I often like to say that making a terrarium is a lot like baking a cake.
In this case, it’s because a soggy bottom can quickly ruin both of them…
Too much moisture in a terrarium substrate can create an anaerobic environment, leading to root rot, mold overgrowth, and more.
Plus, unlike with cakes, it’s not always possible to fix the problem by drying and or/removing parts.
With terrariums, prevention is the best medicine. So you can build in a fail-safe solution instead. Enter the false bottom! (Otherwise known as a drainage layer).
It’s ideal for terrarium plants that don’t like water around their roots, but this approach works in just about any project! So they’re perfect for the overzealous plant parent too.
Let’s dig in.
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Benefits of a False Bottom
False bottoms may be a simple concept, but they come with a variety of potential benefits.
- Provides a buffer so that your terrarium can balance some degree of overwatering (emphasis on some – it can’t hold back a flood).
- Reduces the risk of root rot by allowing excess water to drain and helping the roots to breathe effectively.
- Adds a purification opportunity if charcoal is used somewhere in the false bottom.
Plus, though a drainage layer shouldn’t ever be full of water (if it is, you’ve already gone way too far), there are arguably a few subtle benefits of having a little bit of extra water in there.
- Creates a reservoir of moisture for your plants. It might seem counterintuitive, but the right material (e.g., leca) can sequester water away for roots to find later.
- Boosts humidity levels by creating a supply of water to saturate the air (which tropical plants will always appreciate).
So, do you absolutely need a drainage layer?
Nope – we’ve successfully built terrariums without drainage layers. With perfect watering technique and the right substrate, you can arguably avoid many of the problems a drainage layer is designed to prevent.
That said, we still use them 90% of the time because it’s nice to have the extra protection (perfect watering even harder than it sounds).
So here’s how to make one!
How to Create a False Bottom – The Drainage Layers
1. The Foundation
As the foundation of your terrarium, you’ll need to choose a material that’s strong enough to resist compaction and support the terrarium layers above them.
Make sure they’re non-uniform in shape so they leave plenty of space for water between them.
Being lightweight helps too! As you don’t want to crack the glass.
That’s why I recommend leca clay balls as the ideal drainage layer material. They’re super lightweight and highly porous, so they can actually absorb water too.
In practice, any tough granular material should do the job. Some other common (and potentially more aesthetic) choices are river pebbles, glass beads, sea glass, and aquarium gravel.
Finally, you can also use an egg crate – or some other strong grid-like layer – that can lift the contents of your terrarium away from the bottom of the container.
One note here, be careful not to make this layer too big.
Sure, you want to create a place for water to pool when needed, but you don’t actually want a tonne of water sitting there. Stagnant water never leads to good things…
2. Activated Carbon (The Purifier)
Dropped leaves and decomposing plant material can lead to toxins and gasses building up over time.
It’s totally natural and true of any regular ecosystem. It only becomes an issue in closed terrariums because they can’t escape, so they can end up impacting plant health (and just causing a stink).
That’s where activated charcoal/activated carbon can help – though it’s hard to quantify how much.
Activated charcoal is used in all kinds of filters and cleansers because it’s super reactive and loves to bind to toxins and impurities. So, a layer of activated charcoal can help to absorb pathogens and smells.
Most often, people like to put this layer above the mesh layer (myself included), but in all honestly it doesn’t matter.
You can buy activated charcoal in several forms, including fine powders and larger chunks. Though the latter is arguably better for drainage layers.
👉 Check out the activated charcoal on our shop.
3. Mesh/Screen or Fibrous Layer (The Substrate Barrier)
The final part of the puzzle is a barrier that guarantees separation from the substrate layer.
After all, you don’t want any soil/substrate falling between the cracks and clogging up your reservoir. This can only work if it’s clean and ready to receive water.
Nor do you want your plants’ roots growing into the drainage layer and defeating the purpose.
There are generally two approaches to this problem.
- The natural approach uses a fibrous barrier of sphagnum moss (or something similar).
- The artificial approach uses a thin mesh or screen. You can use various potential materials here, but arguably the easiest and most effective approach is a fiberglass screen like this one (just don’t choose a metal one that can rust).
There are advantages and disadvantages to both.
A fiberglass screen is an unnatural element, so it’s not to everyone’s taste visually. But if you cut and place it perfectly, you can minimize its visibility from the outside.
Whereas a sphagnum moss layer is natural but needs to be much thicker to perform the same function. Plus, adding such a thick layer of absorbent material can significantly alter the flow of water through a terrarium.
Drainage Layers in Different Terrariums
A false bottom is useful in almost every terrarium setup, but it can serve a slightly different purpose depending on your setup.
In open terrariums, there’s no water cycle to facilitate or humidity to regulate.
In fact, the likes of succulents and cacti typically don’t want any water sitting around their roots. So the only job of that drainage layer is to allow water to freely drain away from the substrate.
For this reason, false bottoms in open terrariums tend to be much larger and more straightforward.
On the other hand, when it comes to vivariums, false bottoms can get technical real fast.
The truth is, there’s an added level of complexity to keeping animals in a terrarium vs. just plants.
Often they get watered more regularly (especially if there’s an automatic misting system), and there are additional needs for refreshing substrates for animals and such.
So, quite often, drainage layers must be built with the ability to actually drain the drainage layer…
A Flexible Approach to Drainage
Honestly, the false bottom process is pretty straightforward, right?
Beyond the outlined steps, everything is exactly the same as a typical terrarium build. There’s plenty of flexibility in the model, too, so you can adapt it to your needs.
Realistically, any material can be swapped out if the replacement performs the same function.
As with all terrariums, exploring different options through trial and error is a big part of the process. This isn’t the only way to make a terrarium, but it might be the best way for you!
In my build for the Essential Guide to Tropical Terrariums, I decided not to add a barrier to my drainage layer, but I paired it with a substrate with lots of sphagnum moss instead.
So you don’t necessarily NEED a mesh screen or fibrous layer to have a functioning false bottom if your substrate is super spongy, as it won’t fall through the gaps.
More recently I have to say I prefer using a mesh screen, though. If you cut it accurately you won’t be able to see it, and I like the peace of mind it gives.
False Bottom Terrarium FAQ
If you have a false bottom in place, it’s difficult to drain a terrarium without disrupting the whole setup. It would be easier to simply open up your terrarium and allow it to dry out naturally.
There are a variety of branded terrarium mesh barriers, but they’re still fiberglass screens. You’re best off just buying a roll of standard fiberglass screen instead.
There’s no exact formula for the optimal depth of a terrarium false bottom (it depends on the size of your container) but it should be at least 1-2 inches to work properly.
Now It’s Your Turn
Do you follow the same process for your false bottom terrariums?
Everyone has their own ways of doing things, and I’m sure terrarium-making is no different.
Let me know in the comments.