How to build a closed rainforest terrarium.

In 1829, a London surgeon named Nathaniel Bagshaw Ward sealed a fern frond and some soil inside a covered glass jar and watched it survive on its own for the better part of four years. He hadn't intended to. He'd been trying to study a moth pupa and noticed, almost by accident, that water condensed on the glass each morning and rained back down on the soil at night. The fern thrived. The Wardian case — the direct ancestor of every modern closed terrarium — went on to ferry tea plants from China to India and rubber from Brazil to Malaya, and it changed the global plant trade more than any single object since the wheelbarrow.

Two centuries later, the principle is unchanged. Build the loop right and a closed terrarium runs on the same engine: water transpires from the leaves, condenses on the inside of the glass, and falls back to the soil. There is no watering schedule, no rotation under a grow light, no fertiliser regime. There is only the jar, the plants, and a small amount of attention paid to it once or twice a season. What follows is the full eight-step build — vessel through long-term care — researched against current horticultural literature and the practice of working terrarium builders. Read it once before you start. Half the failures in this craft come from skipping a layer or rushing the planting.

1. Choose the vessel

The jar is the climate. Anything sealable, transparent, and large enough to fit your hand inside the opening will work — which means an old pickle jar with a clamp lid does the job as well as a hand-blown geodesic. What matters is glass clarity (frosted glass starves the plants of usable light), a tight-but-not-airtight closure (the lid should close, but the seal should leak air slowly enough to allow gas exchange over weeks), and a depth of at least 12 cm so you can build the four functional layers without crowding the plants against the lid.

For first-time builders, choose something with a wide opening. Working through a narrow neck with chopsticks is a separate skill, and the most common reason a beginner abandons the project halfway through. A 4-litre apothecary jar with a swing-top lid (about 25 cm tall, 18 cm wide) is the sweet spot: enough volume to host a small ecosystem, wide enough to plant by hand. Wash it with hot water and a drop of plain dish soap, rinse three times, and let it air-dry upside down. Skip antibacterial soaps. Residue on the glass will set back the microbial community you're trying to build in step three.

2. Build the drainage layer

A closed terrarium has no drainage holes. Every drop of water that enters stays inside until it transpires back out, and any water that pools at the bottom and touches the roots will rot the plant from below within a fortnight. The drainage layer — sometimes called the false bottom — is what keeps the substrate above the standing water. It's the single most important structural decision in the whole build.

Use one of three materials, in this order of preference: LECA (Lightweight Expanded Clay Aggregate, sold in 5 mm to 16 mm balls), lava rock (porous, dark, slightly heavier), or aquarium gravel (the cheapest option, but inert and heavy). LECA is the standout choice because it's lightweight, draws excess moisture into its pores by capillary action, and slowly releases it back into the substrate during dry stretches — effectively turning the false bottom into a passive humidifier. Pour 2.5 to 3 cm of LECA across the base, then tap the jar gently against your palm to settle the balls into a flat, even layer.

Don't skip the depth. A 5 mm sprinkle of pebbles is decorative, not functional — it'll wick water straight up into the soil within hours. You want enough depth that when the lower 1 cm fills with condensate over time, the upper 1.5 cm still acts as a buffer.

3. Add activated charcoal

Activated charcoal is the difference between a terrarium that smells like a clean forest floor and one that smells like a stagnant pond. Drop a single 5 mm layer (about half a tablespoon for a 4-litre jar) directly on top of the LECA. Use horticultural-grade activated charcoal, not the briquettes from a barbecue bag — barbecue charcoal contains binders and accelerants that will leach into the substrate and kill the moss. The horticultural kind is sold in plastic pouches at any garden centre and looks like coarse black sand.

Charcoal does two things at the molecular level. First, it adsorbs (literally pulls into its surface area) ammonia and other small molecules produced by decomposing organic matter — the leaf litter that will accumulate in the jar over the months. Second, the porous structure shelters the beneficial microbial colonies that break down that same litter. It's the carbon filter for the closed loop, and it lasts for years before it needs replacing — which, in practice, you almost never do.

4. Lay the mesh barrier and substrate

Cut a circle of plastic mesh (the kind sold for window screens, or a piece of cheesecloth folded twice) to fit the inside of the jar with a 5 mm overlap up the sides. Press it flat over the charcoal. The mesh is what stops the substrate from washing down through the LECA over time and turning the false bottom into compacted mud. Without it, the build will run beautifully for three months and then begin to fail. With it, it will run for years.

On top of the mesh, lay 4 to 6 cm of substrate. The mix matters. Bagged "indoor potting mix" is too rich — the fertiliser will burn the roots inside a sealed jar. Mix your own: equal parts coconut coir (for water retention), orchid bark (for aeration), and a small handful of dampened long-fibre sphagnum moss (for slow-release moisture). Avoid anything with peat in the first two ingredients; peat compacts under repeated condensation cycles and chokes the roots within a year.

Pour the substrate in slowly and slope it. The traditional builder's trick is to mound the back of the jar 1.5 to 2 cm higher than the front. The slope creates depth when you look through the glass — the plants in the back read as further away — and it also improves drainage on the rear-quarter of the jar where condensate tends to pool. Press the soil down gently with the back of a spoon. It should be firm, not packed.

5. Choose your plants

The closed jar is a tropical climate in miniature: 80–95% relative humidity, stable mid-twenties Celsius, indirect light. Choose only plants that prefer that climate. The classic mistakes are succulents and cacti (they rot within a month), Pilea peperomioides (it gets leggy and pale), and most commercial supermarket houseplants (chosen for dry-air tolerance, not humidity).

Five plants worth knowing, all reliable in a closed terrarium with no fuss:

• Cushion moss (Leucobryum glaucum) — pale green, forms a dense pillow, fills empty soil quickly. The single best ground-cover for any beginner build.
• Button fern (Pellaea rotundifolia) — small dark fronds, slow-growing, copes with low light. A classic mid-height anchor.
• Fittonia (nerve plant, Fittonia albivenis) — pink or white veined leaves; thrives in trapped humidity, sulks in dry rooms.
• Peperomia 'Hope' or rosso — succulent-looking but actually high-humidity tolerant; adds a different texture to the canopy.
• Baby's tears (Soleirolia soleirolii) — tiny round leaves on creeping stems; will carpet the jar floor in a few months.

Diana's signature combination, repeated across every house build at Aether & Earth, is cushion moss + button fern + a single Venus flytrap (Dionaea muscipula) + an African violet (Saintpaulia) + a speckled granite pebble. The flytrap is the conversation piece; the violet is the colour; the moss and fern do the structural work. Buy plants from a nursery, not a supermarket — supermarket plants are forced under heavy fertiliser and often carry latent pest infestations that explode in the closed environment of a sealed jar.

6. Plant and arrange

Tip each nursery plant out of its pot, gently squeeze the root ball to break up the dense commercial soil, and rinse the roots under a slow tap until about half the original soil is gone. This sounds aggressive — it is the right thing to do. Commercial soil is heavy with peat and slow-release fertiliser pellets, and if you transfer it whole into the closed jar you'll be feeding the wrong organisms (bacterial slime, mould) for months.

Use a long-handled spoon or a pair of bamboo chopsticks to dig a planting hole the size of the rinsed root ball. Set the largest plant in first, slightly off-centre toward the back of the slope. Work outward and forward, smaller plants in the front. Leave 3 to 4 cm of breathing space between mature canopy lines — these plants will grow, and a tightly-packed initial arrangement turns into a tangled mat by month four.

When everything is positioned, tear the cushion moss into thumb-sized pieces and tuck them around the bases of the plants and across any visible substrate. Press each piece firmly down so the underside of the moss is in good contact with the soil. Add a single small stone or piece of cork bark as a hardscape anchor — one per build, no more. The eye needs a single still object to rest on; clutter ruins the depth illusion the slope creates.

7. First mist and seal

Use distilled water, rainwater, or boiled-and-cooled tap water for the first mist. Do not use straight tap water — the chlorine and dissolved minerals will leave a white residue on the inside of the glass and on the moss within a week. Mist gently across the leaves and substrate until the soil is uniformly damp but not standing in water. For a 4-litre jar, this is roughly 25–35 sprays from a small atomiser.

Wipe the inside of the glass clear of any soil splashes with a clean cloth wrapped around a chopstick, then close the lid. The terrarium is built. Place it in indirect light — a north-facing windowsill, or two metres back from any south or west window. Direct sun on a sealed jar will cook the plants inside an hour.

8. Long-term care

The condensation pattern is the diagnostic. For the first ten days you'll see a thick fog on the inside of the glass each morning that clears partly by afternoon — that's the system finding its loop. After ten days the condensation should appear as a thin film of droplets on the upper third of the glass overnight, then dissipate by mid-morning. That's the running state.

Three things to watch for:

• Heavy condensation that doesn't clear by midday — too much water. Open the lid for an hour to let some evaporate, then re-seal.
• No condensation at all for two consecutive days — too dry. Mist five sprays and re-seal.
• White or grey fuzz on a leaf or the moss — mould. Cut out the affected leaf with sharp scissors, increase the air gap by leaving the lid slightly open for 48 hours, and the colony will retreat.

Beyond that: prune any leggy growth back to a node every few months, and replace the cushion moss when it browns at the centre (usually after 12 to 18 months). Don't fertilise. The closed loop generates exactly enough nutrients from leaf-fall and microbial breakdown to sustain the plants you started with — adding synthetic feed will trigger an algae bloom on the glass and a soft-rot in the lower leaves within a fortnight.

A closed terrarium built well is one of the lowest-input objects you can keep alive. The Wardian case in Kew Gardens that ran from 1833 to 1962 was opened twice in 129 years. Yours probably won't last that long, but it shouldn't need attention more than once a season — and that, exactly, is the point.