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How big your terrarium should be according to the experts

Your terrarium container is going to be one of the first things that you’re going to need to figure out before building your terrarium. Probably the most important factor you’ll need to consider is the size of your container. But how exactly are you supposed to figure out what size you should use?

A typical terrarium fitting three 2″ plants should be 6″ in width or diameter. A larger container will enable you to fit larger sizes and larger quantities of plants in your terrarium. Smaller containers will work best with smaller plants that have small, more defined foliage.

So that’s one example and a couple of rules of thumb. Let’s get into the details of how you can best apply those rules to build your terrarium.

How terrarium size affects your plant selection

To help me answer this question, I reached out to the terrarium experts at City People’s Garden Store and Glasswing (my local garden shops here in the USA). 

How many plants can fit in your terrarium?

According to Parker from City People’s Garden Store, one of the first things you should think about is if your plants will even fit inside your terrarium container.

The size of your terrarium container and the plants inside of it is a crucial –and sometimes overlooked– step. When shopping for houseplants normally, most people tend to look for the biggest, most beautiful, show stopping plant. This is where I struggled in the beginning, none of the plants I chose seemed to fit in my container!

Parker, City People’s Garden Store

Tassy from Glasswing also said something to the same effect:

In general, you want to make sure you’re leaving room for your plants to grow and you don’t want them crushed against the sides of the vessel. For a 6″ container, that means roughly 3qty mini 2″ plants. For an 8″ container, approximately 4-5qty 2″ plants, etc.

Tassy, Glasswing

So if you want to do some quick calculations, all you would have to do is measure the size of your container and compare it to the size of your plants (ideally at their full size). Just add up the sizes of your plants to see if it’s about equal to the size of your container and you should be good to go.

A simple eyeballing way of doing it is to just look at the diameters of your plants and compare them to the length or width of your container.

The mathematical way of fitting as many plants as possible into your terrarium would be to calculate how many circles you can fit into a rectangular surface area. In my opinion, it’s not really necessary to go that far unless you’re really into math. Just eyeballing it should be good enough in most cases.

Here’s a visual:

If you like math, here’s a simple equation you can use to figure out container size or size and quantities of plants that can fit in a container you have already:

Another factor I should mention here: you will also need to make sure to give plenty of space above your plants to account for the drainage layer and substrate that will be supporting your plants.

As another rule of thumb, you will probably want to aim for about 2-3 extra inches of space in addition to the predicted height of your tallest plant. Just keep in mind, larger plants will require a deeper substrate layer, which means you may need to add an extra buffer for larger plants.

Just as a baseline reference for what you might reasonably expect, the height of small terrarium plants range from 1-6 inches, medium plants range from 6-12 inches, and tall plants are anywhere above 12 inches.

The University of Missouri has a list of different types of plants and their heights that you can check out here for some actual measurements (if you scroll to the bottom).

Will your terrarium fit in your home in a spot with good lighting?

Just in case you haven’t thought about this, you will also want to ensure your container size will actually fit somewhere in your home. A measuring tape might come in handy for this.

And not just anywhere. You will want to place your terrarium in a location with decent lighting. This means somewhere near a window.

If you don’t have any space at home where there’s good lighting, you will either have to make space or reconsider the size your terrarium should be.

The size of your terrarium container should scale with your plants

Aside from simply fitting plants into your terrarium, keeping “scale” in the back of your mind will also help you determine what plants you’ll want to add to your terrarium. Essentially, your terrarium should aim to emulate a scaled-down version of nature in the natural world.

Essentially, the name of the game is “scale”…The purpose of a terrarium is to create a mini ecosystem that embodies the spirit of the natural world. When a terrarium is complete, it should look like it is in it’s own little world, or a piece of nature put into a container.

Parker, City People’s Garden Store

To best embody this aesthetic in your terrarium, you will preferably want to choose smaller plants (2 inch pot or smaller) with small, yet defined, foliage. Plants with too large of foliage may cause your terrarium to look overgrown because of the scale of your plant foliage relative to the terrarium.

You might be able to get away with larger foliage plants if you are using a larger container for your terrarium.

When shopping for terrarium plants, you want to find small (2 inch pot or smaller) plants with small, yet defined, foliage. If you choose a plant with too large of foliage, you run the risk of affecting the scale of your terrarium. Generally, this leads to a terrarium that looks overgrown, and it becomes more difficult to appreciate all of the beauty inside and hard work you put into it. 

Parker, City People’s Garden Store

I guess technically, you don’t have to pay attention to scale if you aren’t too concerned with how your terrarium looks. I don’t think foliage size will be an indicator of how healthy a plant will be in a terrarium.

However, it can be a helpful guide to guestimate how well your plants will fit in with the aesthetic of your terrarium.

How the size of your container influences the health of your plants

From an engineering perspective (I have an engineering degree, just FYI), the size of your terrarium can also influence your plant health. There’s a number of reasons for this.

If you are working with a mini vial-sized terrarium, it’s going to be difficult to capture all of the requirements for a fully self-sustaining ecosystem in a bottle.

Part of that is due to how water behaves at smaller scales (in fluid dynamics, this is described by the Reynolds Number in case you’re curious). When you scale down too far, you won’t be able to achieve adequate water drainage, evaporation, and condensation relative to that of a larger terrarium.

Smaller container size will also mean you will be more restricted when it comes to what plants you’re able to pick from. This means it may be more difficult to get your hands on plants that will both work well together and work well in a terrarium. Just due to the sheer fact that you have less options to choose from.

You also wouldn’t be able to add nice elements like springtails or drainage enhancing components to your substrate.

Larger terrarium sizes, on the other hand, provide some benefits.

Aside from being able to fit more plants in your terrarium, the extra space can provide a buffer against negative symptoms like overwatering, mold, disease, and rot.

That’s mainly because it takes more time for these negative symptoms to spread across a greater distance. It’s also because the “average” state of the terrarium will not change as much with relatively small outbreaks. You can think about it like adding a drop of water to a glass of water vs. adding a drop of water to a swimming pool.

Do terrarium plants stay small?

Not all small plants will stay as small as they were when you bought them. So it’s always wise to do a little bit of research ahead of time to make sure you are getting plants that are most appropriate for a terrarium.

It’s tempting to grab whatever cute little plant that you come across in your gardening store and throw it into your terrarium. But you’ll want to plan for the future to avoid having plants that outgrow the terrarium.

To learn more about that, check out my article on why not all plants can be added to a terrarium.

Besides not giving you the aesthetic you’re trying to achieve, plants that outgrow their containers may show visible signs of unhappiness like droopy, yellow leaves. Plant foliage that is leaning against the container walls may lead to some rotting issues because of the constant soaking in condensation on the walls.

If possible, it’s best to stick to plants that are known to stay small even at full growth.

Some small plants that Parker has used in his terrariums include: 

  • ficus pumila
  • fittonias of varying colours
  • purple tradescantias
  • smaller pileas (such as the silver sparkle variety)

He recommends choosing plants that prefer high humidity and lower light conditions with similar requirements for best results:

Also, you will want to choose plants that are more tolerant of humidity and lower light conditions (since most containers feature opaque lids rather than glass ones). So, this basically means to avoid succulents (unless it is an open terrarium, which is a different story)!…You want to make sure that the plants you have for your terrarium are similar in care and in requirements, or you might eventually lose some in the future. Remember, you are creating a closed ecosystem that should theoretically take care of itself!

Parker, City People’s Garden Store

large terrarium Container Ideas

If you’re looking to build a large terrarium, you have a few good options to choose from:

  1. Plastic storage bins
  2. Large glass jugs
  3. Repurposed fish tanks
  4. Glass vase

My recommendation is to go with a large, transparent plastic storage bin. They are usually cheaper than containers of equivalent size that are made of glass. You can also get them in bulk at most department stores and on Amazon.

I also like that plastic storage bins are uniform in size, designed to be carried and moved, and can be used/repurposed for storage around the house. They’re just really convenient all around.

The only downside is they might not capture the terrarium aesthetic/vibe you might be looking for.

Glass containers are the way to go if you want to optimize for aesthetic and design. You might be able to find some at department stores and Amazon, but it might be more difficult and/or expensive to get them in bulk (which may not be an issue if you just want to make one terrarium).

You might even be able to find glass containers that have been crafted into interesting shapes, which might be appealing to you.

Check out my recommended container ideas If you’re interested in browsing more terrarium container ideas.

That’s pretty much all there is to it when it comes to terrarium container size. Just to recap:

  • Bigger is usually better
  • You can compare your plant sizes to your container size as a rough estimate of how many plants can fit in your terrarium
  • Smaller containers should have plants with smaller foliage

By the way, big thanks to City People’s Garden Store and Glasswing. Definitely give them a visit if you’re ever in Seattle.