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Is A Terrarium A Closed System? Open vs. Closed Terrariums

green fruit on clear glass jar

When you hear the word “terrarium”, I’m guessing an image of plants in a jar comes to mind. Does that mean that all terrariums are closed systems? Do all terrariums have to be closed?

Not necessarily. There are two types of terrariums: open and closed terrariums. Closed terrariums are closed systems, but open terrariums are open systems. An open terrarium has an opening that lets air and water escape, hence an open system. Closed terrariums on the other hand are indeed closed systems.

Now that you’ve heard of the two types of terrariums, let’s get into the details of what exactly they are and the advantages and disadvantages of each type.

Should terrariums be opened or closed?

To answer that question: yes.

Opened and closed terrariums have their own unique advantages and disadvantages. Each type will be suitable for different types of plants. Deciding between an opened and closed terrarium ultimately comes down to what types of plants you want to grow and perhaps how much maintenance you want to take on.

Here’s a table listing the key differences between open and closed terrariums:

Open Terrarium

Closed Terrarium
Is an open system: it has a wide opening for air and water vapors to escapeIs a closed system: it’s tightly sealed to allow water and air to recycle themselves within the terrarium
Best suited for plants that enjoy a dry arid environment with plenty of sunBest suited for plants that do well in high humidity environments with indirect light
Plants can grow as big as they want without issuesRequires less maintenance
More versatile decoration potentialPlants can overheat in direct sunlight
Can be more susceptible to pestsIs more prone to overwatering
Proper drainage is often critical for plant survivalPlant size is limited by the size of the container
Will require more maintenance for pruning and wateringMay require tools and a little more effort for maintenance depending on the size of the container’s opening

As you can see from the table, there are pros and cons to both open and closed terrariums. There isn’t one that is inherently better than the other, although you might have a personal preference for one.

For me personally, I like going with a closed terrarium just because there’s much less maintenance involved. I also am fascinated with the concept of a self-sustaining ecosystem in a bottle. Of course as always, you are free to have both.

What do you need for an open or closed terrarium?

All terrariums consists of five essential components:

  1. The container
  2. The drainage layer
  3. The separation layer
  4. The substrate
  5. The Plants

The main differences between open and closed terrarium materials comes in a few places:

  1. The container type
  2. The substrate choice
  3. The type of plants

Let’s walk through each of these components and their unique function for an open terrarium.

The container

For an open terrarium, the key distinguishing factor that differentiates it from a closed terrarium is the container.

An open terrarium container will have a large opening for your plants to be exposed to the open air. For the most part, that’s the only requirement for an open terrarium.

Closed terrariums on the other hand, will require a closed container. It’s going to be tightly sealed to prevent any water or air from escaping the system, although it can be opened for maintenance if needed.

In terms of what material you decide to use for your container, pretty much anything will work. All it needs to do is hold your plants and the terrarium layers in place while allowing light to reach your plants.

This means you can use anything from a bucket to a large bowl or a glass bottle to a large plastic bin. If you want something that’s aesthetically pleasing like something you might find on Pinterest, you can check out a lot of different container types on Etsy.

For more ideas and recommendations on containers, check out my recommended containers here.

The drainage layer

The drainage layer, sometimes called a false bottom, is a very critical component to an open terrarium that grows arid plants (and an important one for closed terrariums).

Most of the plants that tend to do well in open terrariums will be very sensitive to overwatering. That’s where the drainage layer comes to the rescue. The drainage layer prevents water from pooling around the plant roots and creating space for water to drain away from the soil.

The drainage layer is essentially a layer of rocks or pebbles whose sole function is to allow water to drain from the soil. The material you choose to use for the drainage layer doesn’t have to be anything specific, just as long as it retains its non-uniform shape (i.e. doesn’t dissolve or collapse on itself) and lets water flow through. You can use anything from rocks, marbles, or beads.

Technically, if you’re growing common house plants, you actually don’t need a drainage layer. Your terrarium essentially would be just a pot. No need to do anything special.

The separation layer

The separation layer is going to function the same way for both open and closed terrariums.

If your plant roots are reaching into the drainage layer, that will lead to problems with root rot due to roots sitting in stagnant water.

The separation layer solves that problem by stopping your plant to roots from growing into the drainage layer while still allowing water to pass through.

It also helps to prevent soil from falling into the drainage layer. If you’re using a glass container, that’s going to make your terrarium more aesthetically pleasing with clear distinct layers visible from the side.

As for what material you use for the separation layer, most synthetic material will work. As long as the openings are small enough to stop roots from passing through, but large enough to let water pass through, you should be good to go.

This means you can use anything from a plastic bag with some poked holes to a piece of window screen that you cut out. Another equally effective option is to put some sphagnum moss as your separation layer.

The substrate

The substrate is basically another word for the soil or whatever material you are using in place of the soil. This is the material that your plants roots are going to get their nutrients from.

This is another piece that might differ slightly between open and close terrariums. The difference mainly comes from the fact that the type of plants you will be growing are going to be different and have different needs/requirements.

In an open terrarium, your plants aren’t going to be limited by space and can grow as large as they want. This means that you are free to use fertilizers and potting mix, especially if you’re just growing common house plants.

However, if you’re growing plants that require an arid environment, you’re going to have to use soil that promotes drainage. This means that substrates will typically contain mixtures of sand and rocks. 

I recommend using a mixture of perlite and coir (or sphagnum moss) for a combination of having great nutrition in the substrate and allowing for sufficient drainage capabilities. Aquarium soil is another great option.

In a closed terrarium, drainage is not as critical since the plants will typically be adapted to a humid environment with plenty of moisture. This doesn’t mean that you can just soak in with water, but it does mean that you don’t have to be as careful about having drainage capabilities in the substrate.

For a closed terrarium, my go-to is to use native soil. You can essentially go to a local park, collect some dirt, and add it to your terrarium. Just be careful not to track in any unwanted guests or microbes. If you want to be safe, you can give it a quick 30-minute bake at 350° to sterilize it.

The one thing you want to stay away from in a closed terrarium is any fertilizers. You actually don’t want to give your closed terrarium plants too much nutrition so that they won’t grow too large. This will stop them from crowding each other out and from touching the sides of the container, which can cause your plants to rot from excessive contact with condensation.

The Plants

The plants are where the differences between open and close terrarium start to shine.

Open terrariums will be more suitable for plants that prefer an arid environment, while closed terrariums will be best for plants that grow in shady, high humidity environments.

Moss will only grow in the closed terrarium and will have no chance of surviving in an open terrarium. Likewise, succulents will only grow in an open terrarium, but will not do well in a closed terrarium.

You will not be able to mix and match them. That would be almost like trying to mix oil and water. It’s either one or the other, you can’t make them both happy in a single terrarium.

Some plants that you might find in an open terrarium include:

  • Succulents
  • Cacti
  • Flowering plants
  • Lithops
  • Aloe
  • Artichoke
  • Perennials

In a closed terrarium, you might find plants like:

  • Moss
  • Ferns
  • Ficus pumila
  • Polka dot plants

By the way, if you’re looking for a complete guide to the five basic terrarium layers, check out this article.

Do you need charcoal for a terrarium?

If you’ve done a little bit of browsing on what needs to go into a terrarium, you might have come across some people mentioning a charcoal layer.

I wrote another article on this that gives a really in-depth look at what activated charcoal is and if it’s really necessary for your terrarium if you want to find out more.

But if you just want a quick answer to this question, the answer is no (although it might be helpful).

Charcoal or activated carbon is supposed to serve as additional filtration for your terrarium. Basically, it’s supposed to purify your water by adsorbing any toxins generated by bacteria in your terrarium. It also helps to reduce any odors you might smell in your terrarium.

From my research and my own experience, I’ve found that charcoal isn’t a requirement to have a terrarium that thrives. 

In the context of comparing between an open and closed terrarium, charcoal is really only necessary in a closed terrarium. That’s where toxins have the potential to build up within a closed container and pose any real threats to your plants.

In an open terrarium, any toxins generated by your terrarium is free to diffuse into the open air. If you think about it, plants found in nature seem to be doing just fine without any charcoal. There really isn’t much difference for the plants to sit in an open container versus sitting out in the open in nature when it comes to exposure to toxins generated from nature.

So if you do decide to go with charcoal, I would only consider getting it for a closed terrarium. Even then, I wouldn’t say it’s necessarily a requirement to have a successful terrarium.

As far as deciding what type of charcoal to add to your terrarium, it’s important to make sure you select the appropriate type. You definitely don’t want to add barbecue briquettes to your terrarium because of all the chemicals and additives included which can be harmful to your plants.

If you can, stick to horticultural charcoal, lump charcoal, or aquarium charcoal. These types of charcoal are specifically designed and intended to be used for growing plants.

How often should you water an open or closed terrarium?

One of the big differences between open and closed terrariums is the level of water retention. 

In an open terrarium, you have an open system. Water can easily leave the terrarium and your plants will need to be watered more often. 

2 to 3 weeks is a common rule of thumb for watering an open terrarium, but that largely depends on what type of plants you have. On top of that, certain plants may prefer to be watered differently than others. Some plants might like to be misted while others prefer to be soaked.

On the other hand, in a closed terrarium, you have a closed system. Water cannot escape from the terrarium easily and instead recycles itself within the terrarium. So naturally your closed terrarium will need much less watering.

Depending on how airtight your terrarium is, you might find yourself watering your closed terrarium anywhere from once every few months to once every few years. For the most part, a decent misting or a few spurts of water will do.

But one challenge you might face with a closed terrarium is balancing the water levels. Closed terrariums can be more prone to overwatering. If there is too much water in the system, it’s easy to start getting problems with mold growth and rot. Because the terrarium is a closed system, this water has nowhere to go and will continue to wreak havoc in your terrarium until you let it out.

That’s why you have to monitor your closed terrarium to check on the water levels (at least initially). Mainly you want to be looking at the condensation levels. 

There should only be condensation in the container in the mornings and evenings. If there is constantly condensation on the walls of the container, there is probably too much water. Likewise, if you never see condensation in there, it could probably use a little more water.

You also may also want to feel the substrate once in a while. Ideally, the substrate should feel slightly moist, but not soggy or dry. If it’s soggy, there’s too much water. If it’s dry, there’s too little.

How long do open and closed terrariums last?

This is a tough question to give a straight answer to. Technically, all plants will die eventually, although there’s no specific lifespan for most plants. It largely depends on the type of plants that you are trying to care for.

Most typical indoor plants (including perennials) last about 2 to 5 years. Certain plants might last less time if they are annual or biennial plants. Some plants might even last much longer than that.

For example, the oldest known terrarium is about 60 years old and is owned by someone named David Latimer. He planted a single seed in a sealed jug back in 1960 and has only opened it once for watering in 1972. Since then, it hasn’t been opened and his plants are still looking good. There really is no universal cap on the lifespan of a terrarium. 

The other thing that you want to keep in mind is that you can always replace plants within your terrarium that start to die out. If you do this, technically you can keep your terrarium alive as a whole as long as you want.

Long story short, a terrarium is going to last a long time. So don’t worry about having to replace it every few months.

Thanks for reading this far. If you’re interested in learning more about terrariums, feel free to check out some of my other posts.