A false bottom is one of the basic building blocks of a working closed terrarium. Without a false bottom you'll risk standing water which will drown your plants' roots. It also encourages mold to grow which will steal nutrients from your plants. By using a false bottom, water can drain better. Thus, your plants' roots will have more oxygen to breathe and will grow better.
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What is a false bottom?
A false bottom is there to make room under your substrate for excess water to drain into. Your plants' roots shouldn't grow into this layer since then they'll just drown a floor lower. A false bottom also helps to distribute water better and make the air more humid. How a false bottom looks and how you can make one is explained right here. But before you keep reading: The effectiveness of your false bottom is largely dependant on the composition of your substrate. Learn more about substrates in this article.
What are false bottoms made of?
Are false bottoms is made of two parts, simply speaking. The buffer down below and the barrier uptop.The buffer makes up the bottom most part of your closed terrarium and can consists of multiple different materials. It's only important that they cannot decay.
Possibility #1: The passive variant
The simplest way is to use small stones like gravel from outside. Just put a small layer of them on the bottom of your closed terrarium. Depending on the size of your closed terrarium, the thickness will of course have to differ. Generally speaking it should make up about 10% to 20% of the closed terrariums entire height. You really don't have to be exact here. Other materials you can use are glass beads, porcelain shards or aquarium gravel. All these materials have in common that they do not absorb any water and are only just a buffer. Glass and porcelain are also interesting since they can have pretty much any color you want, opening up many design choices. If you're going to use sand as the main component of your substrate, why not try white porcelain as a false bottom?
Possibility #2: The absorbing variant
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Another possibility are puffed clay balls. They work just as well as a buffer and have the added ability to absorb water. This lets your closed terrarium have a bigger amount of water. You'll also have an easier time watering your closed terrarium and don't have to be quite as exact. Puffed clay balls work well for tropical plants since they need a lot of water but cannot stay in standing water for too long. Their red brownish color fits well for many colors of substrates. You won't find them outside like gravel, but can buy them quite cheaply in your local hardware store or online.
Possibility #3: The active variant
The super hero between all false bottoms is active carbon. It has the special ability to bind toxins and keep them away from your plants. This can reduce plants dying and prevents a bad smell (a healthy closed terrarium should smell similar to a forest). Active carbon has a large surface area since it is very porous. You can think of it like a sponge of volcanic stone (which you can use as a buffer as well). Chemically speaking it's very reactive and thus can bind many different substances.
Maybe you've already heard of my first closed terrarium in a candy jar and noticed that I didn't use active carbon but charcoal instead. It's not as reactive since there are no added chlorides during the manufacturing process but works just as well (in my experience). It also has the advantage that your probably already have some at home. You'll just have to make sure to break it up into small pieces (less than 5mm in diameter).
Bonus tip for your substrate
I recommend you not to use charcoal or active carbon completely by itself but instead to add gravel or puffed clay balls. I also have a final tip for your substrate. If your patient you can grind up some charcoal into dust (or take some from the bottom of the bag) and mix it into your substrate. This reduces the chance of mold. You'll probably not even notice the added charcoal dust when looking at your closed terrarium.
We need a permeable barrier!
Now you know quite a bit about the buffer down below. You're also going to need a barrier permeable to water. Here you have several options as well.
Possibility #1: The natural variant
The easiest method is using dried sphagnum moss. In the best case you can find it outside. Dried sphagnum moss has the trait of being resistant to compaction no matter how much water flows through it. That's important so your barrier doesn't turn into a thick layer impermeable to water. That would make your entire false bottom useless. You'll of course have to keep in mind that dried sphagnum moss is a dead plant and will decompose over time. It will stay for a long time but not forever. That's not problematic as long as your amount of water is close to the optimal amount. After a while the roots of your plants will reach into the buffer layer. If they have survived that long, you're probably close to the right amount of water.
Possibility #2: The long lasting variant
An alternative to dried sphagnum moss is any kind of mesh. It's holes should be about 1mm to 2mm in diameter so than water can flow through easily. Although mesh is not as natural as dried sphagnum moss you can hide it quite easily below your substrate so it's almost invisible. When choosing your mesh it's important to choose one that cannost rust. Fly screen is pretty cheap and is usually made out of fibre glass so you won't have to worry about it. Meshes made out of iron or steel can have a negative impact on your plants in the long run.
Possibility #3: The fluffy variant
If you neither want to use mesh or sphagnum moss you can look at felt. It works well as water can flow through it as if it wasn't there. If you have any hairy pets you can even use their own felt. Hair is made out of keratin and will take a very long time to decay.
Now you know why you need a false bottom for your closed terrarium and how you best construct one. If you want to start right away, read the ultimate guide to constructing a closed terrarium.
Without a false bottom in your closed terrarium your plants will most likely drown or be eaten up by mold. A false bottom can be easily constructed by using a barrier and a buffer beneath. For the barrier you can choose fly screen, dried sphagnum moss or even felt. For the buffer you can use any non-decaying material.
By the way...
By the way: If you've built your closed terrarium and want to share it with the world, send me a photo to email@example.com or on Instagram @terrarium.blog. I'll gladly share nice photos and link your profile. You'll also be on the homepage of the Terrarium Blog!