Closed terrarium - what is it and how does it work?

How does a closed terrarium work? A closed terrarium is a closed ecosystem and works due to three basic cycles. Those are the water cycle, the oxygen cycle and the nutrient cycle. If one of these cycles permanently comes out of balance your whole closed terrarium will stop working. Learn more about these cycles here.

Ewige Terrarien in seltsamen Gefäßen
Closed terrariums in weirdly shaped containers

How you can build a closed terrarium yourself and what materials, plants and animals belong in there is already explained in the ultimate guide to building a closed terrarium. All these components form the three cycles. Now you'll learn how they all work.

The water cycle

One of these cycles is the water cycle. All life on earth needs water to survive and thrive as you probably know. Although water is a very limited resource in a closed terrarium, you made sure to have exactly the right amount of water so nothing drowns or dries out. Your plants absorb the soils water and use it to transport nutrients through their stems into their leaves and blossoms.

Der Wasserzyklus in einem ewigen Terrariums
Water cycle in a closed terrarium

As the water reaches the leaves it gets out again through a process called transpiration. Outside the leaves it can evaporate into the air and the humidity rises. The warmer the air is, the more water it can take. At night it gets a little colder and the air has to lose water again. You'll notice this in the morning as your container's walls will be fogged up and the leaves wet. This is condensate or just dew. Due to gravity the water flows back down into the sail where the plants' roots can absorb it again.

The oxygen cycle

Oxygen cycle in a closed terrarium with animals
Oxygen cycle in a closed terrarium

Another important cycle is the oxygen cycle. If you're using soil from your garden or a nearby forest you'll likely have brought some small animals into your closed terrarium. For example, springtails, ants, woodlice or other isopods and worms or snails. Learn more about animals in closed terrariums here. They need oxygen of course and would suffocate without it. Just like you and me, they turn the oxygen into carbon monoxide and exhale it. Your plants use carbon monoxide and water to turn both into oxygen and sugar which they use as a source of energy. So far so good, right? Well, almost.

Sauerstoffzyklus in einem Terrarium ohne Tiere
Oxygen cycle in a closed terrarium without animals

Let's assume that you've built your closed terrarium without any animals inside (which is extremely unlikely). With enough nutrients and water your plants will thrive regardless. But shouldn't all the carbon monoxide be used up at some point and turned into oxygen? No, since plants also breathe oxygen, albeit only in darkness when they can't photosynthesize. Instead they need oxygen and form an entirely independent cycle.

Fun Fact! This is also the reason why it is often recommended not to leave plants in your bedroom at night. At night they take away some of your oxygen and you might wake up a little more tired or maybe even with a headache. You won't suffocate though as enough air will enter your room through the gap under your door even if your windows are closed.

And lastly the nutrient cycle

The last important cycle is the nutrient cycle. The food chain so to speak. In the beginning you start your closed terrarium with a substrate, plants, animals and stones, all introducing different nutrients into the ecosystem. The plants will start using these to grow.

Nährstoffzyklus in einem ewigen Terrarium
Nutrient cycle in a closed terrarium

The insects will eat your plants and digest them, later returning the nutrients back into the soil. If you don't have any animals (again, extremely unlikely) you plants will reach at point where all nutrients are used up. You'll notice this as they'll stop growing and making their leaves turn different colors. At first they'll start to become a little yellow-ish since not enough chlorophyll can be produced. After that the leaves will start to fall off and are slowly digested by bacteria in the soil turning them back into their components. Now the nutrients have also returned back into the soil, closing the nutrient cycle.

Of course I had to simplify this explanation in order to make it brief. In principle you now know which three basic cycles are necessary for your closed terrarium to live indefinitely. If you want details about each cycle, read these three resources.

Information about the water cycle.
About the oxygen cycle.
And the nutrient cycle.


A closed terrarium works when the water cycle, the oxygen cycle and the nutrient cycle are all in balance. These cycles repeat again and again and provide constant conditions in your closed terrarium on a daily basis. The water cycle correlates to rainfall. The oxygen cycle provides enough oxygen and carbon dioxide to breathe. The nutrient cycle correlates to the food chain and distributes nutrients in your closed terrarium.

One question is still open! Are bottle gardens and closed terrariums the same thing? Find out!

Do you already have a closed terrarium?

Wenn ja, dann schicke mir doch ein Bild an oder auf Instagram bei Schöne Bilder teile ich gerne mit deinem Namen bei Instagram, damit die ganze Welt dein Kunstwerk sehen kann. 🙂

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