Activated Charcoal for Terrariums: Is It (Really) Necessary?

Charcoal comes in many forms, and it can produce some very different results in terrariums.

You’d be forgiven for thinking it’s all just burnt wood. Alas, there’s actually a lot more to it, and it’s worth checking how effective your choice of charcoal is and whether it’s safe to use.

Plus, the concept of “activated charcoal” raises some new questions.

Is the modern “purifying” wonder material all it’s cracked up to be? 

We’re going to answer that exact question as we explore how beneficial charcoal can be to a terrarium and which types you should be using.

So, is charcoal the new black? Let’s find out.

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Charcoal is already commonly used as a soil additive in the wider gardening world. 

It’s said to increase water and nutrient retention, improve soil structure, and even support healthy microbial life. The list of benefits goes on, and all of these benefits transfer nicely to terrariums too.

But these aren’t the reasons that charcoal is so popular in terrariums.

Instead, charcoal is used for its powerful filtration properties. 

That’s why you’ll often see people online recommending a complete layer of charcoal at the bottom of a terrarium. To “clean” any water that passes through it. 

Now, charcoal’s purifying abilities are undeniable (that’s why it’s used in everything from skincare to medicines), but how that plays out in a sealed terrarium system is a little harder to quantify. More on this later.

Why Does a Terrarium Need Purifying?

Terrariums are ecosystems like any other.

Leaves fall, and plants perish, and eventually, that organic material will begin to degrade and break down.

Now, it’s important to mention that these things are entirely normal!  None of these things spell the end of your terrarium. But left unattended for too long, they can pose some potential problems.

1. Decaying organic material releases unpleasant gasses and toxins that can be harmful to your plants (and your nostrils).
2. Increased chances of harmful rot in your terrarium.
3. Potential overgrowth of unwanted mold and mildew.

This is where charcoal comes in. 

From horticultural charcoal and aquarium filter pellets to chunky BBQ charcoal – I’ve seen all kinds being recommended online.

But there’s one that seems to be used universally, and that’s “activated charcoal.”

Otherwise known as “activated carbon,” activated charcoal is a type of charcoal that’s been treated at very high temperatures.

Healthline defines activated charcoal as “A type of charcoal that’s processed to make it more porous. This porous texture is what distinguishes it from other types of charcoals, including the type used for barbecuing.”

This distinction is important.

After all, it’s the increased porosity of activated carbon that allows it to bind impurities more effectively. Studies have suggested that the “activating” process can increase carbon’s binding capacities up to 10x.

“Thus, a 50-g dose of activated charcoal has an adsorptive surface area equivalent to about seven football fields! “Superactivated” charcoals may have a surface area of 2,800-3,500 m2/g”

Activated Charcoal for Acute Poisoning: One Toxicologist’s Journey – Kent R. Olson

Whereas non-activated charcoal may have a surface area of just 400-800 m2/g.

So, when we consider the function of a charcoal layer in a terrarium, activated charcoal would seem to be far more effective.

Activated carbon itself comes in a variety of forms – including powders, pellets, and tubes – but the function is the same. The type you choose is probably going to determine how you use it (e.g., mixed in or separated).

👉 We sell activated charcoal for terrariums (in small chunks) on our store.

If we imagine activated charcoal as a sponge that’s soaking up all those toxins and gasses. Like any other sponge, eventually, it will become saturated. 

Remember, activated charcoal does not have an infinite buffering capacity. 

Where activated charcoal is used in aquarium filters, it’s often recommended that they are changed every 2-4 weeks. Which is a little worrying, considering we want our terrariums to last years…

But there are things to consider here before we assume our plant terrariums are going to be the same.

1. Aquarium filters typically contain a much smaller amount of activated carbon than a full terrarium layer would.
2. In an aquarium, the volume of water pumped through the carbon on a monthly basis is significantly more than will pass through a terrarium layer.
3. Aquariums generally house lots of marine animals (otherwise, it’s just a tank of water?), and they’re going to produce significantly more waste than terrarium plants alone.

I’ve not been able to find any legitimate sources to suggest how long a layer of charcoal in a terrarium might last.

However, working on the basis of an aquarium filter lasting an entire month and the demands of a terrarium charcoal layer being theoretically much lower, I would imagine it would last longer.

Just don’t quote me on that…

2. Open Terrariums Don’t Really Need Charcoal Filtration

If the main purpose of activated charcoal is to trap gas and odors.

They shouldn’t be a problem in an open air terrarium, right?

It’s not a closed system, and gases should just escape into the atmosphere.

Well, binding smelly gases is just one function of a charcoal filter. An important one, but a charcoal layer should still be useful for filtering any water contaminants. It may still be useful for absorbing potential plant pathogens, heavy metals, or toxic substances.

So, I’d argue it’s still useful in an open terrarium – just less effective/necessary.

There are tonnes of charcoal options online.

You can find it in a wide variety of different sizes and granularities. So, whether you’re looking to create a full layer of activated charcoal chunks or mix it in with your substrate, you’ll be able to find something suitable.

Alternatively, the tough activated charcoal tubes or pellets can function well as the drainage material in a false bottom. 

I’ve tried using activated charcoal in all of these ways before, but I prefer the clean look of a separate layer.

👉 Grab a bag of activated charcoal here.

Unsuitable Forms of Charcoal for Terrariums

We’ve discussed which kinds of charcoal are best for terrariums, but there are some forms that are simply not suitable. Mostly because they can bring in extra unwanted substances. 

1. BBQ charcoal briquettes – The Royal Horticultural Society doesn’t recommend using charcoal briquettes with plants because “Modern barbeque briquettes can contain additives or contaminants (coal, tars, resins, and other chemicals) that are not suitable for addition to the soil.”
2. Used charcoal from stoves or fire pits – Additionally, the RHS doesn’t recommend used charcoal as “it may be contaminated with harmful chemicals or microplastics. Also, application of any incompletely burned/charred materials can be toxic and/or decompose in the soil using up nitrogen.”

Well, the short answer? It’s not 100% necessary, but almost certainly helpful.

A terrarium can and will function without charcoal. The real question is whether charcoal has a positive long-term effect on a terrarium’s health.

Unfortunately, there’s no concrete evidence on that front.

In theory, it makes complete sense, and there’s plenty of anecdotal evidence from experienced terrarium builders to suggest it’s a helpful addition. But like many aspects of terrarium building, it’s down to personal choice/trial and error.

From what I can find, there’s no real downside to having charcoal in your terrarium.

It’s going to bring some level of purification. And if you have it mixed in with your substrate or use it as a drainage layer, it’s going to bring the usual benefits too. What have you got to lose?

Either way, it’s an easy addition to your terrarium materials and worth exploring.

I’m really interested to hear if you use charcoal in your terrariums.

Have you found one type to be superior to another? Or do you forgo charcoal entirely and still have a thriving terrarium?

Let us know in the comments below!