A glass vessel gives moss exactly what it likes and we struggle to provide indoors: high humidity, soft even light and still air. A closed terrarium can keep a moss landscape fresh for years with almost no attention.
Open or closed
A closed jar or bottle traps moisture and recycles it, so it suits mosses and other damp-lovers and needs watering only rarely. An open bowl dries faster, wants more frequent misting, and suits plants that dislike constant wet. For moss, closed is the easier and more reliable choice.
The layers
Build from the bottom up: a layer of gravel or clay pebbles for drainage, a thin layer of activated charcoal to keep the water sweet, a mesh or fibre divider if you like, then a few centimetres of substrate. A free-draining, low-nutrient mix is best; rich compost encourages mould under glass.
Which mosses
Cushion species hold their shape beautifully behind glass. Bun moss and the broom fork-moss keep their domed, combed look for a long time. Carpet-forming feather mosses spread to fill gaps and soften the floor. Avoid bog mosses unless you are running things very wet.
Planting and care
Press each piece of moss firmly onto the substrate so it makes full contact, then mist lightly. Stand the terrarium in bright, indirect light, never direct sun, which cooks the inside in minutes. A closed jar should show a light mist on the glass; clear it for a while if it streams with water, and add a spray if it dries out. That is more or less the entire job.
Hardscape and arrangement
A jar of flat moss is pleasant; a little landscaping makes it absorbing. A couple of stones, a knot of driftwood or a piece of cork bark give the eye somewhere to rest and the moss something to climb. Set the taller cushions towards the back and let a carpet run forward to the glass, so there is a sense of depth rather than a flat lawn. Odd numbers of features tend to look more natural than even, and a single bold stone usually beats a scatter of small ones. Tweezers and a long spoon help you place things in a narrow-necked vessel.
The long game
A sealed moss terrarium is one of the few planted things that genuinely thrives on neglect, since the water cycles round inside it and the moss asks for almost nothing. Months can pass between sprays. The usual reasons one fails are a sunny windowsill, which overheats it, and too much rich soil or buried leaf litter, which feeds mould in the still air. Keep it bright but shaded, keep the substrate lean, pull out anything that starts to fur over, and a good terrarium will hold its looks for years. For diagnosing the wobbles, there is a dedicated troubleshooting guide.