Cannonball Fungus

The Cannonball Fungus is one of the smallest fungi anyone is likely to notice, and it is noticed almost entirely because of what it does rather than how it looks. Each fruiting body is a whitish to orange-tan sphere barely one to three millimetres across, embedded in wood chips or rotting wood, and easy to overlook until it opens.

Sphaerobolus stellatus sits in the Geastrales, the same broad grouping as earthstars, and shares with them the trick of splitting the outer wall into pointed rays. Where an earthstar merely exposes its spore sac, this fungus goes much further: the inner wall turns itself inside out with sudden force and launches the entire spore package into the air.

That launch is what makes it distinctive. The dark, sticky ball, called a peridiole, can travel several metres horizontally and stick fast to whatever it hits — siding, fences, car paint, leaves. The hard black specks left behind are usually the first evidence people find, well before they spot the fungus itself.

Fruiting body

Unopened fruiting bodies are near-spherical, 1–3 mm wide, pale cream to buff or faintly orange, and often half-buried among wood fragments. They are firm, smooth and slightly translucent when fresh, and tend to occur in scattered troops of dozens or hundreds across a patch of mulch.

Opening and the star

At maturity the outer wall splits into roughly four to nine short triangular rays, forming a tiny pale star or cup 2–4 mm across. Inside the star sits a single glossy dark brown to black ball about 1 mm wide. There are no gills, pores or teeth at any stage.

The launch mechanism

The inner cup layer builds up turgor pressure and everts explosively, catapulting the peridiole away. What remains behind is an empty, whitish, star-shaped husk with a smooth inner surface — a common sight in old patches, and a useful confirmation that the fungus is present even when no loaded specimens are visible.

Flesh and spores

There is essentially no fleshy tissue to speak of; the structure is a thin-walled multi-layered capsule. Spores are not shed as a free print but stay packed inside the peridiole, which glues itself to a surface and later germinates in place. The black specks are extremely tenacious and dry to a hard, tarry point.

This fungus is a creature of decaying woody debris. Bark mulch and hardwood chip landscaping beds are the classic modern habitat, and it is also found on rotting logs, sawdust piles, old boards, straw and herbivore dung. It favours mulch that stays consistently damp, and often appears on the shaded, north-facing side of a bed where moisture lingers.

Fruiting peaks in cool, wet weather from late summer into late autumn, and it can continue through mild winters in temperate maritime climates. The species is widely distributed across Europe, North America, Asia and Australasia, and has spread further through the international trade in bark and wood-chip products.

Sphaerobolus stellatus is a saprophyte, breaking down lignin and cellulose in woody debris and returning those nutrients to the soil. Its mycelium forms pale strands through decaying chips and can persist in a mulch bed for several years, refruiting each time conditions turn suitably cool and damp.

Its dispersal strategy is unusually deliberate. The fruiting body is phototropic, orienting its opening toward the brightest light — usually open sky or a pale reflective surface — before firing. The sticky peridiole adheres where it lands, survives drying, and germinates later to start a new mycelium. This aiming behaviour explains why the specks cluster on light-coloured walls and vehicles parked beside mulched beds, and why they are often found at heights of two metres or more.

Bird's nest fungi are the nearest lookalikes and grow in the same mulch. Crucibulum laeve and Cyathus striatus both form small cups, but their cups are distinctly vase- or barrel-shaped and hold several flattened, disc-like peridioles rather than one glossy sphere, and Cyathus striatus has grooved inner walls and a shaggy brown exterior. Their peridioles are also splashed out by raindrops rather than launched.

Earthstars such as Geastrum species share the star-shaped splitting habit but are far larger — typically 2–8 cm across — and reveal a papery spore sac that puffs out powder through an apical pore instead of a single ejected ball. Tiny orange discs of Cheilymenia or other cup fungi on the same mulch stay open, saucer-shaped and hollow, never containing a dark sphere. Finally, the black specks on a wall can be mistaken for insect frass or tar, but under a lens they are smooth, domed and firmly cemented, and a matching field of pale empty stars will usually be found in the mulch a few metres away.