A sealed jar with a plant in it is a genuinely closed system — water evaporates, condenses on the glass, runs back down, and the whole thing can go for months without attention. It is one of the few houseplant setups that actually rewards neglect. The catch is that garden centres sell terrarium kits containing plants that will be dead within six weeks.

What the environment is really like

Inside a closed jar the humidity sits near 100%, airflow is almost nil, and temperature swings are damped by the mass of soil and glass. That is a rainforest floor, not a windowsill.

Anything adapted to dry air, bright sun or free airflow will struggle. Anything from a damp, shaded, still environment will thrive to the point of needing pruning.

Plants that actually work

  • Mosses — sheet moss, cushion moss, mood moss. The single most reliable thing you can put in a jar. Slow, forgiving, and they look right.
  • Ferns, specifically the small ones: button fern, maidenhair, lemon button. Maidenhair in particular is miserable in a room and magnificent in a jar.
  • Fittonia (nerve plant). Wilts dramatically on a shelf, never wilts in a terrarium, and comes in red, pink and white veining.
  • Pilea, peperomia and small begonias with textured leaves.
  • Selaginella (spikemoss), which is fussy everywhere except here.

Plants that will not, whatever the label says

  • Succulents and cacti. Sold in terrarium kits constantly. They need dry air and drying-out cycles; in a sealed jar they rot. An open, unlidded vessel is fine for them — a closed one is a death sentence.
  • Air plants (tillandsia). Same problem: they need to dry completely between soakings, and they cannot in there.
  • Anything large or fast. A pothos cutting will fill a jar in three months and then press against the glass and rot.

Building one that lasts

Layers, from the bottom: a couple of centimetres of gravel or leca for excess water to collect below the roots; a thin layer of horticultural charcoal, which keeps the whole thing from going sour; a barrier of fine mesh if you want to keep the layers clean; then a few centimetres of a peat-free terrarium or houseplant mix.

Plant, then water once — lightly. Far less than feels right. The commonest failure in a new terrarium is too much water at setup, and there is nowhere for it to go.

Then seal it, and watch the glass. Heavy condensation that never clears means too wet: take the lid off for a day. No condensation at all means too dry: add a tablespoon of water. What you want is a light mist in the morning that clears by afternoon.

Light, and the one thing that will cook it

Bright, indirect. Never direct sun. A sealed glass jar in a sunbeam is a greenhouse with no ventilation, and the interior temperature climbs fast enough to cook everything inside within a couple of hours. This is not theoretical — it is the most common way a terrarium dies suddenly rather than slowly.

North or east-facing is ideal. A couple of metres back from a south window is fine.

The structure around the jar

A mason jar with a plant in it is a mason jar with a plant in it. A frame around it — a pitched roof, upright members, a base ring — turns the same contents into an object. Our mini jar greenhouse fits a standard wide-mouth mason jar and lifts off in one piece for watering, which is the practical version of the idea: you supply the glass you already own, and the frame does the rest.

For open arrangements that need airflow rather than humidity, the planter range has shallow bowls and individual pots better suited to succulents and cacti.

Maintenance, such as it is

Open it once a month for an hour to exchange the air. Pull anything that yellows before it decays. Trim growth before it touches the glass. Beyond that, a well-balanced closed terrarium can go a year or more without water — there are documented examples that have run for decades sealed.

It is the closest thing houseplants have to a set-and-forget, and it works precisely because it is a small, honest ecosystem rather than a decoration with a plant in it.