Mushrooms that glow in the dark: can you grow them at home?

Walk through a damp forest at night and see a faint green shimmer at the foot of an old stump, and you have not seen a goblin but mycelium. Bioluminescence, light an organism makes itself, occurs in over a hundred mushroom species. Most live in the tropics, but a few live right here. This piece is about what glows, why, and about the question we get a few times every summer: can you grow that at home?

Why does a mushroom produce light?

Through the same kind of reaction as in fireflies: a compound, luciferin, reacts with oxygen under the influence of an enzyme and gives off light in the process. In mushrooms the light is always green, at a wavelength around 520 nanometres, and it burns day and night; you only see it when it is dark enough. Why they do it is not entirely certain. The best explanation so far: the light attracts insects at night that carry spores away, the way flowers attract bees by day. In species where only the mycelium glows, it may also be a by-product of metabolism, a kind of breathing that happens to give off light.

Which ones glow around here?

The best known is honey fungus. Not the mushroom itself, but the mycelium and the black cords it uses to creep through wood give off a faint green light. Freshly broken, colonised wood from a honey fungus stump can visibly glow in a completely dark room after ten minutes of letting your eyes adjust. That phenomenon was called "foxfire" and was described as far back as antiquity. The mycelium of a few bonnet mushrooms glows here too, and in the southern half of Europe the jack-o'-lantern, an orange mushroom whose gills genuinely emit light. That one is poisonous and resembles a chanterelle, which is a reason to know it.

Which species glows brightest?

The strongest glower in the world is a small bonnet from Brazil and Japan, Mycena chlorophos, whose caps are so bright green you can read by them in the dark. In hobby growing, the best known is the bitter oyster, Panellus stipticus, a small fan-shaped fungus that grows on dead broadleaf wood. Note: only the North American strains glow clearly, the European ones barely do. Anyone wanting a glowing strain therefore needs a specific strain from a specific continent, not just the species.

Can you grow it at home?

Yes, with a luminous strain of Panellus stipticus on sterilised sawdust or a piece of beech wood, and with plenty of patience. The fungus grows slowly, gives the most light while the mycelium is young and active, and demands complete darkness and adjusted eyes to see anything: a phone screen nearby is enough to wash it all out. It is not a product we sell, and that is a deliberate choice. It is a curiosity, not a harvest, and most people who start with it are satisfied after the first faint green glow and lose interest by the second month. Anyone who really wants it can find it at specialised culture banks. We stick to edible.

What you can do at home

In October, after a wet week, find an old stump where honey fungus has grown, break off a piece of soft, colonised wood, and take it to the darkest room in the house. Wait ten minutes with your eyes closed, then look. In one of every few cases you will see it. It is the cheapest and most astonishing demonstration of what mycelium is, and for children who want more after a grow kit, this is the excursion they remember.

And edible?

Not a single glowing species is worth eating, and a few are outright poisonous. Bioluminescence and edibility have nothing to do with each other, so a glowing mushroom says nothing about safety. The light is for the insects. We only get to look. Anyone who would rather grow something that ends up on the table can always return to the ordinary luminous oyster mushroom, and that is a joke: it only glows in the pan.