Guide
The 10 mistakes that kill first terrariums
By Paul Reaney · Updated 5 August 2026
Almost every failed first terrarium dies of one of the ten things below — and every one of them is avoidable before you plant a single stem. The first three cause the vast majority of casualties.
- 01
Putting succulents in a closed terrarium
The single most common beginner mistake. Succulents and cacti store water in their leaves and need dry air; the constant trapped humidity of a sealed jar rots them within weeks — it's a matter of when, not if.
The fix: Succulents go in open, unsealed vessels only. If you want a sealed jar, use humidity-lovers: ferns, moss, fittonia.
- 02
Standing it in direct sunlight
Glass focuses and traps heat like a greenhouse. A sealed terrarium on a sunny windowsill can exceed 40°C inside and literally cook its plants in an afternoon.
The fix: Bright, indirect light — near a window, not on the sill in full sun. Only open succulent bowls tolerate some direct sun.
- 03
Overwatering
A terrarium has no drainage holes, so every excess drop stays in the vessel. Waterlogged substrate suffocates roots, and root rot is the number-one killer of new builds. A closed terrarium needs astonishingly little water — it recycles what it has.
The fix: Water sparingly, then watch: a closed jar with lightly fogged glass each morning is perfect. Constantly dripping glass means too wet — open the lid for a day or two.
- 04
Skipping the drainage layer
Without a false bottom of clay pebbles, excess water has nowhere to go except around the roots. The build looks fine for a fortnight, then yellows from the bottom up.
The fix: Always build on 2–4 cm of LECA, with a thin charcoal layer above it in sealed builds.
- 05
Mixing plants from different climates
A fern and an echeveria cannot share a vessel: one needs humid stillness, the other dry moving air. Whatever conditions you provide, one of them is slowly dying.
The fix: Pick the climate first — closed and tropical, or open and dry — then choose every plant from that side of the line.
- 06
Choosing plants that grow too big or too fast
That cute nursery seedling may be a metre-tall species in a 20 cm jar. Fast growers also swamp slower neighbours and demand constant cutting back.
The fix: Favour naturally small, slow, small-leaved species, and check the size tag on every plant here before you buy.
- 07
Tap water on carnivorous plants
Venus flytraps, pitcher plants and sundews evolved in mineral-poor bogs. The dissolved minerals in tap water accumulate in their substrate and kill them over months.
The fix: Rain, distilled or reverse-osmosis water only — and never fertilise them.
- 08
No plan for mould
A warm, sealed, humid jar is paradise for plants and for mould. A little white fuzz on new wood is normal; left unmanaged in a build with no airflow and no clean-up crew, it spreads to the plants.
The fix: Add springtails to sealed builds — they eat mould for a living. Remove any fuzzy patch you see early, and briefly air the vessel if it keeps returning.
- 09
Overcrowding the vessel
A packed terrarium looks lush on day one and becomes an airless tangle by month three. Plants need room to grow into, and air needs room to move around leaves.
The fix: Plant it two-thirds as full as you want it to look. It will close the gap faster than you expect.
- 10
Fertilising a closed terrarium
Fertiliser makes plants grow — which is exactly what you don't want in a sealed jar chosen for staying small. It also feeds algae on the glass and salts up the substrate.
The fix: Don't feed a closed build. The slow decay of the substrate provides all it needs; that's why balanced jars run for years untouched.
The easiest way to dodge most of these at once: start with plants matched to your setup.
FAQ
Why is my terrarium going mouldy?+
Mould thrives on still, damp air and decaying material. A little white fuzz on new driftwood is normal and passes; spreading mould means the build is too wet with no airflow and nothing eating it. Remove visible patches, air the vessel for a day or two, and add springtails — they graze mould continuously in a sealed build.
Why is there so much condensation on my terrarium glass?+
Light morning fog that clears is the water cycle working. Glass that streams with water all day means there is too much water in the system — take the lid off for a day or two to let the excess evaporate, then reseal. If it persists, the substrate was soaked to begin with.
Why are my terrarium plants dying at the bottom?+
Yellowing or rot starting at the base almost always means waterlogged substrate — from overwatering, a missing drainage layer, or both. Rebuild with 2–4 cm of clay pebbles under the substrate and water far more sparingly.
How often should you water a terrarium?+
A sealed, closed terrarium may need a splash only every few weeks or months — watch the glass: light fog is right, bone-dry glass and substrate means add a little. An open terrarium dries like a normal pot and typically wants a small water every one to two weeks, always checking the substrate first.