Common Powderhorn Lichen Identification Guide

The Place You Are Standing

Most encounters with this lichen begin with a change of eye level. Stop scanning the canopy, crouch, and look at the bottom half-metre of tree trunks. In cool temperate woodland across Europe, northern Asia and much of North America, that band of rough, damp, mossy bark is where Common Powderhorn does most of its living. Spruce and pine plantations are reliable. So are birch, oak and alder with acid, furrowed bark, old rotting stumps that have gone soft and fibrous, fallen logs at the crumbling stage, peaty banks, and the shaded flanks of ancient fence posts.

The first impression is of two things happening at once. There is a low crust of tiny grey-green scales lying flat against the bark, and rising out of that crust a scatter of slender, pale, upright horns, usually one to three centimetres tall, tapering to fine points. The whole colony reads matte and slightly dusty rather than glossy or leafy, and in dry weather it drains to a chalky grey-green that almost disappears against the bark.

Understanding the Two-Part Body

Before measuring anything, get the architecture straight, because every Cladonia is built the same way and the identification depends on reading both halves.

The primary thallus is the flat, scaly mat. Individual squamules are small, roughly 2 to 8 mm long, lobed or shallowly incised at the margins, grey-green to olive-green on the upper face and conspicuously chalk-white beneath. Many curl upward at the edges, so a colony viewed from the side flickers green and white. In Common Powderhorn these squamules persist strongly and are usually abundant, forming a continuous scurf around the base of the horns.

The podetia are the horns themselves: hollow, upright, secondary structures that grow up out of the primary thallus and carry the reproductive organs. Their shape is the single most useful character in this species.

Reading a Podetium From Base to Tip

Pick one podetium and work upward with a hand lens.

Height and outline. Typically 1 to 3 cm, occasionally reaching 4. The profile is awl-shaped or subulate: widest near the base, tapering steadily and evenly to a fine point that is very often slightly curved or leaning, as though the horn had bent toward the light.

Branching. Usually none at all. A colony of unbranched, single-pointed spikes is normal. Occasional podetia fork once near the tip, but repeated branching is not this species.

Tips. This is where care pays. The typical tip is simply pointed and closed. A minority open into a very narrow, shallow, often lopsided cup, but the crucial detail is that this cup is barely wider than the shaft supporting it. It is a slight flaring, not a trumpet.

Surface texture from base to apex. The lower part is usually corticate, meaning smooth and skin-like, and frequently clothed with small squamules that climb a short way up the stalk. Above that the surface becomes coated in soredia, and the transition is gradual and vague. There is no sharp line where the smooth part stops and the powder begins; one simply fades into the other.

The Powder Itself

Common Powderhorn is named for its soredia, and their texture matters. Run a fingertip lightly up a horn. The coating should be farinose, as fine as flour, leaving a faint pale-green dust on the skin. Under a lens it looks like a bloom of talc rather than a scatter of crumbs. Soredia that are visibly granular, gritty, or clumped into coarse grains push you toward other species.

Colour of the sorediate zone is pale ash-green to greyish-white, often noticeably paler than the squamules below.

Fruiting Bodies and What Replaces a Spore Print

Apothecia are produced only occasionally in this species. When present they sit at the podetial tips as tiny, pale to dark brown, convex buttons, sometimes in small clusters. Blackish-brown pycnidia are more frequently seen, appearing as minute dark ovals capping the tips and sometimes exuding a bead of colourless jelly when wet.

A conventional spore print, the technique used for gilled fungi, does not apply to a lichen. The spores are produced inside asci within those minute brown apothecia and are never released in quantities you could catch on paper. What replaces it is a squash mount. Slice or crush a fertile apothecium in a drop of water on a slide, apply a cover slip, press gently, and examine at around 400x. You are looking for cylindrical asci each containing eight colourless, simple (non-septate), ellipsoid spores in the region of 8 to 12 micrometres long. That combination confirms the genus rather than the species, so treat it as background rather than the decisive test.

Substrate, Season and Condition

This lichen is perennial and present all year, so there is no season in the fungal sense. There is, however, a right time to look. After sustained rain the squamules swell, green up, and become pliable, and the colony is at its most legible. In drought everything shrinks, pales and turns brittle, the horns become fragile, and the green cast drains out. If you find a dry colony and cannot resolve the surface, mist it with water from a bottle, wait two minutes, and look again.

Substrate preference is strongly acidic: conifer bark, birch and oak bases, decorticated and rotting wood, peat, and mossy acidic soil on banks. It is one of the commonest and most tolerant members of its genus, colonising quickly and often forming the first Cladonia to appear on a new stump.

Separating It From Its Nearest Relatives

Work on structure alone, then confirm with chemistry.

Cladonia ochrochlora is the hardest. Its podetia are similar in size but the lower portion carries a distinctly delimited, smooth corticate zone with a fairly clear upper boundary, above which the soredia are coarser and more granular. Its tips are more often minutely cup-shaped. Common Powderhorn blurs that boundary and keeps its soredia flour-fine.

Cladonia fimbriata is separated by the tip. It makes proper trumpets: regular, symmetrical cups clearly wider than the stalk, evenly coated with farinose soredia. If the horn genuinely flares into a goblet, you are not looking at Common Powderhorn.

Cladonia macilenta and Cladonia digitata are also sorediate, but their podetia are blunter, often stouter, and they characteristically bear scarlet pycnidia or apothecia at the tips. Any red on the tips rules out this species, and the chemistry differs sharply.

Cladonia cornuta and Cladonia subulata are taller, more slender and often branched, and their primary squamules typically wither away, leaving the podetia standing on bare substrate. Common Powderhorn keeps a conspicuous squamule mat.

Lepraria crusts can be confused at a glance because they are the same powdery grey-green, but they never produce podetia at all. If nothing stands up, it is not a Powderhorn.

Chemistry as the tiebreaker. Common Powderhorn contains fumarprotocetraric acid. A drop of para-phenylenediamine solution on the podetial surface produces a prompt brick-red to rusty reaction (P+ red), while potassium hydroxide and bleach solutions give nothing (K-, C-, KC-). Cladonia macilenta reacts K+ yellow and P-, an immediate separation.