Choosing plants for a terrarium: which ones cope and which do not
A closed terrarium is an environment that never changes: 90% humidity, still air, light filtered through glass, and space that does not grow. Which species genuinely thrive in it, which die slowly, and why Alocasias do not belong there.
People building their first terrarium choose plants by how they look. It is the wrong criterion, and it is why almost everyone's first terrarium is, four months later, a glass box holding three dead plants and one moss that is getting by.
The right question is not which plant is beautiful. It is which plant tolerates, permanently and without a break, the four conditions a closed terrarium imposes.
The first is humidity that never drops. Inside a closed container the air sits steadily between 85 and 95 per cent. For a tropical understorey plant that is home; for almost everything else it is a sentence to rot, because the leaves stay wet and the tissue never dries.
The second is still air. Even with a gap left open, there is no wind inside a terrarium. Motionless air is the condition fungi work best in, and it is why velvet-leaved plants — the ones everybody wants — are the first to give way: the velvet holds water against the tissue and will not let it go.
The third is light coming through glass. The glass takes a share of it, and the lid takes another. Anyone keeping a terrarium a metre from the window is giving the plants far less light than they think. A low LED bar solves it, and costs less than rebuilding the planting.
The fourth, the one nobody weighs, is that the space does not grow. The plants do. A planting that looks good in month one is a tangle by month six if the chosen species reach an adult size larger than the container.
What actually belongs
The plants that work all come from the same place: the tropical forest floor, under the canopy, where the air is saturated and the light arrives already filtered by the crowns. Small-leaved rhizomatous Begonias do very well, and they are also the loveliest under raking light. Sonerila and Bertolonia — tiny melastomes, with veins that catch the light — were made for this environment, and outside a terrarium they are hard to keep indoors. Selaginella covers the floor and becomes the carpet everything else rests on. Small ferns supply the height. Fittonia is the entry plant: it costs little, forgives nearly everything, and flags trouble before the others because it wilts dramatically and recovers within the hour.
What does not belong, however much you want it to
Succulents and cacti, obviously: they are the shop-window mistake of every store selling closed succulent terrariums, which are objects designed to rot. But the commonest mistake among people who already grow is a different one: Alocasia. They are the plants everyone wants inside a terrarium, and they are also the most wrong. They grow fast, they reach a size the container cannot hold, and the velvet-leaved ones — Black Velvet, Frydek, Infernalis — take rot within weeks in still air with a permanently wet leaf. The same goes for Monstera and any vigorous climber: in two months they have filled everything, in three they crush the rest.
The size rule, put usefully: choose plants by how large they become, not how large they are now. A plant bought small does not stay small because the container is small. It slows, it distorts, and then it smothers what is around it.
How the planting is built, in three layers
The floor is the carpet: live mosses and Selaginella, covering the substrate and holding the humidity low down. The middle is the body of the planting, and it is where the rhizomatous Begonias and the melastomes go. Above sits a single plant giving height, a fern or a more upright Begonia. One only: two tall points in a small container shade each other, and the terrarium loses its depth.
The number of species is nearly always too high. In a container of twenty or thirty centimetres, three species is already plenty. Five look untidy in month one and compete for real by month three. Better three species in several specimens than six in one each: it reads better, and when one fails it does not leave a hole.
Quarantine is not advice, it is the part that decides whether the terrarium lasts. A closed terrarium is the ideal environment for pests too: warm, humid, no predators and no air to disturb them. A mealybug that comes in with a new plant multiplies, behind glass, at a speed it never reaches in the open. Every plant coming in spends two weeks outside first, on its own, and you look under its leaves.
Cuttings or rooted plants
Only plants that already have roots go into a terrarium. A cutting set to root directly in the planting has a fair chance, because the humidity is right; but if it rots it rots in there, touching the substrate of everything else, and in a saturated environment rot does not stay where it started.
A combination to start from, if it is your first
A twenty-five centimetre container with a lid: a base of sphagnum, perlite and biochar; a carpet of Selaginella; two small-leaved rhizomatous Begonias on opposite sides; a Fittonia at the front, acting as the water sentry; a small fern behind, for height. Five plants, four species. An LED bar fifteen centimetres from the glass, ten hours a day.
Closed or open
Closed means constant humidity and almost no maintenance, but also zero tolerance for mistakes: if a plant rots, the problem stays inside. Open means having to mist, but also being able to intervene. For the species discussed here — Sonerila, Bertolonia, understorey Begonias — closed is the only route that really keeps them well.
What to watch, afterwards
The condensation tells you everything: if the glass is evenly misted in the morning and clears by the afternoon, the balance is right. If it stays misted all day there is too much water and it needs opening for a few hours. If it never mists at all, the terrarium is losing humidity and the seal is not holding. White mould on the wood in the first weeks is normal and passes on its own; grey furry mould on the leaves is not, and comes out at once along with the leaf.