Why is my terrarium full of condensation?
A little condensation on the glass is normal and means your closed terrarium is working. Water evaporates, hits the cooler glass, runs back down into the soil, and the cycle repeats. You only need to act when the glass is so misted up that you can't see the plants for days on end, or when you spot mould.
This becomes a common question in autumn, because the heating goes on and rooms get warmer while windows get colder. That bigger temperature difference means more mist on the glass, even though nothing about your terrarium has changed.
When condensation is fine
Mist that comes and goes is a healthy sign. You'll often see the glass clouding in the morning and clearing by the afternoon, or misting on the side facing the window. As long as the leaves look healthy and you can see into the terrarium at some point during the day, leave it alone. Small droplets running down the inside are also normal. That's the water cycle doing exactly what it should.
When there's too much water
There is a problem if the glass stays completely opaque all day, if water is pooling at the bottom above the drainage layer, or if leaves are turning yellow and mushy. That usually means there is more water in the system than the soil can hold.
The fix is straightforward. Take the cork or lid off and leave it open for a few hours, or up to a day if it's very wet. Wipe the inside of the glass with a dry cloth or a piece of kitchen paper on a stick. Then close it again and watch it for a week. Repeat if needed, but resist opening it every day, because the point of a closed terrarium is that it keeps its own moisture.
When it's too dry
The opposite happens too, usually in winter. If you see no condensation at all for a week or more and the soil looks pale, the terrarium needs a little water. Add it in small amounts, a tablespoon or a few sprays at a time, and wait a day or two before adding more. Overwatering is far harder to correct than underwatering.
What about mold?
White fluffy patches on the soil or on wood are common in a new terrarium and often disappear on their own once the ecosystem settles. Take out anything visibly mouldy along with any dead leaves, since decaying material is what mould feeds on, and leave the lid off for a day to bring the humidity down.
Grey mould spreading across living leaves is more serious. Remove the affected leaves, ventilate for a day or two, and move the terrarium somewhere slightly cooler and brighter, though never into direct sun.
Autumn checks worth doing
- Move the terrarium away from a radiator. Direct heat on glass creates constant misting.
- Keep it off cold windowsills at night, where the temperature swing is greatest.
- Trim any growth pressing against the glass, as leaves touching wet glass tend to rot.
- Wipe the outside of the glass too. It's often outside dust, not inside mist, making things look cloudy.
If your terrarium has been running for a season or two and looks tired rather than wet, a refill kit with fresh moss and plants brings it back without starting over. A few basic plant care tools, particularly long tweezers and small scissors, make trimming inside a narrow bottle far less awkward.
In short
Mist that comes and goes is your terrarium working properly. Only step in when the glass never clears, water is pooling or leaves are going soft, and then air it out rather than starting again. In autumn, keep it away from radiators and cold glass and most of the problem sorts itself out.
