What Is A Terrarium? How Do They Work?

If you’re a keen plant enthusiast, you have probably come across terrariums before, but these interesting creations are curious and slightly unusual things, and that might leave you wondering, what is a terrarium? How do they work? They are a friend to those who love rainforest plants, so let’s look at them in more detail!

The word terrarium comes from the Latin for earth (“terra”) and receptacle/place (“arium”). Terrariums are like an aquarium, but instead of being filled with fish, they are intended for growing plants, and are filled with soil rather than water. They are mostly self-sustaining systems designed for trapping heat and moisture close to the plants.

What Is A Terrarium? How Does It Work?

As the name suggests, a terrarium is a glass enclosure filled with soil. It is intended for growing plants, and tends to be kept as a decorative item within the home. Many plants that will not flourish in a normal house environment will grow happily in a terrarium because it provides different conditions.

The phrase “terrarium” generally refers to a self-sustaining (or almost self-sustaining) ecosystem in which there is little need for human intervention. Some terrariums are sealed so that they do not lose moisture, and the plants inside should continue to grow and thrive for years. Other terrariums depend on more human intervention.

Closed Terrarium

A terrarium is intended to mimic real life conditions on a smaller scale. When plants in the natural world get rained on, they absorb what they need and then expel it back into the atmosphere. It gathers up into clouds and then rains back onto the plants, renewing the supply of moisture.

Inside a home, however, this process doesn’t occur – because we don’t have clouds to rain on our plants. Our homes tend to be quite dry, and instead, we water plants when they are thirsty. This works well for some varieties, but others find the dry air impossible to deal with.

A terrarium allows you to mimic the natural process inside your home. You will add some water to begin with, and the plants will take up what they need. When they expel the water back to the atmosphere, it will gather as condensation on the lid and sides of the terrarium, and then “rain” back onto the plants.

This makes terrariums essentially self-sustaining as long as they are set up well. You may have to interfere at times, but in general, terrariums require little extra care once they are established.

Are All Terrariums Sealed?

No, many people do not fully seal their terrariums – or even seal them at all. Some people create open-top terrariums.

Open top terrariums

However, you should be aware that if you don’t seal it, you will have to water it at times. Water will escape from the container as condensation, getting lost to the surrounding air. If you don’t replenish this occasionally, your terrarium will dry out.

Learning to water a terrarium can be challenging at first because it can be difficult to know how much water to add. To gauge this, look at how much condensation is on the container and what color the soil is.

Your soil should be a rich, dark brown, indicating that it is nice and wet, and there should always be some condensation on the lid and the sides of the terrarium. If the terrarium lacks condensation, it may be too dry and you will need to add some water.

Most people do this by using a spray bottle, as this lets them spread the water throughout, and prevents puddling or marshy areas from forming. The best method is usually to spray along the glass in a circular motion so that the water can run down into the soil, evenly watering the plants.

You should make sure the soil is about 90 percent dark brown after you have watered, with few dry pockets. That might seem like a lot of water, but terrariums are damp environments by design, and being too dry won’t help the plants stay healthy.

Do Terrariums Need Light?

Some terrariums will have grow lamps installed as part of the project, but many do not. All terrariums need light of some form, but most will be happy with reasonably bright, indirect sunlight to keep the plants growing.

Grow Light Terrarium

You shouldn’t place your terrarium in direct sunlight, as this is likely to burn the plants inside. Most plants that prefer the moist environment provided by a terrarium do not like to be in hot, bright sun for long.

The majority of terrariums have moss in them, and moss, in particular, likes indirect light. It will quickly turn brown and die if you leave it in the sun, or if it gets too much direct light every day.

Putting your terrarium on a north or east-facing windowsill is usually a good option that will allow it plenty of light, but rarely give the plants direct sun. If you haven’t got a suitable spot, move your terrarium further into the room, or put up a thin curtain to shield it from the sunlight. This will prevent the plants from burning.

Why Don’t The Plants Suffocate?

Most of us are aware that plants need carbon dioxide if they are to survive, and you might be wondering how this works in a sealed terrarium, where little to no air is exchanged with the outside world.

Anyone who loves science will love the answer – it’s because of the microbes in the soil. The inhabitants of your terrarium will develop a beautiful symbiosis, whereby the microbes that are eating the dead matter dropped by your plants will release carbon dioxide. These microbes, tiny though they are, fill your terrarium with enough carbon dioxide to sustain the plants.

The plants will take in the carbon dioxide and output oxygen, and the oxygen will then be absorbed by the microbes, so that they can continue composting the dead plant matter, turning it into nutrient-rich soil, and outputting carbon dioxide.

Many people who own terrariums are fascinated by this closed-loop process, whereby all the inhabitants of the terrarium support each other and keep the environment stable.

Are Open Lid Or Closed Lid Terrariums Better?

This depends on the kinds of plants that you wish to grow. Plants that prefer a drier environment will thrive best in a terrarium that has an open top, because this will allow water to escape into the rest of the air, and stop them from getting too wet.

Plants that prefer lots of moisture, particularly rainforest plants, like to be kept in a sealed or nearly sealed terrarium, because this ensures that there is sufficient humidity to support their growth. Some of these plants can only be grown in terrariums because they cannot tolerate the dry air found in most homes.

You need to make sure you do research into the kinds of plants that you wish to grow inside your terrarium before you create it. Plants like succulents do not enjoy being kept in any kind of terrarium, because they need well-draining, dry soil.

Any plant that dislikes damp soil will struggle in a terrarium, whether the top is sealed or not. You should choose moisture-loving plants, such as ferns, pothos, prayer plants, and ficus pumila. Many people recommend succulents, but these will not thrive in most cases because they will be too wet and at risk of rotting.

Remember that succulents mostly evolved in the desert, and they trap moisture in their leaves to sustain themselves through periods of drought. Keeping one in a sealed glass tank will almost always result in rotting.

Where Did Terrariums Come From?

Terrariums have a surprisingly long history, and they resulted from the Victorians’ love of exotic plants. They were invented (almost accidentally) by a naturalist called Dr. Nathaniel Bagshaw Ward, who began using his cases to export British plants to Australia and Australian plants to Britain.

The terrariums he made protected the plants during travel and maintained the environments that the plants needed, and the “Wardian cases” were used for decades to transport plants. Dr. Ward proved that plants can be kept in a sealed environment without dying, provided the conditions are right.

What Kind Of Substrate Should Be Used In A Terrarium?

This depends a little on the plants that you wish to grow and the kind of terrarium you are creating. In general, closed terrariums contain perlite, peat moss, and vermiculite, which need to be sterilized before being put in the container. This prevents fungal infections and harmful microbes from growing.

An open terrarium may use substrates that dry out more quickly, such as coconut coir. However, you can also use potting compost or peat moss, depending on what you would like to grow in the terrarium.


Terrariums are wonderful for plant enthusiasts, but they aren’t always easy to get right, so don’t be discouraged if your first terrarium involves a bit of trial and error, or even if it goes completely wrong. There’s a surprising amount of science in a self-sustaining terrarium, and it can take time to understand the balances at play. If it goes wrong, empty it and have another go!