If you picture a moss as a neat green dome, you are picturing bun moss. Its rounded, pale grey-green cushions are among the most recognisable and best-loved of all mosses, and the most useful where you want shape rather than a flat carpet.
What it is
Bun moss, also called pincushion moss, is Leucobryum glaucum and its close relatives. The greyish, almost glaucous colour that sets it apart comes from its unusually thick leaves, built with layers of empty cells that store water and scatter light. It is a classic acrocarpous moss, growing upward in tight tufts that mound into firm, springy hemispheres rather than spreading sideways, which is covered more fully in acrocarpous and pleurocarpous mosses.
Where it grows
Look for it on acid ground in shaded woodland and on heaths, where it forms discrete cushions on the floor, on rotting logs and over rocks. The cushions can grow surprisingly large and dense, and a single one detached and set down elsewhere will often carry on quite happily, which is part of its appeal to growers.
Why terrarium makers love it
Behind glass, bun moss is a star. It holds its domed shape for years where a carpet moss would simply spread flat, so it gives a terrarium hills, structure and a sense of miniature landscape. It tolerates the high humidity of a closed jar beautifully. A couple of bun cushions set as low hills with a feather moss running between them is the backbone of many a good terrarium; the species roundup is in the best mosses for a terrarium.
Keeping it
Give bun moss what it has in the wild: shade, steady damp and acidic, low-nutrient conditions, watered with rainwater rather than hard tap. It resents lime and bright sun. It is slower to spread than the carpet mosses, so treat it as a specimen to place rather than a quick cover, and if you collect it from the wild take single small cushions from where it is plentiful rather than clearing a patch, as set out in collecting moss responsibly.