Some mosses you work out; this one you simply recognise. Knight's plume moss (Ptilium crista-castrensis) grows as a row of neat, evenly graded branches curving over to one side, so each shoot looks like a small green feather that somebody has combed. It ranks among the most ornamental of all our mosses, and in Britain among the harder to find, keeping largely to old woods in the north.
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A moss built like a feather
The structure is what stops you. A main stem runs more or less straight, and off it comes a single regular series of short branches, longest about the middle and shortening towards either end, so the outline of the whole shoot is that of a plume or a fern frond. Stem tip and every branch curve smoothly the same way, all sweeping to one side, a feature botanists call falcate-secund and the rest of us read as a moss bent by a wind that is not there. The colour tends to a glossy golden-green, paler and yellower than most of its neighbours. Nothing else on a British woodland floor is combed quite so evenly and so one-sidedly, and once you have met it the shape lodges in the memory.
Where it grows
This is a plant of cool northern forest. Right across the boreal belt of Scandinavia, Russia and northern North America it can floor whole spruce and pine woods in great golden drifts. Britain sits at the mild south-western edge of that range, so here it is decidedly local. Its stronghold is the old native pinewoods of the Scottish Highlands, the Caledonian remnants, where it grows on humus and over rotting logs among other mosses in the shade of the trees. It shows up more thinly in parts of northern England and here and there beyond, but a gardener in the south is likelier to meet it in a photograph than on a walk. I will be plain about that: I have admired this one far more often on a screen and in the herbarium than come upon it in the field, and it would be dishonest to write as though I trip over it.
The feather mosses it resembles
Several glossy, branched mosses carry a feather name, and a beginner can muddle them. Glittering wood-moss (Hylocomium splendens) raises its fronds in stacked storeys, each year's growth arching out of the last, and its branching spreads more loosely than a rigid comb. Plait moss (Hypnum cupressiforme) also curves its leaves to one side, yet it forms flatter, less orderly mats and never quite reaches the tidy pinnate plume. What clinches knight's plume is that pairing of a strictly single row of branches with the whole shoot swept one way; where the branching wanders or forks, look elsewhere. Both of its usual lookalikes have profiles of their own, under glittering wood-moss and plait moss.
A moss of old woodland
Because it holds to long-settled, undisturbed forest, knight's plume moss belongs to that group of species whose presence tells you something about a wood's past. A floor well furnished with it tends to be one that has stood a long time with a stable, humus-covered ground layer, the kind of unbroken continuity also prized in the mossy Atlantic woods described under temperate rainforest. The same dependence works against it: clearance, drainage and the shrinking of old pinewood have thinned it towards the margins of its range, and it is no moss to gather even where it carpets the ground. Leave it where it grows.
The name
The Latin crista-castrensis means the crest of a camp, a nod to the tall feathered plume that once crowned a helmet, and the scientific and the English names both point at that one picture. Ostrich-plume feather-moss is the other name you will meet, and it lands on the same image of a drooping, evenly barred feather. Names of this sort are a quiet pleasure of learning mosses, and a hint that people have thought this one worth singling out for a very long time. If you are still building the general habit of reading a moss by its shape before anything else, that groundwork sits in how to identify moss.