Care guide
Most terrariums that die are killed by attention. What follows is nearly everything we know, and it is short on purpose.
Light
Bright, indirect light. A north or east window, or a metre or two back from a brighter one. Never direct sun through glass — a sealed jar on a south-facing sill in May will cook, and it happens faster than you would think. If the glass is warm to the back of your hand, move it.
Moss will tell you when the light is wrong before anything else does. Too little and it goes leggy and pale; too much and it bleaches at the tips and browns.
Water, and how little of it
A sealed terrarium is a closed loop. Water evaporates, condenses on the glass, runs back down. If yours is working you should see a light mist on the inside in the morning that clears by the afternoon. That is the whole diagnostic.
Permanent heavy condensation means too much water: take the lid off for a day, let it breathe, seal it again. No condensation at all for several days means too little: mist two or three times, no more, and wait a week before deciding anything. A closed terrarium that has settled may not need water again for six months or more.
Open terrariums — succulents, cacti, air plants — are the opposite. They want a proper soak and then to dry out completely. Air plants: a twenty-minute soak once a week, then shake them out and let them dry upside down so water does not sit in the centre.
Sealing a closed terrarium
Build it damp, not wet. Press the moss down firmly so it is in real contact with the substrate — moss that is merely resting on top will dry and lift. Clean the inside of the glass before you seal it, because you will not get another easy chance. Then put the lid on and leave it somewhere you can see it for a fortnight while it finds its own balance.
Mould
White fuzz in the first few weeks is common and usually passes on its own. It is feeding on something organic that came in with the build — a bit of leaf litter, a scrap of bark.
If it spreads: take the lid off, lift out whatever it is growing on, and let the jar dry a little more than usual for a week. Do not spray anything into it. Fungicide will kill the moss along with the mould, and the moss is the part you wanted. A springtail culture will clear minor mould permanently, and they are nearly invisible.
Trimming
Moss does not need trimming. Ferns do — cut anything browning at the base with a small scissors rather than pulling, and take out the cuttings, do not leave them to rot.
Still stuck?
Send us a photograph. hello@mossbell.shop. We can usually tell what is wrong from the condensation line alone.