How to Make a Closed Terrarium (Step by Step)

A closed terrarium is the closest thing houseplant care has to magic: a sealed glass world that waters itself. Plant it well, put it in the right light, and the same water cycles from soil to leaf to glass and back for months — the oldest known sealed terrarium has been going for decades. As the days shorten and the gardening year moves indoors, it's the perfect autumn-weekend project, and a forgiving one: the plants inside live in a humid microclimate that covers for most beginner mistakes.

How a closed terrarium works

Inside the sealed vessel, water evaporates from the soil and transpires from the leaves. It condenses on the cool glass, runs down, and re-wets the soil — a miniature rain cycle. The plants also recycle air: oxygen and carbon dioxide cycle between photosynthesis and respiration. Your job is only to set the starting conditions: enough (but not too much) water, light that powers the cycle without cooking it, and plants that enjoy 90% humidity.

What you need

• A clear glass container with a tight-fitting lid — a preserving jar, a corked bottle, a cookie jar, an old fish bowl with a glass plate on top. Bigger is easier to balance than tiny.
• Free-draining houseplant potting mix, ideally cut with a little orchid bark or perlite.
• A few spoonfuls of activated charcoal (from garden centres or aquarium shops) to keep the sealed environment sweet.
• Small, slow-growing, humidity-loving plants (see below).
• Optional: moss for carpeting, small stones or wood for landscaping, and a spoon, chopstick, and long tweezers as planting tools.

Tip: you can skip the thick gravel "drainage layer" you see in older tutorials — current advice is that a deep rock layer just creates a stagnant water reservoir. A thin decorative layer of pebbles is fine, but the real work is done by an airy substrate mixed with charcoal, and by not overwatering in the first place.

Plants that thrive under glass

Choose naturally small, slow-growing species that love humidity and tolerate moderate light: nerve plant (Fittonia) for colour, small ferns such as button fern or young Boston ferns, Peperomia, baby tears (Soleirolia), small Pilea species, polka dot plant (Hypoestes), and any cushion or sheet moss. One rule is absolute: no succulents or cacti in a closed terrarium. They need dry air and dry roots and will rot within weeks under glass — give them an open, unlidded bowl instead.

Building it, step by step

• 1. Wash the container thoroughly and let it dry — you're setting up an ecosystem, so start clean.
• 2. Add 4–8 cm of potting mix blended with a handful of activated charcoal, deep enough to take the largest root ball. Slope it higher at the back for depth.
• 3. Unpot each plant, tease away excess soil, and trim overlong roots. Dig a hole with a spoon, set the plant in, and firm gently — tweezers help in narrow bottles.
• 4. Leave growing space between plants; they will fill gaps faster than you expect. Press moss onto the bare soil, add stones or wood, and brush soil off the glass.
• 5. Water sparingly around the edges — for a medium jar, a few tablespoons is enough. The soil should be barely moist, never muddy.
• 6. Put the lid on and place the terrarium in bright, indirect light.

Light: the one thing you must get right

Warning: never put a closed terrarium in direct sunlight. The glass works like a greenhouse; even an hour of direct sun can push the inside temperature high enough to cook every plant in it. Bright, indirect light is ideal — near an east- or north-facing window, or a step back from brighter ones. Rotate the vessel occasionally so growth stays even.

Reading the glass: watering and condensation

The condensation on the glass is your gauge. A light fog on part of the glass for part of the day is perfect. Glass dripping wet around the clock means too much water — take the lid off for a day or two to let it dry out, then reseal. No condensation at all, with soil looking pale and dry, means add a spoonful or two of water. Once balanced, a closed terrarium typically needs water only every few weeks to few months — some tight-lidded bottles go half a year. When you do water, use rainwater or distilled water if you can; tap-water minerals slowly mark the glass.

Maintenance and common problems

• Mould in the first weeks: normal in a new build. Remove fuzzy patches and the leaf they sit on, air the terrarium for a day, and consider adding springtails — tiny soil creatures that eat mould.
• Yellow or dead leaves: pinch them out promptly before they rot, and trim any leaf pressed flat against the glass.
• Plants outgrowing the jar: prune, don't repot. Regular pinching keeps the landscape in scale.
• Feeding: almost never. At most, a quarter-strength liquid feed once a year in spring — a fertilised terrarium is a jungle in six months.

Not sure which of your existing houseplants are terrarium candidates, or what species that mystery moss is? Point PlantMe at it — the app identifies 35,000+ plant classes with 93% top-3 accuracy, and its care cards tell you instantly whether a plant wants humid, closed-jar life or dry air and sun.