Closed or open: the container decides everything else

Choosing the glass isn't just decorative, it sets the terrarium's living conditions. A container that closes holds in humidity and sets up a water cycle: condensation falls back onto the substrate, and woodland plants thrive there for months without watering. An open container lets air circulate and water evaporate: that's what succulents and cacti need, as they rot in a confined space. The Closed or open terrarium guide sets out both regimes in detail.

The format shapes the build

A narrow-necked carboy calls for long tools and patience, but gives the most striking little-world effect. A cloche on a base is put together in the open air and then closed, which is the simplest format for a first attempt. A cork-stoppered jar has a wide opening and reopens easily for trimming. The geometric terrarium, always open, shows off graphic plants at their best.

Volume, glass thickness and position

Allow at least 5 litres for a closed terrarium to settle into balance: below that, humidity swings are too sharp. Check the thickness of the glass at the base, which will bear the weight of the layers, and choose a bright spot out of direct sun, behind a south-facing window, the temperature under glass rises very fast.

To compare formats before buying, read Which container for a terrarium. Once you've chosen the glass, the substrates and drainage layers and suitable plants follow on from that.