Witch's Butter

Tremella mesenterica is the best-known member of the Tremellales, an order of jelly fungi distinguished microscopically by divided (septate) basidia. Its fruitbody has no cap, gills or stipe - simply a shapeless, folded gelatinous mass that oozes out of bark cracks and swells dramatically when wet.

The striking colour, somewhere between egg-yolk yellow and deep orange, makes it one of the easiest fungi to spot in a bare winter hedgerow. In dry conditions it contracts to a hard, dull orange or brownish crust barely thicker than paint, then rehydrates into a quivering lobed blob after the next downpour, sometimes tripling in volume.

The most interesting thing about the genus is where it draws its nutrients from. Tremella species are not wood decayers at all: they are mycoparasites that attack the mycelium of other fungi. T. mesenterica specifically parasitises crust fungi of the genus Peniophora, so the yellow blobs are effectively a signal that a thin, unnoticed crust fungus is already colonising the branch.

Fruiting body

An irregular, gelatinous, lobed and convoluted mass 2-10 cm across and up to 4 cm thick, composed of flattened folds like the surface of a brain or a crumpled ribbon. It has no cap, gills, pores or stipe, and is attached broadly to the wood at one or more points. Several masses often merge along a branch.

Colour and texture

Golden yellow to deep orange when fresh and turgid, often paler and more translucent at the outer edges, becoming darker orange and finally horn-brown and hard as it dries. The texture is soft, slippery and gelatinous, wobbling when the branch is tapped; it tears rather than snaps, and a squeezed fragment releases water.

Flesh & spore print

The interior is uniformly jelly-like with no differentiated layers - the whole outer surface bears the spore-producing tissue rather than an underside only. The spore print is white to very pale yellow. Fruitbodies always emerge from bark, small dead twigs or the cracks of attached branches rather than from soil.

Look for it on dead but still-attached branches and twigs of broadleaf trees and shrubs, and on recently fallen brash. Gorse, hazel, beech, oak, birch, ash and willow are all frequent, and the fungus is common in hedgerows, scrub, heathland and open woodland. It is almost never found on soil or on conifers.

Fruiting is triggered by wet weather and is heaviest from autumn through winter into early spring, when the fungus is often the brightest thing in a leafless hedge. It is widespread and common across Europe, North America and much of temperate Asia and Australasia, with several similar yellow Tremella species sharing the range.

Tremella mesenterica is a mycoparasite. Its hyphae penetrate the mycelium of crust fungi in the genus Peniophora using specialised cells called haustoria, drawing nutrients from the host rather than from the wood itself. Because Peniophora is the actual wood decayer, the yellow jelly is really the second link in a chain of decomposition on the branch.

The gelatinous consistency is a water-management strategy: the fungus can shut down completely in dry spells and reactivate within hours of rainfall, releasing spores whenever conditions suit. Its spores can bud into yeast-like cells, an unusual life-cycle stage among larger fungi that allows the organism to persist and multiply on bark surfaces until it encounters a suitable host mycelium.

Tremella aurantia is nearly identical in colour and form but parasitises Stereum hirsutum and tends to be a slightly duller, more orange, denser mass; the presence of hairy Stereum brackets on the same branch is the practical clue. Dacrymyces stillatus (Common Jellyspot) forms much smaller, cushion-like orange dots a few millimetres across, usually on conifer wood, rather than large folded lobes.

Calocera viscosa (Yellow Stagshorn) shares the colour but is upright, branched and antler-like with a tough, rubbery texture and a rooting base in conifer wood. Exidia species are the same shape but blackish, olive or whitish, and Tremella foliacea is brown and formed of frilly, leaf-like lobes.