Terrarium care: watering, condensation, mould, light and springtails

TUREFF• •0

A closed terrarium is often described as self-sustaining, and a balanced one comes close. But it gets there through a few weeks of adjustment, and it stays there because someone checks on it now and then. Here is how to read what the glass is telling you.

Watering

Closed terrariums recycle most of their water. Many need a light top-up only every few weeks or months. Check the soil: if it looks pale and dry and the glass has had no condensation for days, add a little water with a spray bottle or a small spoon.
Open terrariums dry out much faster. For succulents, water when the substrate is dry all the way down, and give a small amount around the roots rather than a soaking, as there is no drainage hole for the excess to leave.
Watch the drainage layer. If water is visibly pooling in the gravel at the bottom, you have given too much. Leave a closed jar open for a day or two, or soak up the excess with a paper towel twisted into a wick.

Reading condensation

Light misting that comes and goes through the day is a sign of a healthy closed jar: the soil is damp and the cycle is working.
Heavy condensation that never clears, droplets running down the glass all day, means too much water. Take the lid off for a few hours a day until it settles.
No condensation at all for days at a time usually means it is too dry, or the lid is not sealing.

A jar near a cold window or a radiator will show more or less condensation as the room temperature changes, so judge it over several days.

Light and placement

Most terrariums do best in bright, indirect light, such as near a window that does not get hours of direct sun.
Avoid direct sun on a closed jar. Glass traps heat, and a sealed jar on a sunny sill can overheat and cook the plants inside.
Turn the jar now and then so plants do not all lean towards the window.
If your room is dim, a small LED grow light above the jar works well. Keep it on a timer so the plants get a regular day and night.
Open succulent bowls need far more light than closed jars, ideally a bright window with some direct sun.

Mould

A little white fuzz on a new piece of wood or on fresh soil in the first weeks is common and often fades as the jar settles. Persistent or spreading mould usually means too much water, too little airflow or something rotting.

Remove dead leaves, fallen petals and anything soft or brown straight away.
Wipe or lift out mouldy patches with a cotton bud or tweezers.
Air the jar by leaving the lid off for a while each day.
Water less.

Springtails: the clean-up crew

Springtails are tiny soil invertebrates, usually only a millimetre or two long, that live in damp soil and leaf litter. In terrariums they are kept on purpose because they feed on mould and decaying matter and compete with mould for food, which helps keep it in check. They do not harm healthy plants. They are sold in small culture tubs for terrarium and vivarium keepers; tip some in, keep the substrate damp but not soggy, and they usually establish themselves. Some keepers add small isopods too, which break down larger debris, though these suit bigger set-ups better than small jars. Seeing springtails jump on the soil or glass is a good sign, not an infestation.

Pruning and feeding

Prune anything touching the lid or crowding its neighbours. Long tweezers and small scissors make this easy. Remove the cuttings, as they will rot if left.
Feeding: most terrariums need very little. Slow growth is usually what you want. If plants look pale after a year or more, a heavily diluted houseplant feed once in a while is enough.

A simple routine

Look at it every few days: condensation, colour of the leaves, any fuzz.
Every few weeks: remove dead material, wipe the glass inside, top up water if needed.
Every few months: prune and rearrange if anything has outgrown its spot.

Post your condensation puzzles and mould questions, or share a jar that has run for years, in the comments.