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Is Charcoal Essential for Terrariums? Unraveling the Truth

If you’ve looked into how to build a terrarium, you’ve might have heard of people mentioning that you need to add some activated carbon into your terrarium to “purify the water”. But is charcoal actually necessary for terrarium or is it just a nice to have?

Long story short, It’s not 100% necessary. Charcoal is an optional, but potentially beneficial addition if you want to remove toxins and odor from your terrarium. However there’s no data showing that it’s required for a thriving terrarium. There are other ways to remove toxins and orders from your terrarium if you can’t or don’t want to use charcoal.

But before we can give a sweeping statement like that, we will have to look into the inner workings of activated carbon and what specific benefits it provides for your terrarium.

What is charcoal and activated carbon?

Finally a chance to put my chemical engineering degree to use.

Activated carbon is an adsorbent (emphasis on the “d”). It’s basically a material whose surface adheres to atoms, ions, or molecules that are found in a gas or liquid without soaking up too much of the gas/liquid absorbate.

To put it in layman terms, chemicals and toxins “stick” to it, but without it soaking up water like a sponge.

Activated carbon is a strong adsorbent because of its high surface area, micropore structure, and high surface reactivity. It’s used often in a variety of different applications to purify, deodorize, dechlorinate, and filter harmful constituents (a.k.a. toxins) from gases and liquids. It’s used in all sorts of chemical processes in all sorts of industries including food, pharmaceutical, petroleum, nuclear, water treatment, and automobile.

They can even be used as supplements to reduce gas and bloating.

Activated carbon is a form of carbon which has been exposed to temperatures around 800 C in an inert atmosphere to eliminate any oxygen, hydrogen, and nitrogen. This activation process creates highly reactive pores in the carbon which are used to bind to a wide spectrum of chemicals/toxins. If charcoal isn’t activated, it still performs the same function, but to a lesser degree.

If you want to get really technical about it, you can check out this 487 page textbook someone wrote just on activated carbon.

Long story short, activated carbon is scientifically proven to work, and is commonly used for industrial water purification. It’s not just like a healing crystal or something.

What does activated carbon do for a terrarium?

Charcoal absorbs “toxins” from your terrarium that supposedly build up over time. It also absorbs unpleasant odors by adsorbing to them.

In theory, this is supposed to help keep your terrarium fresh and your plants healthy. Specifically, it should reduce your chances of running into things like mold, rot, and disease in your terrarium by controlling the growth of bacteria and fungus.

It also helps to absorb excess moisture in the event of over-watering. While it doesn’t absorb water like a sponge necessarily, it has been shown to reduce the humidity levels. In my opinion, however, you can achieve the same effect with a traditional drainage layer in your terrarium. 

What toxins are present in a terrarium?

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Toxins can be produced by bacteria residing in your terrarium. 

Examples of toxins include:

  • Organic acids
  • Amino acids
  • Antibiotics
  • Gibberellin
  • Nitrites
  • Hydrogen sulfide

For more details, check out this research paper from 1964.

Like with many things, the effect these chemicals will have on your plants’ health is complicated and difficult to narrow down to “this always causes that”. Their interaction with your plant’s biology is extremely complex and will vary depending on the ecosystem and the conditions in your terrarium.

Most of these toxic substances are produced by aerobic and anaerobic bacteria. Aerobes and anaerobes produce toxins that limit root growth and promote root diseases. Anaerobes will be especially present if there’s not sufficient drainage in the soil.

Aside from the toxins, there are also pathogens that will directly harm your plants. Pathogens including verticillium, phytophthora, rhizoctonia and pythium fungi can penetrate and decompose living plant tissue, leading to symptoms of rot.

Both of these guys will be having a feast whenever there’s any dying or decaying plant parts that get generated in your terrarium. Naturally, that’s going to lead to an increase in the toxin levels. 

Are toxins bad for your terrarium?

Most likely, these toxins won’t do your plants too much harm, especially if they aren’t accumulating in extreme amounts. But they can create an unpleasant odor. 

Also, just an FYI, not all bacteria is harmful to plants. Most of them are even beneficial and are important in the cycling of nutrients in the plant ecosystem. Just so that you’re aware.

What type of charcoal should you use for a terrarium?

There are several different types of charcoal that you can purchase, but only some of them will be suitable for adding to a terrarium.

Here’s a list of charcoals that are usable:

  • Activated charcoal
  • Horticultural charcoal
  • Lump charcoal
  • Aquarium charcoal

These types of charcoal are specifically designed to be used for plants. This means that most of them will have no chemicals or additives and will be 100% all natural. 

Activated charcoal

Activated charcoal (also called activated carbon) is a black powder/pellet made from bone char, coconut shells, peat, coal, or sawdust. This is the type that is processed at very high temperatures leading to a more porous structure to maximize filtration/purification.

Horticultural charcoal

Horticultural charcoal is made from pyrolyzed organic material, mainly hardwoods. It’s commonly used as a soil additive to help retain water, nutrients, and oxygen. But it can still be used as a separate layer in a terrarium for filtration purposes. Unlike activated carbon, horticultural charcoal is heated at a much lower temperature, meaning it’s a little bit less porous, but can retain more oxygen.

Lump charcoal

Lump charcoal is basically wood that was burned slowly without any oxygen present until all impurities and moisture has been removed from the wood. For the most part, these are used for barbecuing, but unlike charcoal briquettes, these don’t have fillers or additives and can be safely used in a terrarium. If you’re on a budget, these are the way to go.

Aquarium charcoal

Aquarium charcoal is no different from activated carbon, other than the marketing. Sometimes these might come in mesh bags that make it handy to insert into your terrarium if it happens to be the right size.

You can find a lot of these online at Amazon or Etsy. You might come across the term “biochar”, which basically means a carbon that’s derived from biomass. If you see that word, that’s a good sign that it’s suitable to be used for plants, although it’s not necessarily a requirement.

Charcoals that you should stay away from

Here’s a list of charcoals you should not use:

  • BBQ charcoal briquettes
  • Charcoal from stoves or fire pits
  • Coal

The reason you want to avoid these charcoals is that there’s often a lot of chemicals that are added that will likely be harmful for your plants, such as ignition accelerants. While these chemicals might help make the charcoal burn hotter or longer, they have no place within a terrarium.

Where can I buy charcoal for a terrarium?

There are plenty of places where you can buy charcoal for a terrarium. With even just a little bit of searching you can probably find it without too much difficulty.

Here’s a list of places you can buy at online:

  • Amazon
  • Etsy
  • Josh’s frogs

If you would rather pick it up in person, here’s a list of places you can get it:

  • Home Depot
  • A local plant nursery
  • Walmart
  • Lowe’s

How to add charcoal to a terrarium

Your charcoal can be placed over the drainage layer or be used as the drainage layer material itself. Basically, as long as water is flowing through and around your charcoal, it should be serving its purpose.

From my research, all sources recommend adding the charcoal as a separate layer, rather than as an additive to the soil as for a traditional garden plant. I haven’t seen a reason why, but if I had to guess, it would be to limit the nutrients in the soil to prevent your plants from growing too fast or too large.

It’s the same reason why most sources recommend against adding fertilizer to a terrarium. If your plants grow too big, they will start crowding each other out and start touching the walls which can lead to some rotting. 

How to handle your terrarium charcoal

Depending on how your charcoal is when it’s shipped to you, you may have to break it down a bit into smaller pieces. This will make it easier to work with and spread around when you add it to your terrarium. Just make sure not to grind it into dust, that’s a bit too small.

A funnel would be really handy here, especially if your opening is particularly small. They can help you target where you’re adding your charcoal, rather than having it all end up in a pile in the center. Although, you can always spread it around with a fork if you need to.

If your charcoal happens to come in the form of a mesh filter, that makes your job easier. Just cut out a shape that will fit into your container and place it above your drainage layer. That will double as both a purifier and a separation layer.

Just one word of warning: charcoal is very dusty, so just be careful not to stain your clothes black. It may be worthwhile to give it a quick rinse to reduce the dustiness. That’s also going to minimize the amount of carbon dust that stains your container walls.

Can you use activated charcoal powder in a terrarium?

In theory, activated charcoal powder should perform the same as any other types of activated charcoal.

The only concern I have about using a powder rather than pellets or small chunks is that the powder may easily wash away into your drainage layer and collect at the bottom.

If that happens, that’s going to minimize the amount of contact it’s going to have with water. Rather than having water pass through a carbon layer, it’s going to sit on top of it, with much of the water evaporating before your carbon can work its magic.

One thing you could try is mixing it with your substrate. That’s typically what’s done with traditional gardening soil when it’s used. However, just be aware that this may give your plants a little bit more nutrients than they need, causing more growth than you might want.

Does activated charcoal get used up?

This is probably the biggest complaint that I’ve seen against using activated carbon in a terrarium. Since chemicals are adsorbing to the carbon, once those pores get filled up, activated carbon can no longer do its job of binding to toxins. There’s just no more space for them to bind to the carbon.

From what I’ve read, activated carbon filters when used for aquariums need to be changed out every few months at least. In the case of a terrarium, there’s probably much less toxins and chemicals that will be generated and need to be filtered out than an aquarium, especially when compared to the waste and bacteria that’s generated by living animals.

In that sense, there’s a good chance your activated carbon will be good for a few years, assuming your terrarium isn’t generating too many toxins.

Ultimately, it will run out. You’ll either have to replace it or start over with a new terrarium if you’re obsessed about having your activated  carbon stay active.

What can I use instead of activated charcoal in a terrarium?

Activated carbon is not the only way to get filtration in a terrarium.

Here’s a list of ideas:

  • Swap charcoal for moss
  • Switch to using distilled water for watering your terrarium
  • Avoid over watering your terrarium
  • Add springtails to your terrarium

The main alternative you can look to is moss.

Live moss can help absorb odors and excessive water like activated charcoal. They can help stabilize humidity levels in case you have plants that love high humidity.

It’s also going to give you a more natural look than having black pellets or powder in your terrarium.

The other factor you might want to consider is which type of water you are using to water your terrarium. If you are using tap water, that can potentially introduce unwanted chemicals to your terrarium like chlorine and lead.

If you want to be extra cautious, you will want to use either filtered water or distilled water. This will minimize the chances of having minerals being accumulated in your terrarium.

You’ll also want to make sure not to over water your terrarium. That’s probably going to be the number one cause of unwanted fungus and mold growth in your terrarium.

The last consideration you may want to think about is adding springtails to your terrarium. These guys will feed on any fungus, mold, and decaying matter in your terrarium. They’re like little janitors that help clean up your terrarium (for free).

Final verdict: is activated carbon really necessary for a terrarium?

Short answer: no. 

Activated carbon is not 100% necessary for your terrarium. It’s unlikely that having carbon or not having carbon in your terrarium is going to make or break whether or not your plants will survive in the next month. There are plenty of other factors that will play a bigger role on your terrarium’s overall health.

While activated carbon does do its job, it’s unclear how much of an effect that has on the final state of your terrarium.

I’ve seen many terrarium enthusiasts find success without using activated carbon for their terrarium. I’ve even seen someone try it with and without activated carbon and found that it didn’t make much of a difference.

It’s true, it may help minimize the number of toxins and pathogens that are accumulating in your terrarium. But really how harmful are those toxins and pathogens to your terrarium? These are found naturally in the soil, and most plants seem to be doing just fine in their natural environment.

In my opinion, any issues caused by these toxins and pathogens are relatively rare. I personally would rather save myself a few bucks and take my chances than try to play it extra safe.

Even if you did decide to use activated carbon, it’s only going to last for a limited time. After that, it effectively has no value. It’s almost like a depreciating asset.

The only case that I can justify using activated carbon in a terrarium is to reduce any odors. If you’re someone who likes to smell your terrarium to check on its health, having some activated carbon in there will make this check a little bit more pleasant on your nostrils.

For these reasons, I can’t recommend having charcoal or activated carbon as a necessity for every terrarium. Although, it can be a nice comfort knowing that you’re giving your terrarium the best care that’s out there by having some charcoal in your terrarium for purification.