Skull-shaped Puffball Identification Guide

The first impression

This is a mushroom you notice from a distance because it is large, pale and shaped like nothing else in the leaf litter. Picture the edge of a hardwood wood in late summer: a path margin, a grassy clearing, a stand of oak and maple with rich humus underfoot, or a shaded roadside bank. Sitting on the ground is a smooth, buff-coloured object the size of a fist or a grapefruit, broad and rounded above, narrowing abruptly below into a stout, pleated, stalk-like base that grips the litter. The overall silhouette really is skull-like or pestle-like, and the head often leans slightly. Fresh specimens are white to cream; older ones go tan, then dull ochre-brown, and finally the whole top disintegrates and blows away, leaving a persistent cup-shaped base standing on the ground like a broken eggshell for months afterwards.

Size and shape

Measure the head and the base separately. The fertile head is typically eight to fifteen centimetres wide and can reach twenty, and the whole fruit body is often taller than it is wide, six to twenty centimetres in height. The upper part is broadly obovoid or top-shaped. Below it, the tissue contracts into a thick sterile base that is chambered and spongy inside and that often shows deep vertical folds or wrinkles at the point of narrowing. There is no ring, no volva and no stipe in the ordinary sense; the sterile base is simply the lower half of the same structure. Rooting cords of white mycelium usually run from the very bottom into the litter.

The outer skin

Run a hand over the surface. It is thin, soft, and smooth to very finely felted or minutely granular when young, without the coarse spines, warts or pyramidal scales seen on other puffball groups. As it matures the skin becomes areolate: it cracks into a fine mosaic of shallow patches, and it often develops a wrinkled, brain-like or crumpled texture over the upper half. Colour begins white to pale cream and shifts through buff and tan to a dull greyish-brown. The critical structural point is that this outer layer is fragile and does not open by a single neat pore at the apex. Instead it breaks down irregularly across the top and flakes away in sheets, exposing the spore mass wholesale, so mature specimens look as though their crown has been torn off.

Cut it in half: the single most useful step

Every puffball-shaped object should be sliced from top to bottom with a sharp knife before anything else is decided. In a young Skull-shaped Puffball the entire fertile head is filled with a uniform, pure white, firm gleba with the consistency of a marshmallow, and there is no internal structure of any kind: no outline of a cap, no gills, no stipe, nothing but homogeneous white tissue. Beneath it, sharply demarcated, sits the sterile base, which is also whitish but visibly chambered, like a fine sponge or honeycomb.

As the fungus ripens, the gleba passes through cream to a distinctive yellow or greenish-yellow phase, then becomes olive-brown, and finally a dry, dusty, brown to olive-brown powder threaded with fine fibres. The sterile base stays paler and firmer and remains as a persistent cup long after the spores have gone. That colour sequence, white to yellow to olive-brown, is worth memorising, since the yellow phase is a useful confirming detail.

Spores and prints

There is no spore print to take in the conventional sense, because the spores are produced inside the gleba rather than on an exposed surface. Instead, tap a mature specimen and note the colour of the smoke that puffs out: olive-brown here, rather than the deep purple-brown of some relatives or the almost black of others. Under a lens or microscope the spores are tiny, globose and very finely warted, and the sterile threads mixed among them, the capillitium, break easily into short segments.

Substrate and season

Calvatia craniiformis is a saprotroph of rich, humus-laden soil in and around deciduous woodland. It is a species of eastern North America and eastern Asia, common in oak and mixed hardwood woods, along wooded edges and paths, in grassy openings and on shaded lawns near trees. It appears from midsummer through autumn, often after heavy rain, sometimes singly and sometimes in small scattered groups or loose arcs. It does not grow on wood; a puffball-like body attached directly to a log or stump is something else.

Separating it from look-alikes on morphology alone

Giant Puffball, Calvatia gigantea, is the first comparison. It is far larger, often thirty centimetres or more, essentially a smooth white ball or slightly flattened globe with almost no sterile base and no pinched, wrinkled stalk-like lower portion, and it sits on grassland and field edges rather than woodland humus.

Purple-spored Puffball, Calvatia cyathiformis, shares the top-shaped outline and the persistent cup, but its mature gleba turns a distinct purple-brown rather than olive-brown, its skin cracks into larger polygonal plates, and it is a species of open grassland, pasture and lawns.

The Lycoperdon group is smaller, usually two to six centimetres, and their surfaces are covered in spines, warts or granules that rub off to leave a patterned skin; crucially they open by a single neat apical pore rather than crumbling wholesale. Stump Puffball grows in dense clusters directly on wood with white cords at the base.

Earthballs, Scleroderma species, are the important structural contrast: their skin is thick, tough and leathery, often coarsely scaly and yellow-brown, they have no distinct sterile base, and their interior turns firm, marbled and purple-black to almost jet black very early, quite unlike the soft white gleba here.

Finally, the vertical cut also separates a true puffball from the unopened egg stage of a stalked mushroom. An Amanita button, sliced lengthways, shows the silhouette of a developing cap, gills and stipe wrapped in a veil; a Skull-shaped Puffball shows nothing but uniform white tissue above a chambered base. Any internal outline at all means it is not this species.