Terrarium Troubleshooting
A LAB Jar terrarium — most of what looks alarming inside one is a normal part of its ecosystem settling in
A closed terrarium runs on its own little weather system, and that means it sometimes looks like something's wrong when it isn't. Here's how we'd read the most common things customers ask us about.
Heavy condensation on the glass
Condensation happens when cool air outside the jar meets the warmer, moister air inside, and on its own it's completely normal — a light misting on the glass is actually a good sign the humidity cycle is working. It only becomes a problem if it builds up so much you can't see the plants. If that happens, take the lid off for a couple of hours to let some moisture escape, then put it back on. Avoid leaving the lid off too long, especially in a dry room, or you'll tip the balance the other way.
White, fuzzy spots on the soil are usually saprophytic fungi — they break down organic matter and aren't harmful to your plants. Scoop out what you can see, air the terrarium out for a few hours, and check you're not overwatering.
Mould on a leaf, or small white spots on a bonsai trunk
If you spot actual mould growing on a leaf, wipe it off and prune the leaf if it's already damaged — left in place it can give off a faint rotten smell as it decays. The same goes for small white specks on a bonsai trunk: that's mould too, and a clean cloth and an airing-out session will usually sort it. After removing it, leave the lid off for a couple of hours to dry the air out slightly, and repeat over the next week or two if needed while the terrarium settles.
Tiny mushrooms appearing in the soil are different and nothing to worry about — they're a sign of a rich, healthy substrate and will die back naturally within a couple of weeks. Leave them be as long as your plants still look healthy.
Small flies around the soil
Fungus gnats hatch from eggs laid in damp soil and are common across houseplants, not just terrariums. An organic gnat powder sprinkled just under the top layer of soil, or a sticky trap placed nearby, will bring the population down. They're more a nuisance than a real threat to the plants.
Plants looking weepy, pale or dry
This is your terrarium asking for a small drink. Open the lid, feel the soil and moss with a finger, and if it's genuinely dry, add a couple of tablespoons of water with a pipette or syringe at the base of the plants — avoid the leaves where you can. A looser-fitting lid will need topping up more often than a tight seal, since more moisture escapes between waters. Never let it swing between bone dry and soaking; the goal is a steady, modest amount of moisture.
Plants growing too tall for the jar
Open the lid and prune the stem a couple of leaves down, cutting just below the node rather than trimming individual leaves — this gives the plant room to keep growing without outgrowing the glass. Ficus Ginseng will release a sticky white latex when cut; it seals itself quickly, just wash your hands afterwards.
When to actually worry
- Normal: light condensation, the occasional white fungal fleck, a passing mushroom, a few fungus gnats.
- Worth acting on: mould actually on a leaf or trunk, soil that's gone properly dry, or condensation so heavy you can't see the plants at all.
- The golden rule: observe daily, make small changes, and avoid drastic swings between very wet and very dry.
Still stuck on something? We offer a Terrarium A&E service for a deeper fix.
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