Charcoal for Terrariums – Is It Necessary?

Charcoal comes in many forms, and when it comes to terrariums it can be difficult to know what’s best to use.

It can be tempting to use whichever kind of charcoal you already have in your garage, but hold up. You may want to first check first how effective it is and how harmful it could be to your plants.

So, is it even worth it?

In this article we’re going to discuss how charcoal can be beneficial to a terrarium, which types are suitable, and ultimately whether you should be using it in your terrariums.

Let’s get stuck in.

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The Potential Benefits of Charcoal in Terrariums

Unlike in general gardening, charcoal is not often used as a soil additive in terrariums.

Instead, almost everyone online seems to recommend a complete layer of charcoal near the bottom. 

It’s key function? Filtration.

Because after all, terrariums are ecosystems, and every ecosystem has an unpleasant side that needs clearing out. Leaves fall off, plants die 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 they 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 moulds and mildew.

This is where charcoal (allegedly) helps. 

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”.

What is 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 carbons 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

Where 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).

Activated charcoal for terrariums
These charcoal “tubes” can be good in false bottoms (probably best broken up a little).

Potential Downsides of Charcoal in Terrariums

Let’s start with the facts. Activated charcoal is very effective at binding a wide range of toxins and impurities.

This much is not up for debate. 

What’s not so clear is the long term effectiveness of the material in a terrarium, and whether a terrarium needs such a filter at all.

Activated Charcoal Doesn’t Remain “Activated” Forever

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..

Is Charcoal Filtration Needed In an Open Terrarium?

If the main purpose of activated charcoal is to trap gas and odours. They shouldn’t be a problem in an open 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 at filtering any water contaminants. It may still be useful at 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.

Where to Buy Activated Charcoal for Terrariums

So, are you convinced of the power of activated charcoal to cleanse your terrarium?

If so, here are the available kinds:

  1. Activated charcoal for terrariums” like this one on Etsy. You still have to apply some critical sense (it’s not like this stuff is “certified for terrariums”) and you can’t always get a good sense of its quality/source but it’s a good place to start in your search.
  2. Aquarium filter charcoal is specifically made for filtering water, and so I tend to put my faith in it. Here’s a tub on Amazon for a bargain price. It often comes in a fine gravel-like size that’s easy to spread as a layer, or can be mixed in with a substrate.  
This is what I’ve been using but I don’t think the brand is important. Made for large aquarium and pond filters.

Other Suitable Charcoal for Terrariums

Though activated carbon is by far the most effective form of charcoal filtration, other types still have toxin binding capacities.

These types of charcoal tend to be cheaper and more readily available, but may be significantly less effective than activated charcoal.

  1. Lumpwood charcoal is a type of BBQ charcoal that’s often available in supermarkets (or from stores like this one on Etsy) and can be safely used in terrariums.
  2. Horticultural charcoal (biochar) has been used in horticulture in various forms for years and can be sourced from industry stores or from Etsy.

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. These include:

  1. BBQ charcoal briquettes – the Royal Horticultural Society don’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 don’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”.

So, Is Charcoal Necessary for Terrariums?

Well, the short answer is, probably not.

Not 100% necessary, but almost certainly helpful to some degree.

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

Unfortunately, there’s no concrete evidence to show how effective charcoal is at filtering out unwanted impurities from a terrarium.

In theory, it makes sense – and there’s 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.

Personally, I don’t tend to use it anymore. In the project for my Essential Guide to Tropical Terrariums, I chose not to bother with charcoal and instead relied on springtails to keep my terrarium clean. Honestly, it’s still my healthiest yet.

That being said, from what I can find there’s no real downside to having a charcoal layer and it’s an easy addition to your terrarium supplies if you do want to use it.

If you’re building terrariums using the false bottom approach, charcoal is still useful as a drainage element. So if it’s sat between your drainage rocks and your substrate then it won’t be interfering with your plants much anyway.

If you’re not keen on adding charcoal, try adding more terrarium moss! Moss naturally filters water anyway, and can even remove things like arsenic.

What Do You Think?

I’m really interest 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!

9 thoughts on “Charcoal for Terrariums – Is It Necessary?”

  1. Tilde Andreasen

    Bought this item for my moss terrarium and works great! Prevents any odors and helps with the overall system of the terrarium The charcoal does have a very strong distinct smell at first, but it clears up in a few days. The size is just right for what I need. Not tiny dusty chips, but just the right size

  2. While activated carbon is engineered to have the most surface area in order to “bind” the most passing contaminants and will generally yield better, more dependable results, natural lump wood charcoal still “binds” contaminants and WILL perform the same function as Activated (granted, less effectively) – especially when broken and pulverized into varying sizes from chunky to nearly dust. It’s commonly used in successful terrariums and vivariums all over the world. Activated charcoal is a better, cleaner, more expensive alternative. Lump wood charcoal is fine if untreated, natural, etc. Never use charcoal briquets, period.

    1. Thanks so much for your insightful comment. I honestly hadn’t been able to find evidence to back up the idea that horticultural charcoal binds toxins (despite it being a commonly held belief). After reading your comment I have been able to find studies that suggests it’s about 1/4 the efficacy of activated charcoal. I’ll update the article to reflect that. Cheers!

    1. People tend to run with 5-10% of the overall mix, but I’d imagine a small handful would be enough for a terrarium of that size 😀

  3. Hey, I’m currently setting up a 12x12x12” terrarium/vivarium that will eventually hold isopods, millipedes, & darkling beetles. Some of the isopods are in already, along with two plants (waiting for more to arrive). I didn’t know to put a bottom layer of activated charcoal in, so the substrate is sterilised organic compost, peat, coconut coir, worm castings, & a little sand. There’s no way I could get – layer under the substrate now — should I just mix it through the substrate? Or should I add moss instead? Or both?? Thanks!

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