White fuzz or a soft, dark stem in your terrarium? Learn the difference between mould and rot and how to fix both quickly and safely.
White or grey fuzz on the soil, moss or wood usually points to mould: it's typically harmless and clears up on its own once you give the terrarium some fresh air. Soft, dark, collapsed stems or leaves, often with an unpleasant smell, point to rot instead: that means water has been sitting around the roots for too long and you need to step in. In this article we explain the difference, how to spot both problems, and exactly what to do, step by step, to get your terrarium healthy again.
Mould forms on the surface: on the potting soil, between the moss, or on a piece of wood, usually as a white or pale grey haze. It feeds on organic matter in humid, still air and is, in most cases, harmless to the plants themselves. Rot, on the other hand, develops inside the plant - in the stem, the roots, or the underside of the leaves - and results from prolonged excess moisture around the roots without enough drainage. Where mould usually disappears on its own with a bit of extra air circulation, rot needs real intervention to stop the whole plant from being lost.
Mould shows up as a thin, white or grey fuzzy layer on the soil, on moss, or on pieces of wood inside the terrarium. It usually appears after a period when the terrarium has stayed closed for a long stretch - right after setting it up, for example, or during a cold spell when you've opened it less often. Mould rarely smells unpleasant and usually doesn't harm the plants themselves; it's mainly a sign that there's temporarily too little air circulation.
You can recognise rot by a stem or leaf stalk that feels dark, soft and collapsed, often right above the soil. In more advanced cases you'll notice a distinctly musty or sour smell coming from the terrarium. Rot develops because the roots have sat in overly wet soil for too long, usually because the drainage layer of clay pebbles and activated charcoal beneath the potting soil is no longer working properly, or simply because there's too much water in the system.
Open the terrarium for a few hours to a full day to let air circulate; that's often enough to make mould disappear. Gently remove any visible fuzz with tweezers or a cotton swab, then let the surface dry briefly before closing the terrarium again. Our Sven XL Bonsai terrarium already comes with a layer of clay pebbles and activated charcoal beneath the potting soil by default, which helps drain away excess moisture and limits mould formation.
Cut away affected stems or leaves with clean scissors, remove as much rotten material as possible, and check whether the drainage layer is still doing its job. If the layer of clay pebbles and activated charcoal is saturated or has become too thin, refresh it with fresh material from our terrarium supplies collection, so moisture is properly absorbed again before it can sit around the roots.
Sometimes a plant can no longer be saved: if rot has already reached the main stem or most of the root system, trimming off a few leaves won't do much good anymore. In that case it's wiser to remove the plant completely before the rot spreads to the rest of the terrarium, and fill the empty spot with a new plant that suits a closed, humid environment. That way, the rest of your mini-ecosystem - such as the fern and the Fittonia Ruby Lime in the Sven XL Bonsai terrarium - stays healthy and unaffected.
Most mould and rot problems occur because a terrarium stays closed uninterrupted for too long, or because too much water was added when setting it up. So open your terrarium for a few hours about once a month as standard practice, don't use more water than necessary, and keep the drainage layer intact. Wondering whether the moisture in your terrarium is properly balanced? Our article on why your terrarium is full of condensation explains how condensation relates to moisture balance, and when it is or isn't a problem.
Usually not. Mould on the soil or moss is superficial and often disappears on its own once you let the terrarium air out for a while. Only if the mould spreads to the plant itself does it need action.
A musty or sour smell usually points to rot deeper in the potting soil or at the roots, caused by too much retained moisture. Check the stems right above the soil for soft, dark spots.
That depends on how saturated the layer is, but check it at least as soon as you repeatedly notice rot or an unpleasant smell. Fresh clay pebbles and activated charcoal absorb excess moisture more effectively again.
Yes. Mould mainly forms in still, humid air, so airing it out regularly - about once a month for a few hours - keeps air circulation up and significantly lowers the chance of mould.
Not sure whether your own terrarium is still healthy? Go through the steps above and use the Sven XL Bonsai terrarium as an example of what a good drainage layer looks like.
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