How does a closed terrarium sustain itself? Discover the water, gas and nutrient cycles behind this fascinating self-sufficient mini-ecosystem.
A closed terrarium keeps itself alive thanks to three natural cycles running at the same time inside the same glass enclosure: the water cycle (evaporation and condensation), the gas cycle (photosynthesis and respiration), and the nutrient cycle (organic matter broken down by soil life). As long as these three processes stay in balance, a terrarium needs almost no maintenance at all. In this article we explain how each of these cycles works and why a well-composed terrarium can keep going for years as a result.
During the day the glass warms up and moisture evaporates from the potting soil and the plants' leaves. That water vapour rises, meets the cooler glass higher up, and condenses there into small droplets - much like clouds form in the atmosphere. Once the droplets are heavy enough, they run down the glass and 'rain' back onto the soil. This way, almost all the water ever added keeps circulating inside the closed system instead of evaporating into the outside air, which is why watering is often needed only a few times a year.
During the day, plants use light to convert carbon dioxide and water into oxygen and sugars (photosynthesis), then use some of those sugars and oxygen back up at night for their own respiration. Soil life - tiny organisms that break down dead plant matter - also consumes oxygen and produces carbon dioxide. In a healthy, balanced terrarium these two processes complement each other, so hardly any gas exchange with the outside air is needed. This is exactly why enough (indirect) light remains essential for a closed terrarium, even though the system otherwise appears fully sealed.
Fallen leaves, old roots and moss are slowly broken down inside the terrarium by bacteria and fungi in the soil. That breakdown releases minerals and nutrients, which the plants then reabsorb through their roots. This creates a closed nutrient loop, similar to the floor of a tropical forest, where dead organic matter is continuously converted into new nourishment for living plants.
The idea of a self-sufficient ecosystem in glass isn't new. Back in 1829, British physician and botanist Nathaniel Bagshaw Ward accidentally discovered that plants could keep growing for years inside a sealed glass case without ever being watered - the so-called Wardian case. This early discovery of the water cycle inside a closed glass environment laid the foundation for what we now call a terrarium, and the underlying scientific principles haven't changed since.
These three cycles only work well together if the layer structure inside the glass is right. At the bottom, a layer of clay pebbles provides drainage so excess water doesn't sit around the roots. Above that, a layer of activated charcoal stops waste products and odours from building up in the standing water. Only then comes the potting soil with the plants themselves. Our Dome XL Bonsai terrarium is built with exactly this structure, featuring a Ficus Ginseng Bonsai, a fern and a Fittonia Ruby Lime that naturally work well together inside this kind of closed system.
Not every plant fits equally well into these cycles. Plants that naturally grow on a humid forest floor - such as ferns and fittonias - have slow, compact growth and a photosynthesis rate well suited to the indirect light inside a terrarium. Choose a plant adapted to dry, sunny conditions instead, and the cycles fall out of balance more quickly: excess moisture builds up that the plant can't use, for example, leading to rot and mould. Browse our full terrarium collection for ready-made ecosystems already tuned to this balance.
As long as the three cycles stay in balance, a closed terrarium can keep growing almost independently for years. That said, it's common practice to open the glass for a few hours about once a month: that refreshes the air, stops carbon dioxide or moisture from building up too much, and gives you a chance to remove any dead leaves before they throw the nutrient cycle out of balance. Want to read more about how such a fully sealed system works in practice? Our article on a sealed garden ecosystem in a glass bottle shows what these cycles look like in a fully closed setup.
In practice, hardly any. Daytime photosynthesis and nighttime respiration largely complement each other in a balanced terrarium, so little gas exchange with the outside air is needed.
Without light, photosynthesis can't happen, and without photosynthesis the oxygen production that the plants and soil life depend on stops. So light stays essential, even though the system is otherwise sealed.
You'll usually first notice excess moisture (condensation, mould) or a nutrient shortage (yellowing leaves). The wrong plant choice or a damaged drainage layer are the most common causes.
With a proper layer structure and the right plants - such as the Ficus Ginseng Bonsai, fern and Fittonia Ruby Lime in the Dome XL Bonsai terrarium - a closed terrarium can keep growing almost independently for years.
Want to see these three cycles at work for yourself? The Dome XL Bonsai terrarium shows exactly how the water, gas and nutrient cycles come together in balance inside a single piece of glass.
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