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How a closed terrarium works: explained by an engineer

So you’ve heard of these magical things called terrariums. Supposedly, plants are supposed to grow in a sealed container with no need for maintenance or interaction on your behalf. How exactly does a closed terrarium do that?

A closed terrarium is a self-sustaining ecosystem enclosed in a small container. Inside of the terrarium, plants rely on a self-contained water cycle, carbon dioxide produced by plant respiration, sunlight, oxygen produced by photosynthesis, and nutrients produced by soil bacteria or insects. 

Let’s break down all of the components that go into a terrarium to demystify how that ecosystem is created in this magical container. I’m an engineer, so I like breaking things down to see how they work.

Breakdown of the Basic Material Components in a Terrarium

To understand a little bit more about how this mini-ecosystem is created, it helps to understand all of the components that make up a terrarium.

I wrote another article that goes into this in a bit more detail, but I’ll give you a brief rundown here.

Terrarium Container

The terrarium container is one of the key components that defines a closed terrarium.

The two basic requirements for a closed terrarium container are:

  1. It must be sealed to prevent water from escaping
  2. It must be transparent to allow sunlight into the container

And that’s pretty much it. 

The whole purpose of the container is to keep water within an enclosed environment to create that water cycle. If there’s an opening for water to escape, your closed terrarium would then become an open terrarium, which is something completely different.

At the same time, the container must also allow sunlight to reach the plants within. Plants will require sunlight to perform photosynthesis to provide food for themselves.

The material, size, and shape can be anything you want as long as it meets those two criteria (and can fit the plants you want inside). But typically the most commonly used materials are either glass or plastic, with some kind of lid, cap, or cork to seal it up.

Drainage Layer / False Bottom

The drainage layer is a collection of materials at the bottom of the container that creates extra space for water to drain into.

Typically, this would be something like rocks or charcoal. Basically, the whole idea behind a drainage layer is to allow excess water to drain freely away from the soil.

This replicates soil in nature because water doesn’t usually absorb into the soil and sit there. It usually drains away into deeper layers of the ground or wherever the downhill direction is.

Depending on the type of plants in your terrarium (for example a moss only terrarium), you may not technically need a drainage layer. However, it’s generally recommended if you have some kind of plants that have roots growing into the soil to avoid root rot and reduce the risk of overwatering.

It’s also recommended to include a separation layer (like a mesh screen) that prevents soil from falling into the drainage layer.

I wrote another blog post about this if you want to read more about false bottoms.


Soil is another key component for sustaining life in a terrarium (and plants in general)

The soil is a major piece of where the magic happens for plant life. It provides plants with nutrients, an even supply of moisture, and provides the roots access to oxygen

It also hosts billions of tiny microorganisms and bacteria that play a key role in the plant life cycle by producing nutrients and carbon dioxide for plants to use.

For the most part, the soil in a terrarium is no different from the soil that you would use for house plants. However, the only key is that the soil must be sufficiently well draining to prevent overwatering and clumping of the soil. That’s why potting mix is usually preferred over gardening soil in a terrarium.

Terrarium Plants

The last piece of the puzzle is of course the plants that reside in the terrarium.

No, not all plants will be suitable for a closed terrarium. Especially succulents.

These conditions that are created inside of a terrarium only suit certain types of plants. Closed terrariums usually will have high moisture, high humidity, and low light inside of the container. 

Generally, the plants that tend to do well in closed terrariums are of the tropical or woodland variety. These plants naturally grow in conditions where there’s high moisture, high humidity, and low light. Exactly the conditions you would see in a terrarium.

Some examples of these types of plants include:

  • Moss
  • Ferns
  • Polka dot plants
  • Aluminum plants
  • Prayer plants
  • Pothos
  • Creeping fig

Why closed terrariums are usually airtight: the water cycle

Closed terrariums are usually airtight to keep water from escaping. This allows the water to stay inside the container to preserve and maintain the water cycle. 

Water in the soil drains into the drainage layer and evaporates over time. In the right conditions, water vapor eventually condenses on the walls of the container, to later triple back down into the soil for plants to use and repeat the cycle all over again.

Variations in temperature throughout the day help promote each step in the water cycle. In the mornings and late evenings, temperatures are cooler allowing water vapor to condense on the walls of the container and trickle into the soil. Warmer temperatures throughout the day in combination with increased sunlight cause water to evaporate. 

Since water is constantly cycling inside of the container, the conditions in the terrarium are pretty moist. That’s why it’s not recommended to grow succulents and other dry, arid-loving plants inside of a closed terrarium.

That’s also why terrariums tend to be characterized as low-maintenance enclosures. Ideally, you should never have to water your terrarium if it is completely sealed. Closed terrarium plants should have everything they need to survive in the container without outside intervention.

Carbon dioxide and oxygen are also being cycled/exchanged by bacteria in the soil and by plant respiration and photosynthesis. Although you’re completely free to open it up once in a while to get your plants some fresh air and carbon dioxide if you like.

But that’s pretty much how closed terrariums work. They are essentially closed, self-contained ecosystems in a bottle. The plants should have everything they need to survive without the need for external support or maintenance.