King Alfred's cakesDaldinia concentrica

King Alfred's cakes (Daldinia concentrica), also called cramp balls or coal fungus, are one of the easiest fungi to spot on a winter walk. They have no cap, gills or stem: each one is a hard ball 2 to 8 cm across, fixed firmly to a dead branch or trunk, most often of ash. Young ones are reddish brown, mature ones black, like a lump of coal or a burnt bun. It's an ascomycete, related to candlesnuff and the woodwarts rather than to ordinary mushrooms.

How do you identify King Alfred's cakes? Cut or break a mature ball in half: inside you'll see alternating silver-grey and dark concentric bands, like the rings of a tree. They were long thought to mark seasons of growth, but a 2015 study found that all the bands form within a single season, so you can't count years from them. Another clue is a black dusting on the wood around the ball, where ripe spores have been shot out sideways.

Where does it grow? It grows on dead and dying broadleaved wood, above all ash, less often beech, alder and birch, in woods, hedgerows and parks. The balls last for months, so you can find them all year, with fresh brown ones appearing from spring to autumn. True Daldinia concentrica favours the mild Atlantic climate of western Europe, where it was first described, from Halifax in England, and further east, in Poland and Ukraine, specimens once given this name turned out to be other Daldinia species.

Are King Alfred's cakes edible or poisonous? They aren't edible: they're as hard as charcoal. There's no evidence that they're poisonous to touch, but there's also no evidence for any health benefit, despite lab studies of unusual compounds from Daldinia species.

What can you mistake them for? Brittle cinder (Kretzschmaria deusta) is also black and crumbly, like charred wood, but it forms flat, wavy crusts with no rings inside, and grows at the roots and base of living lime, beech and oak, where it causes a dangerous rot. Woodwarts such as beech woodwart are tiny, just a few millimetres across, and plain inside. Black bulgar is black but soft and rubbery, like a button, and stains your fingers. Black witches' butter turns to black jelly in wet weather.

The name comes from the legend of King Alfred, who, hiding from the Vikings in the 9th century, was told to watch a woman's cakes by the fire, let them burn and was scolded for it. Carrying one in a pocket was once thought to ward off cramp, and the Horniman Museum in London holds just such a charm on a string, from East Sussex, acquired in 1912. Dried mature balls catch a spark from flint and steel and smoulder for a long time, so they were used to carry fire. At La Draga, a Neolithic lakeside village in Catalonia about 7,300 years old, archaeologists found a store of these balls, probably kept as tinder. Collecting them for fire-lighting is now discouraged, as ash trees are declining. The larvae of the concealer moth Harpella forficella feed inside.