The order of the layers isn't decorative

Under the moss and the plants, a closed terrarium stacks four layers, each with its own job. The clay balls at the bottom create a water reserve below the roots. Horticultural charcoal holds back what ferments and cuts down on bad smells. The substrate, aerated and free-draining, holds the roots without ever waterlogging them. The leaf litter on top feeds life in the soil. Skip a layer and you're only putting off the problem by a few weeks: the Which substrate for a terrarium guide sets out the thicknesses.

Tropical or mineral: two regimes

A tropical substrate blends coconut fibre, fine bark and sphagnum moss: it holds moisture without compacting, which is exactly what woodland plants need. A mineral substrate for cacti works the other way round: volcanic rock, coarse sand and very little organic matter, so water drains through in seconds. Getting it wrong shows up as rotten roots, not slow growth.

Going bioactive

A bioactive terrarium adds a clean-up microfauna to the substrate, springtails and isopods, which eats plant waste and early mould. The soil starts to work like a woodland floor: the litter is recycled in place, and the terrarium no longer needs emptying and rebuilding. This calls for a permanent litter layer, constant humidity and a little patience: the colony takes a few weeks to settle in.

What this changes about upkeep

A well-balanced bioactive terrarium needs less intervention than a classic one: fewer dead leaves to pick out one by one, less mould to deal with. In exchange, it can't cope with either drying out completely or fungicide treatments, which kill the microfauna. For the full set-up, follow How to build a closed terrarium.