Square Cube: A Glass Box Terrarium Without a Single Round Edge

Flat glass changes the rules. A cube gives you four clear viewing faces and a flat substrate surface, which means you can design the planting as a plan view rather than a slope — and it means bad irrigation shows up as a rectangle of dry moss instead of a soft gradient. This build leans into that precision.

By The Terrarium DeskPublished 2026-10-02

Stratum by stratum

Coarse silica base

2.5 cm of coarse silica sand rather than gravel. Sand packs tighter and spreads weight evenly across a flat-bottomed cube, so the substrate above it does not slump toward one corner over time.

Charcoal seam

0.5 cm charcoal. Keep the line perfectly level — in a cube, an uneven charcoal seam is visible from all four sides and is the first thing anyone notices.

Peat and bark substrate

Five centimetres of peat mix with milled bark, no loam. Loam compacts under its own weight in a flat cube and turns to brick; peat and bark stay open and let the fine surface roots of Soleirolia run freely.

Pellia skin

Press a pellia thallus over the whole surface and push its edges against the four glass walls. When the edges meet the glass, you see no substrate at all from the side — the cube reads as a solid block of green.

Plan-view planting

Arrange clubmoss in a diagonal band across the centre, Soleirolia in the two opposite corners, and the fern gametophyte tucked against one wall. Water only at the corners with a skewer-guided trickle, and cover with the glass lid.

Growing on

Soleirolia is the ground cover and will fill any gap you leave; it is the plant that makes this cube look finished, and also the one that will smother everything else if you do not pinch it back monthly. Golden clubmoss has the most specific requirement in the build: it wants consistent humidity and will turn olive-brown at the tips if the lid is left off for a day. The fern gametophyte is a slow grower and works as a permanent accent rather than a spreading plant.

Upkeep rhythm

A glass lid on a flat cube is a poor seal — expect to mist every four or five days rather than monthly. Wipe the inside of the lid with a lint-free cloth whenever condensation builds into visible droplets, since those droplets fall straight down onto the planting and create wet spots that turn into mould. Rotate the cube a quarter turn weekly because a flat-sided vessel in a window gets very uneven light. Reposition pellia edges against the glass every few months as they lift.