Few plants are as quietly everywhere as Hypnum cupressiforme. Glance along a fence rail, a churchyard wall or the shaded side of an old apple tree, and the flat green mat pressed against the wood or stone is very often this single species, reckoned the commonest moss across much of the temperate world.
Recognising it
Plait moss creeps. Its shoots run along the surface and branch as they go, so it builds a flat, trailing mat rather than a standing dome. Each shoot is clothed in small curved leaves that all hook over to one side, like teeth on a comb or the foliage of a cypress, which is where the older name cypress-leaved comes from. The mat takes on a faint gloss as it dries, brighter than most mosses, and shades from fresh green in deep shade to a bronzed yellow-green out in the open. It sits squarely among the creeping, branching mosses, the group set out in acrocarpous and pleurocarpous mosses.
The moss that grows almost anywhere
Tolerance is its whole trick. You will find it on bark, dead wood, fence posts, roof tiles, brick, bare rock, gravestones and packed soil, coping with sun or shade and with ground that is acid or limey, in clean country air and in town. Hardly any other moss spans so many situations, which is exactly why it turns up so reliably. Botanists have long divided it into several varieties and look-alike segregates according to the surface it grows on, so the plait moss on a tree trunk may not be quite the same plant as the one on a rock; for anyone gardening with it, though, they behave as one.
The sheet moss of terrariums and walls
When a supplier sells "sheet moss" for terrariums, floristry or a living wall, a Hypnum is very often what is in the bag. The flat growth lifts away in coherent sheets, drapes obediently over a curved surface and knits back down wherever it meets damp bark or soil, which makes it ideal for lining a living moss wall or carpeting the floor of a glass case. That same low, even habit is why it features so heavily in the terrarium species roundup, usually as the carpet that runs between the cushion mosses. Living or preserved, it is one of the staples of the moss trade.
Growing and keeping it
Because it asks for so little, plait moss is among the easiest mosses to establish. Lay a sheet moss-side up on firm, weeded ground, press it into close contact and keep it damp with rainwater, and it will usually take hold and start creeping outward within a season. Behind glass it revels in the steady humidity and wants only soft light and the odd misting. The conditions it dislikes are few: stagnant deep shade with no moving air, where it thins and grows leggy, and a thick sodden wad of itself, which rots from the middle as any moss eventually will. Thin it, give it a little light and air, and it largely sees to itself.
Worth learning first
For all its commonness, plait moss repays a close look. It is the species that trains you to read a surface, because once you can name it on sight you begin to notice the scarcer mosses growing in among it. Anyone learning to tell mosses from their look-alikes could do worse than start here, using its familiar glossy mat as the green against which everything less ordinary stands out.