Collared Earthstar

Earthstars are among the most sculptural of all fungi. They begin as an onion-shaped body buried or half-buried in litter, then the outer wall (exoperidium) splits from the top downwards into pointed rays that fold back and lift the fungus clear of the ground, exposing a papery inner spore sac. Geastrum triplex is the largest and most frequently encountered species of the genus in temperate broadleaf woodland.

Its common name comes from the feature that separates it from every relative: as the rays flex back, the thick fleshy inner layer of the outer wall cracks in a circle around the base of the spore sac, so the sac appears to sit in a raised bowl or wear a collar. The specific epithet triplex refers to this three-layered construction.

Expanded specimens are tough and persistent, often surviving as recognisable brown stars on the woodland floor for many months, so the species can be recorded long after fruiting has finished.

Unopened stage

A firm, rounded to onion-shaped or pointed-topped body 3-6 cm across, pale buff to ochre-brown, sitting on or partly buried in leaf litter, sometimes described as resembling a small bulb or an unopened fig.

Rays and collar

The outer wall splits into 4-8 broad, thick, pointed rays which arch back and downward, raising the fungus and giving an expanded span of 5-12 cm - the largest of the common earthstars. The thick fleshy inner layer of the rays then characteristically cracks in a ring around the central sac, leaving a raised, bowl-like collar or saucer beneath it. This collar is present in most, though not quite all, mature specimens.

Spore sac

A globose to slightly flattened, pale grey-brown to buff papery ball, 1-3 cm across, seated in the collar. It is sessile or nearly so - there is no distinct stalk between sac and collar. At its apex is a single conical pore (the peristome), fringed or finely fibrillose, and often surrounded by a faint paler circular zone or halo.

Interior and spore print

White and firm when young, ripening to a dry, dark brown, cottony spore mass surrounding a central sterile column (the columella). Spore mass and print dark brown; spores spherical and warted.

This earthstar is a litter species of broadleaf woodland, showing a marked preference for beech, but also occurring under oak, ash, hazel, hornbeam and in mixed woodland, shrubberies, old hedgerows, parkland and even mature gardens. It favours deep, undisturbed leaf litter on moist, often calcareous or humus-rich soil, and is frequently found on shaded banks and in the accumulated litter at the foot of large trees.

Fruiting bodies form in late summer and autumn, but because the expanded stars are tough and weather-resistant they persist through winter and can be found in almost any month. Individuals commonly appear in small scattered troops or rings. The species is widespread in temperate Europe, North America, Asia and parts of Australasia.

Geastrum triplex is a saprotroph of leaf litter and humus. Its mycelium permeates the decaying leaf layer beneath broadleaf trees, breaking down cellulose and lignin from fallen foliage and contributing to the formation of woodland topsoil. It has no mycorrhizal partnership with trees; the strong beech association reflects the depth and chemistry of the litter beech produces rather than a root relationship.

The star-shaped design is a dispersal adaptation. Splitting rays lift the spore sac above the litter into moving air, and the elastic sac then works as a bellows: raindrops striking its surface compress the papery wall and puff spores out through the single apical pore in fine clouds. In some earthstars the rays are hygroscopic, curling shut when dry and reopening when wet, though in G. triplex they are largely fixed once expanded.

Other Geastrum species share the star form but differ in detail. The Rayed Earthstar (Geastrum fornicatum) arches on long rays whose tips remain attached to a buried basal cup, standing the spore sac up on stilts. Geastrum striatum has a clearly stalked spore sac with a collar-like apophysis and a sharply grooved, pleated beak-like peristome, quite unlike the fringed cone of G. triplex. Geastrum rufescens is smaller, flushes pinkish-brown when fresh, and lacks a well-formed collar.

The Barometer Earthstar (Astraeus hygrometricus) has thick, tough, strongly hygroscopic rays that close over the sac in dry weather and a coarsely cracked upper surface, and it grows in open sandy ground rather than deep broadleaf litter. Unopened earthstars can be mistaken for small puffballs, but slicing one vertically reveals the layered outer wall destined to split into rays, which no true puffball possesses.