Almost any moss survives in a closed terrarium, but some look far better and behave far better than others, and choosing well at the start saves a great deal of fiddling later. A short guide to picking the right ones.
Cushion or carpet
Mosses fall into two broad habits, and a good terrarium usually wants both. Cushion mosses, the acrocarpous kind, grow upright in tight tufts that mound into domes, and they hold that shape behind glass, giving a planting height and small green hills. Carpet or feather mosses, the pleurocarpous kind, creep flat and branching to cover ground and soften the floor between the features. Most good terrariums use one or two cushions for structure and a carpet to knit the scene together, the same pairing that makes a planting read as a miniature landscape rather than a green smear.
Reliable choices
- Bun moss (Leucobryum): the classic pale-green dome, holds its shape for years.
- Broom fork-moss (Dicranum scoparium): springy combed cushions, very photogenic.
- Mood moss (Dicranum types sold under this name): larger clumps for a wilder look.
- Sheet or carpet moss (Hypnum and relatives): the flat green floor.
- Fern moss (Thuidium): fine, frond-like, lovely as an accent.
How they behave under glass
The whole point of a closed jar is constant humidity, and the mosses that reward it are the ones that meet high humidity in the wild: woodland floor and shaded rock species rather than sun-baked wall mosses. Cushion mosses such as Leucobryum keep their domed form for years under glass, where a wall moss that is used to drying out and reviving may sulk in permanent damp. Carpet mosses spread to fill the gaps and will creep a little way up damp wood or stone, which is worth planning for when you place them. All of them grow slowly behind glass, so what you plant is close to what you get; there is none of the waiting for a carpet to fill in that you get outdoors, and no need for it either.
Where to get it
You can collect small amounts from your own garden, walls and paths, which is free and local; take a little from several places rather than stripping one patch, and rinse off soil and creatures. Specialist terrarium suppliers sell named, clean cushions if you want a particular look or to avoid introducing pests. A single cushion of bun moss and a handful of carpet moss will furnish a good-sized jar, so you rarely need much.
What to avoid
Skip bog moss (sphagnum) unless you are deliberately running a wet, boggy setup, as it wants saturation. Be wary of moss lifted with a lot of soil and leaf litter, which brings in mould spores, slugs and springtails; a closed jar will happily incubate all of them. A quick rinse and a look-over saves trouble later.
Settling it in
Whatever you choose, press each piece firmly onto the surface so its base makes real contact, mist it in, and give it bright but indirect light. A cushion set loosely on top will lift and dry at the edges; one bedded down knits in and stays put. See the terrarium guide for building and care, and if a planting starts to look sorry for itself, the causes and cures are set out in why terrarium moss dies.