Walk into a craft shop or a terrarium supplier and the moss on offer falls under two broad headings: sheet moss, sold as flat mats that peel up like turf, and cushion moss, sold as firm rounded domes. The labels are trade shorthand rather than species names, but the split behind them is real, and knowing which one does what will save you a good deal of wasted moss.
What the two names mean
Sheet moss is the flat, spreading kind. It grows as a carpet of creeping stems that branch sideways and root down wherever they touch, so it lifts in coherent sheets and lays back like turf. Most bags labelled sheet moss hold a Hypnum, the plait moss, though feather mosses such as Thuidium reach the shelves the same way. Cushion moss is the domed kind, a colony of upright shoots packed so densely they mound into a firm hummock that keeps its shape when lifted. The pale grey-green domes are nearly always Leucobryum, the bun moss; the springier deep-green mounds often sold as mood moss are Dicranum, the broom fork-moss. Why one creeps and the other clumps comes down to their growth architecture, laid out in acrocarpous and pleurocarpous mosses; for choosing between them at the counter, the shape in your hand tells you most of what matters.
What sheet moss is good for
Anything that needs covering. Because it drapes over curves and knits back down where it meets damp ground, sheet moss is the material for a kokedama ball, which asks the moss to wrap a sphere and stay put; the classic bag sold for kokedama is exactly this. It carpets the floor of a terrarium, runs between stepping stones in a miniature landscape, backs a wreath frame and clothes a living wall, closing broad areas faster and more evenly than any dome could. Laid on damp ground and pinned flat, it spreads at the edges and fills its own gaps in time. Whenever the aim is an unbroken green surface, at whatever scale, this is the moss that delivers it.
What cushion moss is good for
Height and focus. A cushion holds its rounded form for years, so it reads as a hill or a small green boulder, the kind of relief a flat carpet can never supply. Inside a terrarium a few domes give the planting its skyline, rising behind a floor of sheet moss so the eye travels over contours instead of a level lawn. On a bonsai tray or in a kusamono planting, one firm cushion at the foot of the trunk settles the whole composition. What a cushion will not abide is being flattened: wrap Leucobryum around a kokedama ball and it goes brown at its core, resenting the squeeze, which is why the doming mosses belong in open dishes and under glass rather than on a sphere.
Humidity and handling
The two headings carry a rough guide to their needs as well. Cushion mosses mostly hail from humid woodland floors and shaded rock, so they want steady moisture and repay the close air of a sealed jar; Dicranum in particular revels in high humidity and can sulk in a dry room. Sheet mosses tend to be more easygoing, tolerating a more open setting and quicker to bind into fresh ground. Both dislike direct sun and both revive from dry, though a cushion left to bake will brown at its heart long before a thin carpet shows any distress. Whichever you bring home, soak it before use, lay it on something that stays damp, and give it shade while it settles in.
Using them together
The best plantings rarely pick one and drop the other. A terrarium or a dish garden gains most from both at once, cushions for relief and sheet for the ground between, the domes as hills and the carpet as the valley floor. Buy a little more sheet moss than you expect to need, since it is what you use to hide edges and blend the seam where dome meets floor, and keep a spare cushion or two for the focal points. If you want one dependable choice for a closed jar while you find your feet, the forgiving woodland domes described in bun moss are the place to start; for everything that wants covering, reach for the sheet.