Terrariums are really cool enclosures that showcase the natural beauty of small plants. But one piece of the terrarium that is often overlooked is the bottom-most layer, the drainage layer.
At the bottom of the terrarium is a layer of rocky material known as the drainage layer. While the drainage layer is technically not a hard requirement, it provides many benefits for a terrarium, including reducing the risk of over-watering, oxygenating soil, and minimizing odors.
To help clarify whether or not you need a drainage layer in your terrarium, let’s break down exactly why drainage layers are recommended and what problems you may run into without one.
What is the purpose of a drainage layer?
Long story short, the primary purpose of the drainage layer is to reduce the chance of over-watering by allowing excess water to drain away from the soil. But why do we need to drain water and what are we draining water from?
So obviously plants need water for photosynthesis, cooling, and transportation of nutrients and minerals from the soil.
At the same time, plants can be overwatered. Over-watering is a situation where an overabundance of water suffocates the plant roots by preventing them from receiving oxygen (pretty much like drowning). When this happens, the root cells can start to die off, impacting the ability of the plant to receive nutrients, minerals, water, and oxygen from the soil.
Overwatering also greatly increases the risk of developing root rot, a disease characterized by softening and decaying of the roots. Usually, you’ll see the roots be a dark brown or black color as opposed to their bright white/yellow healthy appearance.
That’s where the drainage layer steps in.
The drainage layer is the first, bottom-most layer of the terrarium. It’s usually composed of small rocks or stones that create space at the bottom of the terrarium to allow excess water to drain away from the soil.
Unlike typical house plant pots, a terrarium container usually does not have drainage holes. So without a drainage layer, there’s nothing stopping water from accumulating and waterlogging the soil. It’s kind of like a substitute for drainage holes that you would find in a houseplant pot.
The drainage layer also provides a few other secondary benefits.
The drainage layer oxygenates the bottom of the soil. Having a layer of empty space at the bottom of your terrarium means that there’s another direction from which oxygen can penetrate into the soil. This can be beneficial for your plants by giving them plenty of oxygen for respiration, which is the process that converts glucose into energy.
The drainage layer can reduce the chance of bad odors accumulating in the terrarium. Most of the bad odors in a terrarium come from anaerobic bacteria that produce sulfur (which smells like rotten eggs). Having a drainage layer prevents your terrarium from developing oxygen-poor regions at the bottom of the soil that could promote anaerobic bacteria growth. No anaerobic bacteria = no bad odors.
The drainage layer can serve as a visual indicator of water levels in the terrarium. In my opinion, this is a highly underrated benefit of the drainage layer. As soon as you can see water accumulating in the drainage clear, that’s an immediately apparent sign that there’s too much water in your terrarium. It usually means your terrarium has reached its saturation point, where the humidity and soil water retention levels have maxed out.
What should you put in the bottom of your terrarium?
Let’s talk about what should actually go into your drainage layer.
I actually wrote an in-depth article on this that you can read here if you want to learn more. But I’ll summarize the main points below.
For the most part, one to two inches of any material that you choose will probably work. This is where you have the most flexibility and the most options in your terrarium.
Here’s a table listing some common materials and prices that I’ve come across:
| Material | Cost / cu ft | Min bulk price | Pros | Cons | Source |
|---|---|---|---|---|---|
| Egg crate | $5.55 | $16.65 / 4'x2'x0.375" panel (3 cu ft) | -Cheap (by volume) -Maximum drainage -Lightweight -Retains shape | -May not be aesthetically pleasing -Requires cutting and/or zip-tying | Home Depot |
| landscape rocks | $10 | $5 / 30 lb bag (0.5 cu ft) | -Cheap -Looks natural -Easily obtainable -Can be gathered for free in nature | -Somewhat heavy -May require rinsing | Home Depot |
| Sand | $10 | $5 / 50 lb bag (0.5 cu ft) | -Can be gathered for free in nature -Cheap -Looks natural -Can use without separation material | -Water may soak back up into substrate -Does not provide maximum drainage | Home Depot |
| Perlite | $19 | $5 / 8 qt bag (0.27 cu ft) | -Some of this can be added to soil to: -Improve drainage and aeration -Prevent soil compaction | -Might give your plants fluoride burn if in contact with roots | Home Depot |
| LECA / hydroton | $22 | $39 / 50 L bag (1.76 cu ft) | -Tough -Can retain water to increase terrarium humidity -Can also be used in plant soil | -Bulk price may be expensive | Home Depot |
| Filter floss | $24 | $12 / 12"x72"x1" roll (0.5 cu ft) | -Filters water passing through -Easy to cut and shape | -Might not retain shape | Amazon |
| Foam pond filter | $30 | $14 / 23.6"x17"x2" cut (0.46 cu ft) | -Easy to use -No separation material required -Easy to cut/shape | -Might not retain shape | Amazon |
| Turface | $45 | $6 / 4 qt bag (0.13 cu ft) | -Holds moisture -Provides great drainage -Adds acidity to water | -Pricey -Not abundantly available | Glassbox tropicals |
| Aquaponic substrate | $120 | $16 / 1 gal bag (0.13 cu ft) | -Lightweight -Looks natural | -Must buy online -Pricey -May affect water pH levels | Amazon |
| Charcoal | $160 | $8 / 2.25 qt bag (0.08 cu ft) | -Purifies your water and air from toxins and chemicals -Reduces odor -The best material money can buy for your terrarium health | -Pricey -Might be a little messy | Amazon |
| Marbles | $593 | $9 / 100 0.64" marbles (0.015 cu ft) | -Looks cool -Maintains shape long term | -Expensive -Your child may choke on them | Amazon |
Basically, what you’re looking for is some type of material that is sturdy and will retain its shape in the presence of water. It needs to sturdy because you don’t want it to collapse in on itself and block water drainage. Since it’s going to be at the bottom of the terrarium, it has to support the weight of all that soil and your plants on top.
My usual go-to option is some type of rock or stone. Rocks tend to fit in with the natural aesthetic of your terrarium. It just seems like something you would see out in nature.
In addition to the sturdy material that you add at the bottom of your terrarium, you usually want to include a separation layer to prevent soil and roots from getting into the drainage layer.
Without one, you run the risk of eliminating all of the benefits of the drainage layer by Blending together the soil layer and your drainage layer together. If the soil is in the drainage layer, it’s almost becomes like there’s no drainage layer at all. You still run the risk of having water logged soil and roots reaching in to where excess water accumulates.
Do you really need a drainage layer in your terrarium?
This is where the topic of drainage layers becomes controversial.
Long story short, the drainage layer is not required, but highly recommended.
In theory, you could get by without a drainage layer and survive. There are plenty of people who have done that and had successful terrariums. In fact, you probably don’t need one if your terrarium only consists of bryophytes (like moss) or plants that are known to be extremely resistant to overwatering/prefer to grow in water.
The only problem that I see with this is that it requires some experience to tell when you’ve reached the correct water levels without a drainage layer. It’s super easy to overwater your terrarium, especially when you’re used to watering traditional house plants.
Without a drainage layer, there’s no visual cue of how much water you actually have in your terrarium other than the condensation levels, which may not be immediately obvious to someone without a lot of experience building terrariums.
Having a drainage layer in your terrarium it’s just a really quick and simple step that should greatly reduce your risk of overwatering your plants. It’s a low-cost, high benefit addition to your terrarium. I would especially recommend a drainage layer for open terrariums or for anyone who wants to water their terrarium often.