Activated charcoal is not really that useful in terrariums. Photo: themultitaskingwoman.com
Terrariums are currently very much in vogue again. Unfortunately, modern terrarium designers seem to have picked up quite a bit of misinformation from back in the previous heyday of terrariums back in the 1970s. And one of these concerns a soil additive called activated charcoal.
You’ll read that adding activated charcoal (charcoal that has been heated to increase its adsorptive power) to the soil in a terrarium is supposed to help keep the soil fresh, to remove toxins from the soil, even control bacteria and fungus. Hey, according to these claims, it will even keep the terrarium smelling fresh. (Who, I wonder, actually sticks their head into a terrarium to smell the soil … but I digress!)
I suppose that the above advantages would be wonderful if only they made any sense! But what does “keep the soil fresh” even mean? And, yes, activated charcoal might well absorb toxins if there were any worth mentioning, but for how long? Two months? Three? By about that time, it will have absorbed all it can absorb and won’t be activated anymore, will it?
And exactly what toxins are expected? Given the fact that terrariums require much less added water and fertilizer, two possible sources of added toxins, than other gardening techniques, that’s hard to figure. Plus most “toxins” would break down on their own in a terrarium setting … into elements that plants would actually be able to use for their growth.
As for bacteria, well, if they don’t like the conditions near a piece of activated charcoal (although I don’t see why they wouldn’t), they can just move a couple of microns further away. And fungus will actually move into charcoal and start to (slowly) decompose it.
Some sources even go as far as recommending adding a layer of activated charcoal at the bottom of terrariums, still without any proof this does a thing for the plants that grow there. Are they being paid by the activated charcoal industry or something?
Water Less for Good Results
The cause of all negative effects mentioned, other than “toxins” (I still don’t know what that’s supposed to mean), like harmful fungi and bacteria and sour-smelling soil—in fact, terrarium failure in general—, can all be traced back to one single factor: overwatering. Only water terrariums when their soil starts to dry out and even then, only add water by the spoonful, not with a fully charged watering can, and your terrarium will do just fine.
One positive thing activated charcoal can actually do is “lighten” the growing mix, that is, increase aeration. And yes, it will do that, but so will perlite and vermiculite, probably already present in the potting mix you bought in the first place. So, there is already plenty of air space… unless you overwater (read above). Too much water will indeed fill in spaces between particles, yes, even particles of charcoal, and that can certainly lead to rot.
If you feel your mix needs more better aeration, you could simply add more perlite or vermiculite: they’re much cheaper than activated charcoal. But I don’t think you’ll need to.
I don’t know that any serious study has ever been done into the effect of activated charcoal in terrariums. I suspect researchers just think the whole idea is silly and look for some more reasonable project. However, they say the proof is in the pudding, and thousands of terrariums are made without using activated charcoal, yet suffer no ill effect.
I used to make terrariums for a living and did add this fairly expensive ingredient to my first arrangements. But then I joined an association of terrarium-lovers and the consensus was that adding activated charcoal was a waste of time and money. So I experimented, creating identical terrariums with and without activated charcoal and was unable to see any difference in results, even after a full year, so I stopped using it. I kept certain charcoal-free terrariums for over 5 years and they did just fine … until I got bored and redid them. And the world’s oldest known terrarium (over 50 years old) has been doing fine, in spite of the designer having not included any activated charcoal in preparing it.
I know the above is going to shock some people (who likes being told they’re wrong?), but just show me the proof that activated charcoal really does anything truly useful when added to a terrarium. You won’t be able to find it, because it doesn’t exist.