Making a spore print from your own mushroom: the best experiment after the harvest

Anyone who has left an oyster mushroom on the block a day too long knows the white powder that then covers everything beneath it. Those are spores, and they are the whole reason a mushroom exists. With a cap from your own grow kit, a sheet of paper and a night of patience you can turn them into a spore print: an image of the gills in millions of microscopic dots. Growers and mycologists do it to identify species; at home it is the finest experiment there is after the harvest.

What is a spore print?

From the moment its cap opens, a mushroom releases spores from the gills or pores on its underside. They fall straight down. Lay the cap underside-down on paper and they land in the pattern of the gills, forming a print that looks like a finely drawn flower. The colour of that print is fixed per species, which makes it one of the most important characteristics used to identify mushrooms. White, pink, brown, black: it says more than the colour of the cap.

What do you need?

• A ripe mushroom with the cap just opened. An oyster, shiitake or wine cap from your own growing is ideal, because you know it for certain.
• Paper in two colours: a white sheet and a black sheet, or a white sheet half coloured black. White spores only show on dark paper, brown ones only on light.
• A glass or bowl that fits over the cap.
• A knife, and a night.

Step by step

1. Cut the stem off just below the cap so the cap can lie flat with the gills facing down.
2. Place the cap on the border between the white and black paper, half on each colour. That way you see the print whatever colour the spores are.
3. Let a drop of water fall on the top of the cap; it helps the spores release.
4. Put the glass upside down over the cap so no draught blows the spores away.
5. Wait a night, at least six hours. Lift the glass, lift the cap carefully straight up, and look.

Which colour do you see for which species?

Oyster mushrooms give a white to pale lilac print, so fine that on black paper it glows like a galaxy. Shiitake gives white. Wine cap gives dark purple-brown, almost black. Nameko and chestnut give rusty brown. Lion's mane and maitake are trickier: they have no gills but spines or pores, and give a thin, whitish print you only see on black. Compare with the table below, and with what you find in the woods: a mushroom that looks like an oyster but gives a brown print is not an oyster.

Why does this matter to growers?

Because it is the one test you can do without a microscope that never lies. Spore colour is fixed per species, regardless of how the mushroom was grown. At the nursery we use it the other way round: a print on sterile paper is also the starting point of a new culture, because from those spores you can grow a new strain on agar. That is how a wild mushroom becomes a strain, as described in our piece on the culture library. You need not try that at home, but it is good to know what is lying on your paper: not dust, but the beginning of a mushroom.

Keeping and framing

A fresh print wipes away in an instant. To keep it, spray it carefully from half a metre with hairspray or artist's fixative, in two thin coats. Framed on black paper, an oyster mushroom print is a gift people ask for. And with children, see the children's piece, this is the moment the question "where do mushrooms come from" gets a tangible answer: there they are, a hundred million at once, on a sheet of paper.

The safety rule

A spore print is harmless, with one caveat: do not do it with a mushroom from elsewhere that you are not certain of, and do not leave the paper lying around in a kitchen where someone with a spore allergy or asthma works. With your own oyster from your own grow kit there is nothing to worry about. That is exactly why it makes such a good pupil.