A closed terrarium rarely needs watering, but how do you know when it's time? Here's how the water cycle works, which signs to watch for, and how to top it up without disturbing the ecosystem.
A closed terrarium typically needs watering only once or twice a year. That's the short answer, and it's exactly why so many people choose a closed terrarium in the first place: the system largely regulates its own water and humidity levels. Still, "once or twice a year" isn't a fixed rule that applies to every jar in every room. How much water your terrarium actually needs depends on the type of glass, the plants inside it, and where you place it. In this article you'll learn how the water cycle inside a closed terrarium works, how to recognise when a top-up is needed, and how to do it the right way without disturbing the ecosystem.
A closed terrarium is essentially a mini ecosystem in glass. During the day, water evaporates from the soil and through the leaves of the plants (transpiration), rises against the glass, and condenses into small droplets as the temperature drops. Those droplets then trickle back down to the soil, where the plants and the drainage layer absorb the water again. This process repeats day after day as long as the terrarium stays closed. It creates a cycle that closely mirrors the natural water cycle outdoors, just on a tiny scale inside a sealed glass environment.
Because this system is so efficient, a closed terrarium barely loses any moisture to the air outside. In an open terrarium, or with a regular houseplant in a standard pot, moisture constantly escapes into the surrounding air, which is why you often need to water weekly. Inside a sealed glass environment, that moisture largely stays within the system, which explains why topping it up is so rarely necessary.
A houseplant in a pot sits in open contact with the air in your home. That air is often drying, especially with heating or air conditioning running, so the plant steadily loses moisture that you need to replace. A terrarium with a lid or dome doesn't face that exposure. The glass acts like a permanent mini-greenhouse that traps moisture inside instead of letting it escape.
While the general rule of thumb is "once or twice a year", a few factors determine whether your terrarium sits on the higher or lower end of that estimate.
In short: two identical terrariums can end up on a completely different watering rhythm in two different homes. So always look at the signals your own terrarium gives off, rather than sticking to a fixed calendar.
The great thing about a closed terrarium is that the glass shows you exactly how the ecosystem is doing.
If you've checked the signals and concluded a top-up is needed, take it slow and add small amounts at a time.
Many ready-made terrariums, including our Dome XL, come with the recommendation to open the terrarium for a few hours once a month. That doesn't just let your plants breathe; it's also a good moment to check straightaway whether a top-up is needed.
Most problems in closed terrariums aren't caused by too little water, but by too much. Because the system is so sealed, excess moisture has nowhere to go; it keeps circulating and can eventually lead to soggy soil, root rot, or mould growth. Not sure yet whether your terrarium needs a little extra moisture? It's better to wait a few more days, or even a week, and check the signals again than to top it up too soon. A closed terrarium usually bounces back just fine from a stretch without water, but struggles more with too much moisture all at once.
Not every terrarium in our collection of terrarium DIY kits behaves identically. Larger models with a more generous soil layer tend to buffer more water, so they go longer between top-ups. Terrariums with plants that naturally grow in humid environments, such as moss varieties or fittonias, show signs of needing moisture sooner than a terrarium with a bonsai or a cactus-like plant, which are used to drier conditions. It's worth checking which plants are included in your terrarium when you buy one, so you have a rough idea of what to expect in terms of watering needs.
No, even a closed terrarium loses a small amount of moisture over time, for example through the small gap around the lid or while it's open to air out. That's why an occasional top-up, on average once or twice a year, is usually still needed.
Watering too often is the most common cause of problems in a closed terrarium. The soil becomes soggy, roots can start to rot, and there's an increased risk of mould because the system has no way to get rid of the excess moisture.
A light, changing layer of condensation that appears in the morning and clears up again during the day fits a healthy water cycle. If the glass stays completely fogged up all day, that's often a sign there's too much moisture in the system.
Distilled or filtered water contains less lime and fewer minerals than tap water, which lowers the chance of white limescale building up on the glass or soil. Both types of water work in principle, but distilled water gives a clearer result over the long term.
Still not sure which terrarium best fits your space and your plants? Take a look at our collection of terrarium DIY kits and discover which model matches the watering rhythm you feel most comfortable with.
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