Walk round a garden centre's houseplant benches and you may meet a "peacock moss" with an oily blue sheen, or a "clubmoss" sold as ground cover for a bottle garden. Both look the part, low and green and feathery, and neither is a moss at all. They belong to a much older and quite separate branch of the plant world, and knowing what they really are helps you grow them, because they do not behave the way a true moss does.
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Why the name stuck
The word moss has always been generous. Gardeners and country people handed it to almost any small, soft, ground-hugging green thing, so a lichen became reindeer moss, a seaweed became Irish moss, and these low ferny plants became spikemoss and clubmoss. Spikemoss is the genus Selaginella; clubmoss covers Lycopodium and its close kin such as Huperzia. Botanists group them, along with the odd little quillworts, as lycophytes, and the lineage is astonishingly old. Three hundred million years ago their tree-sized relatives, Lepidodendron among them, built the swampy forests whose remains we now dig up as coal. The scaly plants creeping under your fern today are the survivors of that vanished giant clan.
What really separates them from moss
A true moss is a bryophyte, with no woody plumbing inside, no proper roots, drinking and breathing over its whole surface. Spikemosses and clubmosses are built on a different plan. They carry vascular tissue, the internal pipework of xylem and phloem that lifts water up the stem and moves food around, and they anchor with genuine roots rather than the simple gripping threads a moss uses. That places them, for all their mossy looks, far closer to the ferns than to anything else on this site. The knot that ties everyone up is reproduction: like mosses and ferns, they scatter spores instead of setting seed, so people reasonably assume that spores mean moss. Spores are only the older way of doing things, shared across several unrelated groups, and by themselves they say nothing about kinship.
The ones you can buy
Most spikemosses in cultivation are grown for a terrarium floor, and they look wonderful there. Peacock or blue spikemoss (Selaginella uncinata) throws out flat sprays that catch the light with a metallic blue-green shimmer, a structural colour thrown off by the leaf surface rather than any pigment in it. Krauss's spikemoss (Selaginella kraussiana) makes a bright, quick, trailing carpet, so vigorous that in mild damp gardens it can slip out and turn weedy. Best known of all is the resurrection plant or rose of Jericho (Selaginella lepidophylla), which curls bone-dry into a dead-looking brown fist and unfurls green within hours of a watering, a trick it will repeat for years. Every one of them wants the warm, close, humid air of a covered case; where a hardy garden moss shrugs off frost and a dry sill, these will not.
Clubmosses out on the hill
The clubmosses you meet outdoors are wild plants of heath, bog and mountain, not shop stock. Stag's-horn clubmoss (Lycopodium clavatum) trails long scrambling stems clothed in tiny incurved leaves across acid upland ground, sending up forked stalks tipped with pale cones; fir clubmoss (Huperzia selago) stands in stiff little tufts like a miniature conifer. Their spores were once a minor industry. Lycopodium powder, so fine and oily that it flashes when thrown through a flame, served as stage lightning, as the flash for early photography, and as a dusting to keep pills from sticking. These are slow, particular plants that take years to build a colony, so it is kindest to admire them where they grow; a pulled clump almost never settles anywhere new.
Sorting them from the real thing
In the hand the difference is not hard once you know to look. A clubmoss or spikemoss shoot is firmer and more wiry than a moss shoot, often flattened into fern-like fronds, its leaves set in neat ranks or overlapping like fine scales; tease the base apart and you find true roots where a moss would show only a felt of hair-like rhizoids. Clubmosses give themselves away with those little club-shaped cones held up at the branch tips, which is exactly where the English name comes from. If you enjoy running lookalikes down, the same patient attention sorts moss from the liverworts and hornworts it is genuinely related to, and the broader puzzle of what counts as a moss at all is one I take up under is moss a plant?