Beginner's Guide to Terrarium Care: Watering, Light & Common Mistakes
A terrarium is one of the lowest-maintenance ways to keep something green and alive in your home — but the few rules it does have are worth getting right. Here's everything we tell our Masterclass guests about keeping theirs thriving.
Open or closed?
The first thing to know about your terrarium is which kind you've got, because it changes everything else. A closed terrarium — like the cork jars, glass eggs, and our Eden Project domes (yes, even with that beautiful glass dome, the cork lid still seals it shut) — works like a tiny self-contained ecosystem. The moisture inside evaporates, condenses on the glass, and rains back down on the plants, so it needs very little watering once it's settled in. An open terrarium, like an uncovered glass bowl or dish garden, breathes more freely and dries out faster, closer to a normal houseplant.
If you're not sure which you have, look for a lid. Cork stopper, snug glass dome, or anything that seals = closed. No lid at all = open.
Our glass egg terrariums are a good example of a closed system — the curved glass seals tight, so the moisture cycle does most of the work for you.
Watering: less is more
The single biggest cause of terrarium trouble is overwatering, by some distance. We never mist a closed terrarium — misting wets the leaves and the air rather than the roots, and encourages mould. Instead, water directly at the root of each plant with a narrow nozzle spray or a pipette, in small amounts. Done properly, a closed terrarium can easily go several months without needing any water at all.
Rather than watering on a schedule, watch for the signs your terrarium is actually thirsty: the plants start to wilt, the moss fades from a healthy green to a light, dry brown, or the terrarium itself simply feels dry rather than holding any moisture. Any one of those is your cue to water — not the calendar.
Open terrariums dry out faster and need more regular attention, similar to any small houseplant — water at the roots when the top of the soil feels dry to the touch, then let it drain.
Whichever type you have, water only at the base of the plants, never over the leaves, and never let water pool at the bottom of the glass for long. A thin drainage layer of small stones or activated charcoal under the soil — the same layer we build in during our Masterclasses — makes a real difference here.
Light: bright, not direct
Terrariums love bright, indirect light — a spot near a window but out of the direct midday sun. Direct sunlight through glass acts like a magnifying lens and can scorch leaves or cook the inside of a closed terrarium surprisingly quickly, especially in summer. If your terrarium sits on a windowsill, a few hours of gentle morning or evening sun is usually fine; midday sun is the one to avoid.
Too little light shows up just as clearly: leggy growth, pale colour, and moss that turns brown and crisp rather than soft and green. If that's what you're seeing, move it somewhere brighter before you reach for the watering can.
Common mistakes to avoid
- Watering on a schedule instead of by eye. Terrariums don't read calendars — check the glass and the soil, not the date.
- Skipping the drainage layer. Without stones or charcoal beneath the soil, water has nowhere to go but straight to the roots.
- Trimming too rarely. In a closed terrarium especially, plants can quickly outgrow the space. Pinch back new growth before it presses against the glass.
- Removing the lid \"to let it breathe.\" A closed terrarium is meant to be closed — taking the lid off regularly defeats the self-watering cycle and dries everything out.
- Ignoring early mould. A little white fuzz on soil or bark is common and usually harmless, but if it spreads, it's a sign of too much moisture or too little air movement. Crack the lid for a day or two to dry things out.
Want to build your own from scratch? Our Terrarium Masterclasses walk you through the whole process, drainage layer and all.
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