Star Earthball | Scleroderma polyrhizum Identification Guide

First impression: a pale ball breaking out of hard ground

The habitat is the first clue and it is an unusual one. This is not a species of soft woodland litter. You meet it on compacted, poor, disturbed mineral ground: the shoulder of a track, a gravelly lawn, a hard-baked path edge, an old field, a bare patch under scattered oaks. Early on it is entirely subterranean, and what you actually notice is a raised dome cracking the soil surface, looking like a whitish or buff leathery ball half-buried in the ground. Later, when it has opened, you see a thick pale star lying flat on the earth with a dark powdery centre, and at that stage it is unmistakable from ten metres away.

Step 1: the closed stage

A young fruit body is 5 to 15 cm across, sometimes larger, and irregularly rounded rather than neatly spherical. The surface is off-white to buff or pale tan, often smeared with adhering soil, and cracked into low, flat, scurfy scales or a fine mosaic. Press it: the whole thing is hard and unyielding, quite unlike the soft give of a true puffball. There is no pore, no ostiole and no neck at the top. Everything about the closed stage says thick rind rather than papery sac.

Step 2: the star

As the fungus matures and dries, the thick outer wall splits from the apex downward into four to eight (typically six or seven) thick, blunt, irregular rays that peel back and lie on the soil. The opened fruit body can span 15 to 20 cm. Take a ray between finger and thumb. It is 3 to 8 mm thick, leathery and tough, whitish on the inner face and greying or browning outside, and it will tear rather than fold. Old rays crack further, curl and blacken in place, and can persist for months as a dark rosette.

One structural point matters more than any other here. When the rays open, there is no separate papery bag sitting in the middle of them. The dark spore mass exposed at the centre is simply the contents of the wall that has just split. That single observation removes the entire earthstar group from consideration.

Step 3: the spore mass

Section a fruit body at various ages and you will see the sequence. Young material is firm and uniformly white inside, dense and unbroken. It then becomes marbled with fine dark veins running through pale tissue, then olive-brown and doughy, then a loose, dark purple-brown to almost blackish powder. Once fully mature the powder blows away in dry wind, leaving a shallow bowl of rays with a dusty floor.

Step 4: the base and the cords

Dig, do not pull. The base tapers into a dense knot of white to pale buff mycelial cords, and those cords bind soil into a solid plug that comes away with the fruit body. This mass of roots is what the species epithet refers to and it is a genuine field character worth confirming, because it is far better developed here than in most relatives. There is no stalk of any kind, and no sterile stem-like column inside the base.

Step 5: cut it in half and watch for change

A fresh, young fruit body cut vertically shows a wall 2 to 6 mm thick, white in section, sometimes with a faint pinkish or rusty line just under the surface, and a uniform white or slightly marbled interior. Exposed cut flesh may flush slowly pinkish and then dull grey over some minutes. There is no blue reaction anywhere and no rapid dramatic bruising. The wall stays firm and rubbery-hard, never becoming soft or spongy.

Step 6: reading the spores

There is no spore print in the gilled sense. Instead, tap a mature open fruit body over white paper. What falls is a dark brown to blackish-brown dust, not olive, not yellowish, not pale. Under a lens the dust looks uniform and fine rather than granular. Under a microscope the spores are globose, roughly 4 to 8 micrometres, brown, and covered in short spines, with at most a faint partial net between them. Globose, spiny, brown spores confirm the earthball genus and eliminate the smooth or pitted spores of unrelated groups.

Substrate, partners and season

Despite its liking for bare compacted ground, this is an ectomycorrhizal fungus, associating with oaks and other broadleaf trees. Fruit bodies frequently appear many metres from the nearest visible trunk, along verges and in mown areas where roots run out under hard ground. The season is summer into late autumn, and because the tissue is so tough, opened rosettes persist as weathered remains long after fruiting, sometimes right through winter. Recording those old rosettes is a good way to map where to look next year.

Separating it from look-alikes by structure

Scleroderma citrinum. Stays closed and then breaks apart irregularly rather than forming neat rays. Its surface is yellow-ochre with coarse raised warts and scales, and it sits in acid woodland litter and moss with a short, poorly developed base.

Scleroderma areolatum and S. verrucosum. Smaller, with a distinct stalk-like sterile base, and a surface patterned with small dark scales in a fine mosaic. Neither forms a broad thick star.

True earthstars (Geastrum). The decisive difference is the separate inner spore sac, a thin papery bag with a defined pore or fringed mouth, sitting in the middle of the opened rays. Geastrum rays are also thinner and more flexible.

Astraeus hygrometricus. Thick rays, but they are hygroscopic: they close up tightly when dry and open again when wetted. It also has a distinct papery inner sac and is much smaller.

Pisolithus arhizus. Club-shaped with a long rooting stalk, and its interior is packed with distinct pea-like chambers set in a dark matrix rather than a uniform mass.

Large puffballs (Calvatia, Lycoperdon). Soft to the touch, with thin walls that disintegrate or open by a pore, an olive to yellow-brown spore mass, and no thick leathery rays.

Field sequence

1. Hard compacted ground, path edge or lawn, broadleaf trees within reach.
2. Hard ball with a thick pale rind, no pore.
3. Splits into four to eight thick blunt rays, no inner papery sac.
4. Wall 2 to 6 mm thick in section.
5. Base a knot of white cords binding a soil plug.
6. Mature dust dark purple-brown to blackish; spores globose and spiny.