mushrooms may produce long, spindly stems with undersized caps as the organism “searches” for light it expects to find at the soil surface.

So the practical answer for home growers depends on the species. If you are growing button mushrooms, you can do it in a closet. If you are growing oyster mushrooms, shiitake, or most other gourmet species, plan on providing some indirect light during the fruiting stage, roughly the amount you would get near a window with no direct sun. The mycelial colonization phase, when the white threads are spreading through your substrate, works fine in the dark regardless of species.

Spore Release and the Rhythm of Light and Dark

Even after a mushroom has formed, light and darkness continue to influence its behavior. Many fungi release their spores at specific times of day rather than continuously, and the light-dark cycle is one of the environmental factors that shapes this timing.¹¹ For some species, darkness actually promotes spore discharge. Research on one wood-decay fungus found that discharge rates were highest under conditions of moderate temperature, high humidity, and either low light intensity or darkness. Direct illumination disrupted the normal discharge pattern.¹²

This makes ecological sense. In a forest understory, nighttime air is typically more humid and calmer than daytime air. Releasing spores at night can help them travel through moist air that keeps them viable rather than drying them out. The relationship between fungi and darkness is not just tolerance; in some cases, darkness is the preferred operating condition for critical reproductive steps.

Mushrooms That Make Their Own Light

Perhaps the most surprising twist in the fungal relationship with darkness involves mushrooms that glow in the dark. Around a hundred species of fungi are bioluminescent, producing their own greenish light through a chemical reaction. Researchers studying one such species, Neonothopanus gardneri from Brazil, discovered that its bioluminescence follows a circadian rhythm, with the glow peaking about ten hours after the transition from light to dark and cycling on roughly a 22-hour period.¹³

The rhythm persists at different temperatures, a hallmark of a true internal clock rather than a simple response to temperature changes. Testing at different temperatures showed the period barely shifted, with a temperature sensitivity measure of just 1.04, confirming genuine circadian control. The working hypothesis is that circadian-controlled bioluminescence evolved because glowing at night, when the light is most visible, attracts insects. Those insects then help disperse the fungus’s spores, which is especially useful on the forest floor where wind movement is minimal.¹³ So here is a fungus that not only thrives in darkness but has evolved to exploit it, turning the nighttime into an advertisement.

Mushrooms, UV Light, and Vitamin D

One area where light exposure matters for mushrooms in a way that directly affects you is vitamin D. Mushrooms contain ergosterol, a compound in their cell membranes that converts to vitamin D₂ when exposed to ultraviolet light, in a process remarkably similar to how human skin produces vitamin D₃ from sunlight.¹⁴ Mushrooms are rich in ergosterol and a related compound, ergosta-5,7-dienol, which can be partially converted to vitamins D₂ and D₄ through UV exposure.¹⁵

When common mushroom species are exposed to UV radiation, whether from sunlight or a UV lamp, they generate nutritionally meaningful amounts of vitamin D.¹⁶ This is why some commercial mushrooms are now sold with “UV-treated” labels and elevated vitamin D content. Conversely, mushrooms grown entirely in the dark, as button mushrooms usually are, contain very little vitamin D. If you buy standard white button mushrooms and want to boost their vitamin D, you can place them gill-side up in direct sunlight for 15 to 30 minutes. The conversion happens even after harvest because the ergosterol is still present in the tissue.

This is a case where the dark-growing habit of commercial mushrooms has a real nutritional consequence. The mushroom itself grows perfectly well without light, but the human eating it misses out on a valuable nutrient unless UV exposure is provided at some point.

How Fungal Networks Navigate Without Eyes

Given that so much of a fungus’s life unfolds underground, in rotting wood, or inside other organisms, fungi have evolved sophisticated ways of navigating and making decisions without any visual information. Mycelial networks are not passive; they actively explore their surroundings, growing toward nutrient sources and retreating from poor ones.

Experiments with cord-forming fungi have revealed that these networks display a form of ecological memory. When researchers introduced a new food source near a fungal network, the mycelium later showed greater regrowth from the side that had originally faced the new resource, implying the network retained some directional “memory” of where nutrients had been.¹⁷ This kind of behavior requires no light at all. The fungus uses chemical gradients, moisture cues, and its own internal signaling to make allocation decisions about where to grow. Light, when present, is simply one more signal layered onto an already sophisticated toolkit.

The honey mushroom rhizomorphs stretching across the mine floor in Minnesota illustrate this ability dramatically. Those structures grew more than twenty feet across bare rock between wood food sources in permanent darkness, somehow locating the next piece of timber without any visual information.⁸ Fungi were solving complex spatial problems in lightless environments long before animals evolved eyes.

When Darkness Goes Wrong for Mushroom Growers

If you grow mushrooms at home, the most common mistake related to light is not understanding which phase of growth you are in. During colonization, when mycelium is spreading through your bag of grain or sawdust, darkness is fine for essentially all species. The problems start during fruiting. Oyster mushrooms grown in total darkness tend to produce elongated stems and tiny, malformed caps. The mushroom is stretching upward because, in nature, the transition from dark soil to open air is accompanied by a change in light levels. Without that signal, the fruiting body does not develop normally.

The fix is simple and low-tech. You do not need grow lights or anything elaborate. Ambient room light or a few hours of indirect daylight is enough for most species. The light does not need to be strong; it just needs to be present. Some growers use a standard fluorescent or LED light on a timer for about twelve hours a day during the fruiting phase, roughly mimicking a natural day-night cycle. Blue-spectrum light is ideal based on the research discussed earlier, but in practice, ordinary white light from any household bulb contains enough blue wavelengths to do the job.

The one species where this barely matters is the button mushroom. If you are growing Agaricus bisporus, you can keep the lights off from start to finish and still harvest normal-looking mushrooms. This is part of why button mushrooms became the dominant commercially grown species worldwide: they are cheap to produce, and dark warehouse conditions are cheap to maintain.