Arched Earthstar Identification Guide

The first impression

An Arched Earthstar looks less like a mushroom than like a tiny four-legged figure standing on the woodland floor. Under a dark hedge, in the deep leaf litter beneath yew, holly, laurel or hawthorn, you find a brown structure five to nine centimetres tall: four stiff, leathery arms bent downward like legs, standing on a shallow cup of matted litter, and lifting a small brown ball high into the air on a short stalk. Nothing else in the temperate fungal flora adopts that posture.

The habitat is worth noting before you touch anything. This is a species of rich humus in shady, sheltered places, frequently in artificial or semi-artificial settings such as old churchyards, parkland, shrubberies, hedge bottoms, mature gardens and estate woodland. It favours base-rich, calcareous or free-draining sandy soils, and it often returns to the same few square metres year after year.

How the structure develops

Understanding the developmental sequence makes the identification obvious. The fungus begins as an unopened, more or less egg-shaped or onion-shaped body sitting in the litter, its lower half fused to a dense pad of soil, leaf fragments and mycelium.

At maturity, the outer wall splits from the apex downward into a small number of rays, usually four, occasionally three or five. Instead of simply folding back and lying flat, as most earthstars do, the rays of this species arch strongly downward and outward while their tips remain anchored in that basal pad. The pad separates as a persistent cup, and as the rays continue to curve they lever the centre of the fungus upward. The result is a spore sac hoisted three to six centimetres clear of the ground on what looks like a set of legs, with the ragged cup left below like a saucer.

If you find a specimen that has been kicked or dislodged, look for the detached litter cup nearby. Finding the cup is strong supporting evidence, because it is produced by only a small group of earthstars.

Reading the rays

The rays are thick, tough and leathery, dark brown to greyish-brown or almost blackish outside where soil and litter adhere. On the inner face, which is now turned outward and downward by the arching, there is usually a paler fleshy layer that darkens with age, cracks, and may flake away in patches, exposing the fibrous layer beneath. Old specimens weather to a uniform dark brown or grey and become hard and woody.

Count them and check their tips. Four is the usual number, and the tips should be attached to, or clearly torn from, the basal cup. Rays that lie flat on the ground with free tips belong to a different group of earthstars.

Reading the spore sac

The spore sac, or endoperidium, is the part that carries the finest detail and the most useful separating characters.

Shape and size. Globose to slightly egg-shaped, roughly one to two centimetres across.

Colour and surface. Greyish-brown, mid-brown to dark brown, often coated when fresh with a fine whitish or greyish frosted layer that rubs off, leaving a smoother darker surface beneath. Weathered material is plain dark brown.

Stalk. There is a short but definite stalk, or pedicel, raising the sac above the point where the rays meet. Above the stalk there is frequently a small collar-like swelling, an apophysis, forming a shallow rim at the base of the sac. Look for it with a lens.

Peristome. This is the mouth at the top of the sac, and it is decisive. In this species it is conical to slightly beaked, and it is fibrillose or silky, meaning that under a lens it resolves into fine radiating fibres rather than into sharp regular grooves. Just as importantly, it is not sharply set off from the rest of the sac by a distinct paler circular zone. The transition is gradual and diffuse.

Interior. In a very young unopened specimen the interior, the gleba, is white and firm and can be cut cleanly with a knife. As it matures it passes through olive and yellow-brown tones to a dry, powdery, dark brown mass of spores mixed with a mesh of fine threads called capillitium. In section you can also see a central column, the columella, running up into the sac.

Spore prints and how this group differs

You cannot take a spore print from an earthstar in the usual way, and there is no need to. There are no gills, no pores and no free-falling deposit; the spores are released through the single apical mouth when raindrops, falling debris or a finger tap compress the sac, puffing out a small cloud.

The practical field equivalent is the puff test. Hold a mature specimen over a sheet of white paper and tap the sac lightly. A fine brown cloud should emerge from the peristome and settle on the paper as a brown dust. Examine that dust with a lens or a microscope: the spores are globose, dark brown and coarsely warted, quite unlike the smooth pale spores of most gilled fungi. Their colour and ornamentation confirm the gasteroid habit at once.

Substrate and season

The Arched Earthstar is a saprotroph of accumulated organic matter. It lives in deep humus and leaf litter, decomposing plant debris in the top few centimetres of soil, and it particularly favours the dry, shaded, litter-rich strip beneath evergreen hedges and shrubs where the litter builds up undisturbed. Yew, box, holly, laurel, privet, hawthorn and old ornamental plantings are frequent associates, and it is a well-known churchyard and old-garden fungus.

Fruiting runs from late summer through autumn into winter, but the mature fruitbodies are tough and weather-resistant, so bleached, hardened specimens can be found standing months out of season. Earthstars often appear in small colonies, and it is common to find several individuals at different stages, some still unopened in the litter, within a small area.

Separating it from look-alikes on morphology alone

Geastrum quadrifidum is the true test, because it shares the arching, cup-anchored habit. Work through these differences: it is smaller overall, usually four to five centimetres tall; the rays are thinner and typically number exactly four; the spore sac is paler, often pale grey to greyish-brown with a persistent frosted bloom; and the peristome, though fibrillose, is surrounded by a well-defined, sharply delimited paler circular zone or court. It is also a conifer specialist, fruiting in needle litter under spruce and pine, while the Arched Earthstar prefers broadleaved and evergreen shrub litter on richer soils.

Geastrum triplex is much larger and does not arch. Its rays fold back and lie flat, and its distinctive feature is that the thick inner fleshy layer cracks around the sac to form a collar or bowl that the spore sac sits in.

Geastrum striatum has a stalked spore sac with a pronounced collar-like apophysis, but its peristome is sharply grooved, or sulcate, with regular radial furrows, and its rays spread flat rather than arching onto a cup.

Geastrum coronatum has a stalked, dark, often frosted spore sac with a fibrillose mouth, but the rays recurve beneath the fruitbody without leaving the tips fixed in a persistent basal cup, so the specimen is not lifted onto legs.

Geastrum saccatum, Geastrum rufescens and Geastrum fimbriatum all have sessile or nearly sessile spore sacs sitting directly on flatly spread rays, with no stalk and no arching.

Myriostoma coliforme is unmistakable once examined: its spore sac has many mouths and stands on several stalks rather than one.

Field checklist

Four arching leathery rays with their tips anchored in a persistent litter cup; spore sac lifted several centimetres above the ground; sac globose, greyish-brown to dark brown, often frosted when fresh; short stalk with a small collar; peristome conical and fibrillose, not grooved and not sharply outlined by a pale court; powdery dark brown spore mass; deep humus under hedges and shrubs on base-rich soil; autumn into winter, persisting.