Walk through a temperate forest on a moonless night, and you might spot an eerie green glow emanating from a rotting log. This isn't magic or swamp gas. It's foxfire—the ghostly light produced by bioluminescent fungi that have fascinated humans since Aristotle first wrote about glowing wood over two thousand years ago.

The Glowing Mushrooms Among Us

More than 100 species of mushrooms worldwide can produce their own light. In North American temperate forests, three species do most of the glowing. The honey mushroom (Armillaria mellea) sends up clusters of tan caps in late summer. The jack-o-lantern (Omphalotus olearius) flaunts bright orange caps up to six inches wide. And the bitter oyster (Panellus stipticus) forms small, shelf-like clusters on dead wood.

These glowing fungi belong to three separate evolutionary lineages. They developed bioluminescence independently, yet remarkably, all three use the same chemical system to make light. This suggests that producing light offers real survival advantages—enough to evolve multiple times.

The greatest diversity of bioluminescent fungi lives in tropical forests. But temperate regions host their fair share. New Jersey's Pine Barrens and oak-hickory forests are particularly good places to spot foxfire. So are the oak forests of Costa Rica's high elevations.

How Fungi Make Cold Light

The light comes from a chemical reaction as elegant as it is efficient. An enzyme called luciferase catalyzes the oxidation of a molecule called luciferin. When oxygen joins the party, the reaction produces oxyluciferin and releases energy as visible light.

This process creates what scientists call "cold light." Nearly 96 percent of the energy becomes light, with only four percent wasted as heat. Compare that to an old incandescent bulb, which dumps 90 percent of its energy as heat. It's one of nature's most efficient light sources.

The light itself glows yellowish-green or bluish-green, with wavelengths between 520 and 530 nanometers. That's the same color range where human eyes are most sensitive at night. Coincidence? Probably not, as we'll see.

Here's something surprising: these fungi glow constantly, 24 hours a day. We only notice at night because daylight drowns out their faint luminescence. Some species glow from their caps or gills. Others light up only their stems. In many species, the mushroom itself stays dark while the mycelium—those thread-like filaments spreading through rotting wood—produces the glow.

Why Bother Glowing?

For decades, scientists debated why fungi would expend energy making light. Three main hypotheses emerged. Maybe bioluminescence was just a metabolic byproduct with no real purpose. Perhaps it deterred animals that eat fungi, or attracted predators that would eat those fungivores. Or maybe it served as a beacon to attract insects that could spread spores.

The evidence now strongly supports that third option. In 2015, researchers in Costa Rica's high-elevation oak forests ran a clever experiment. They set up green LED lights mimicking fungal bioluminescence alongside dark control traps. The green lights attracted significantly more insects—specifically flies capable of carrying and dispersing fungal spores.

Similar studies in Florida and Brazil found the same pattern. Insects, particularly dipteran flies, swarm to the glow. They land on the luminescent gills or caps, picking up spores on their bodies. When they fly to another location, they deliver those spores to new territory.

This makes evolutionary sense. Unlike plants, fungi can't rely on wind or animals eating their fruit to spread seeds. Their spores are microscopic and easily overlooked. A glowing advertisement in the dark forest becomes a powerful attractant. The wavelength these fungi produce—that yellowish-green light—happens to be exactly what insect eyes detect best at night.

Forest Recyclers With a Light Show

Beyond their luminous novelty, these fungi play crucial roles in forest ecosystems. All known bioluminescent species are white rot fungi. That means they can break down both cellulose and lignin—the tough, complex molecules that give wood its strength.

This ability makes them essential recyclers. Dead trees and fallen logs contain locked-up nutrients. As bioluminescent fungi digest this wood, they release nitrogen, phosphorus, and other elements back into the soil. Other organisms can then use these nutrients. Without wood decomposers, forests would become choked with dead material.

The connection between lignin breakdown and light production might be more than coincidental. Breaking down lignin creates toxic peroxide compounds. Some scientists think the bioluminescent reaction helps detoxify these peroxides, making light a beneficial side effect of chemical cleanup.

These fungi prefer humid environments and decaying wood. You'll find them on fallen logs, old stumps, and dead standing trees. The honey mushroom appears from late summer through late fall. The jack-o-lantern follows a similar schedule, fruiting from mid-summer onward. The bitter oyster can appear year-round in milder climates.

From Folklore to Biotechnology

Early travelers encountering foxfire didn't understand what they were seeing. New Jersey folklore attributed the ghostly lights to spirits, fairies, or omens of the Jersey Devil. European traditions linked similar glows to will-o'-the-wisps—mysterious lights that supposedly led travelers astray in marshes and forests.

The name "foxfire" itself likely comes from the French faux feu, meaning "false fire." Other historical names include torchwood, cold fire, and fairy sparks. All capture that sense of wonder at light without heat, glow without flame.

Today, scientists see bioluminescent fungi as more than curiosities. The luciferin-luciferase system has become a valuable tool in biological research. Researchers use it as a reporter gene—a way to track whether specific genes are active by watching for fluorescence. The same mechanism might someday help with medical imaging, allowing doctors to see biological processes inside living tissue.

Seeing the Glow Yourself

Spotting bioluminescent fungi requires patience and the right conditions. You need darkness—real darkness, not just dusk. Your eyes need 15 to 20 minutes to fully adapt. Even then, some species like the honey mushroom produce such faint light that you might question whether you're seeing anything at all.

Humid nights after rain offer the best chances. The fungi glow brightest when actively growing, which requires moisture. Look for areas with lots of decaying hardwood. Old growth forests with plenty of dead logs provide ideal habitat.

The jack-o-lantern offers the most dramatic display, with fresh gills glowing bright enough to read by—at least according to enthusiastic accounts. But even this species requires fresh specimens. As the mushroom ages, the light fades.

The Bigger Picture

Bioluminescence has evolved more than 40 times across the tree of life. It lights up ocean depths in fish and jellyfish. It flashes from fireflies on summer evenings. It glows from bacteria in tropical seas. And it emanates from fungi in forests worldwide.

Each lineage evolved light production independently, solving different problems with similar chemistry. For fungi, that problem appears to be reproduction—how to spread spores when you're rooted in place and your offspring are microscopic.

The solution: become a beacon. Glow steadily through the night, attracting mobile creatures that will carry your genetic material to new locations. It's advertising, fungal style, and it's been working for millions of years.

Next time you're in a temperate forest on a dark night, take a moment to look for that faint green glow. You might be witnessing one of nature's most elegant solutions to an ancient problem—and connecting with a phenomenon that has sparked human imagination since we first noticed light without fire.