Dog Lichen Identification Guide

The Setting and the First Impression

You are almost never looking upward when you find Dog Lichen. You are looking down: at a mossy bank beside a path, an unmown churchyard, the stabilised back of a dune system, a shaded roadside verge, or the grassy lip of an old quarry. What catches the eye is a patch of broad, floppy, ear-shaped flaps the colour of damp cardboard, spreading loosely over moss and soil rather than gripping bare rock. A mature colony can cover the area of a dinner plate, with lobes one to three centimetres wide and several centimetres long, overlapping at the edges and lifting free at the margins.

The second thing to register is how much the organism changes with moisture. Dry, it is stiff, papery, pale ash-grey to greenish-grey and rather brittle, and the margins curl upward so the pale underside shows from standing height. Wet, the same colony turns soft, limp and much darker: grey-brown, olive-brown, occasionally almost slate. Colour on its own is therefore close to useless unless you standardise it. Carry a small bottle of water, wet a lobe, wait a minute, and read the colour then.

Reading the Upper Surface

Lift a single lobe and look across it obliquely against the light. In Peltigera canina the upper surface is matt, never varnished, and finely tomentose, covered in a short felted down that is strongest towards the lobe tips and margins. Under a hand lens the felt resolves into a dense pile of short erect hairs. This tomentum is a genuine field character and it separates canina from several smooth, faintly shiny relatives.

The lobe outline is broad and rounded, with margins entire to slightly wavy and usually ascending or upturned. The lobes are not cut into narrow fingers. Equally important is what is absent: no soralia (powdery grey patches along cracks and edges), no isidia (tiny coral-like or scale-like outgrowths), and no darker wart-like cephalodia scattered across the surface. The absence of all three is as diagnostic as any positive character.

Turning the Lobe Over

The underside is where Peltigera identification is actually decided, so make a habit of flipping a lobe before you do anything else. In Dog Lichen the lower surface is pale, whitish to cream near the margin, and carries a network of raised veins. Those veins are broad, flat-topped and cottony, they are the same pale colour as the surrounding tissue near the lobe edge, and they darken gradually to buff or brown towards the centre of the thallus. Crucially, the transition is gradual: there is no sharp boundary where pale suddenly becomes black. Between the veins lie shallow pale interspaces, so the pattern reads as a soft, blurred web rather than a crisp lattice.

Arising from the veins are the rhizines, tufts of anchoring filaments that grip the moss beneath. In canina these are long, shaggy and pale, often one to two centimetres, simple or gathered into loose bundles. Short, discrete, tufted rhizines belong elsewhere in the genus.

The Spore-Bearing Structures

Fertile material carries apothecia at the tips of narrowed, erect lobes. They begin as small rounded discs and expand into saddle-shaped or rolled structures held vertically like little upright ears, five to fifteen millimetres across, reddish-brown to chestnut, with the pale underside of the lobe visible on the reverse. They stand clear of the thallus rather than sitting flush in it. Many colonies are sterile for years at a time, so their absence proves nothing, but their presence and their upright, saddle-like form confirm the genus immediately.

The photosynthetic partner is a cyanobacterium, which is why the wet thallus goes such a dark, slaty grey-brown rather than the bright apple-green of the green-algal members of the genus.

Spores and the Nearest Thing to a Spore Print

You cannot lay a lichen on a sheet of paper overnight and expect a neat deposit, but you can still work with the spores. Cut a fertile apothecium free, lay it face down on a glass slide inside a closed pot with a damp tissue for a few hours, and a faint pale smear of discharged ascospores collects underneath. Under a microscope Peltigera canina spores are long, narrow and needle-like, colourless to pale brown, distinctly multi-septate with three to seven cross-walls, and forty to eighty micrometres long. That slender, many-chambered shape confirms the genus but will not separate the species; for that you go back to the veins, the rhizines and the tomentum.

Substrate, Season and Associates

Dog Lichen is a ground-dwelling, moss-loving species of nutrient-poor but not sterile soil. Look on mossy banks, in short unfertilised turf, on dune grassland and sand, on old walls capped with moss, on ant hills, and in the mossy shade beneath scattered scrub. It is terricolous and muscicolous rather than saxicolous or corticolous: a Peltigera growing directly on bark high on a trunk is more likely to be one of its relatives. Because it fixes nitrogen through its cyanobacterial partner, it thrives where soil nitrogen is low and is squeezed out by rank grass on fertilised ground, which is why old lawns, verges and dune systems hold it while improved pasture does not.

Being a perennial, it is present year-round and there is no season for it in the way there is for a fleshy fungus. It is simply far easier to find and to interpret in autumn, winter and early spring, when the thallus is hydrated, expanded and dark and the surrounding vegetation is short.

Separating It from Its Closest Relatives

Peltigera membranacea is the usual confusion. It is thinner and more translucent when wet, the upper surface is only sparsely downy, and the underside carries narrow, rather cord-like dark veins with clean pale interspaces and rhizines that are more slender and less matted. Dog Lichen is thicker, more opaque, more evenly felted, and its veins are broad and flat with a soft gradual darkening.

Peltigera praetextata shares the felted surface but produces isidia: small flattened scale-like outgrowths along cracks and lobe margins that make the edges look chewed or frilled under a lens. Dog Lichen has clean, smooth margins.

Peltigera rufescens is a shorter, tougher, drier-country species with small crisped lobes whose margins curl strongly upward and inward, giving a ruffled, almost lettuce-like colony; its veins are darker and more crowded, and it favours open, sun-baked calcareous turf rather than shaded moss.

Peltigera aphthosa and Peltigera leucophlebia turn bright grass-green when wet, because their main partner is a green alga, and they carry conspicuous dark wart-like cephalodia scattered over the upper surface. Neither feature occurs in Dog Lichen.

A Working Field Sequence

Wet a lobe and read the colour. Feel the upper surface for felt. Flip the lobe and check that the veins are broad, pale at the edge and gradually darkening. Confirm long shaggy pale rhizines. Confirm no soralia, no isidia, no cephalodia. Note that it is sitting on moss or soil. If all six agree, you have Dog Lichen.