Sheet moss makes the floor: it spreads, covers quickly, and gives the continuous surface that makes a terrarium look finished. Cushion moss makes the relief: it forms dense domes that stand in for hills and give depth. They do not replace each other — in a container of 20 cm and up, the right answer is to use both.

The comparison

Cushion moss

It grows in tight domes, a pale green with a slight silver cast when the surface dries. Its density is what makes it interesting: set on a mound of substrate, it immediately gives the illusion of a mossy hill seen in miniature.

Its use: to mark high points, underline a piece of relief, set a foreground in front of a root. It is used in touches, not as cover.

Its constraint: it is slow. It keeps its shape and grows very little. It also needs good contact at the base, or it lifts away — hence the importance of the technique described in How to lay moss in a terrarium.

Mind the format: a 6 cm cushion in a 12 cm jar flattens the sense of scale. Keep it for containers of 20 cm and up.

Sheet moss

It spreads as a soft, thinner carpet in a clear green. This is what gives you the floor: the continuous surface on which everything else visually rests.

Its use: to cover, to tie things together, to hide the edges of root balls, to close up a composition.

Its advantage: it establishes and spreads faster than a cushion. Within two to three months the joins between pieces disappear and the carpet becomes continuous. It is also the one that multiplies best from fragments — see Growing moss in a terrarium.

Its limit: on its own it gives a flat floor. Without relief, a terrarium soon looks monotonous.

The mossy stone

A third option, often forgotten: a stone already covered in moss. It brings ready-made relief without having to build a mound of substrate, and it is stable — it will not lift away.

It does not replace a floor moss: it sits on a carpet of sheet moss, as a piece of hardscape. It is the quickest trick for giving volume to a flat composition.

How to combine them

The composition that works, in a container of 20 to 25 cm:

1. Sheet moss over the whole visible surface of the substrate, cut to follow the edges;
2. Two or three cushions set on mounds, never lined up or evenly spaced — nature does not do symmetry;
3. A mossy stone as a high point, if the composition lacks volume.

Always leave a small area of substrate visible, usually at the front: it lets you keep an eye on the moisture, and it gives the eye somewhere to rest.

What they have in common

Both ask for exactly the same conditions, and both fail for the same reasons:

• indirect light every day, without which they pale and then brown;
• high humidity, so a closed terrarium;
• firm contact with the substrate when laid;
• no standing water, which rots them from underneath.

The symptoms and their causes are set out in Moss turning brown in a terrarium.

In short

Small container, under 20 cm → sheet moss alone.

Container of 20 cm and up → sheet moss as the floor, cushions for relief.

A composition that looks flat → add a mossy stone before you add more plants.

The species are covered in detail in Mosses for terrariums, and our selection is on the mosses shelf.

Sources

• Creating a Closed Terrarium — Penn State Extension
• How to Design a Closed-System Terrarium — Mississippi State University Extension