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

How does a closed terrarium work? A closed terrarium is a closed ecosystem and is based on three cycles, simply explained.

These are the water cycle, the oxygen cycle and the nutrient cycle. How these three cycles work in a closed terrarium and how you make sure they stay in balance is what you will learn in this article.

A perpetual terrarium functions through the water cycle, the oxygen cycle, and the nutrient cycle
A closed terrarium like this can survive indefinitely through the three cycles

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 in a closed terrarium

The first cycle in a closed terrarium is the water cycle.

Like everywhere else on Earth, an closed terrarium needs water to exist. While water is a very limited resource, as long as you have just the right amount of water in your closed terrarium, nothing will die of thirst or drown.

Your plants draw water from the soil and use it to transport nutrients from the soil through their stems and into their leaves and flowers.

The water cycle in a closed terrarium
Water cycle in a closed terrarium

Once the water has reached the leaves, it passes through so-called transpiration back to the outside, where it can evaporate. There it is absorbed by the air, increasing humidity. The warmer the air is, the more water it can absorb.

If it becomes cooler at night due to the darkness, the air must release the water again. You can see this in the morning when drops of water hang on the leaves and on the glass pane. This is condensation or simply dew.

The water will then simply flow by gravity back into the soil where the roots of the plants can take it up again.

The oxygen cycle in a closed terrarium

The second cycle of a closed terrarium is the oxygen cycle. If you used soil from your garden or the forest for your closed terrarium, you will surely have some small animals in your terrarium.

These are, for example, springtails, ants, woodlice or other isopods, and worms or snails. They need oxygen to breathe, of course, and would suffocate without it.

Like yourself, they convert the oxygen into carbon dioxide and breathe it out. In turn, the plants use this, together with sufficient water, to produce oxygen and sugar again, which they use as an energy source.

But wait, what if you don't have any animals at all in your terrarium?

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

Let's assume that you have set up your terrarium so that there is definitely not a single animal in it. This is extremely unlikely, since you will always have some microorganisms in the soil, but let's continue the thought.

With enough nutrients and water, your plants will still grow and thrive. But shouldn't all the carbon dioxide have been used up and converted to oxygen at some point?

No, because at night when it is dark, plants can not perform photosynthesis. Instead, they need oxygen and thus form a completely independent cycle.

Oxygen cycle in a terrarium without animals
Oxygen cycle in a closed terrarium without animals

Fun Fact! This is also the reason why it is often recommended not to put houseplants in your bedroom. At night they take away oxygen and you wake up in the morning more tired or maybe even with a headache. But you won't suffocate, because enough air comes through the gap under your bedroom door, even if your windows are closed.

Nutrient cycle in a closed terrarium

The last important cycle of an closed terrarium is the nutrient cycle. The food chain, so to speak. Initially, you bring nutrients into your ecosystem with your substrate, plants, animals and also rocks. The plants first use these to grow diligently.

Nutrient cycle in a closed terrarium
Nutrient cycle in a closed terrarium

The insects will eat your plants and, of course, excrete them later, putting the nutrients back into the substrate.

If you have too few animals in your closed terrarium, your plants will eventually hit a point where pretty much all the nutrients are depleted.

You will see this by the fact that they no longer really grow and some leaves change color. First, they turn yellow because not enough chlorophyll (colored green and important for photosynthesis) can be produced. Then the leaves die, fall to the ground and are converted back into their components by bacteria.

Now the nutrients have also returned to the soil, closing the nutrient cycle.

Of course I had to simplify this explanation very much in order to explain it briefly and clearly. In principle, however, you now have an idea which three basic cycles are necessary for your closed terrarium to last forever. If you want details on this topic, I can recommend the following resources.

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


A closed terrarium works when the water cycle, oxygen cycle and nutrient cycle are in harmony. These cycles repeat over and over again, creating steady conditions in your closed terrarium.

The water cycle corresponds to the rain. The oxygen cycle provides enough oxygen and carbon dioxide for breathing. The nutrient cycle corresponds to the food chain and distributes the nutrients in your closed terrarium.

If you have any other questions, you will most likely find the answer in the following article.

Do you already have a closed terrarium?

If you already have a closed terrarium, send me a photo to or on Instagram I'll gladly share your photo with the world and link your profile.

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