Terrariums have taken the horticultural world by storm over the past decade or so, going from obscure Victorian curiosity to a staple of homeware stores. Yet in almost any advice you will find on how to set up one of these miniature gardens in a sealed glass container – be it online, in books or through in-person courses – you will come across a tip to include activated charcoal in your potting mix.
This substance is more easily tracked down as a fancy food supplement. It can sell for pretty exorbitant prices and is often described as essential to creating a functioning terrarium. But is this really the case? Let’s take a look at the evidence.
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First off, it is plausible that this form of charcoal has measurable benefits as an addition to a potting mix. The tiny air pockets formed in its creation result in an enormous surface area, and with that an impressive ability to absorb compounds dissolved in water-based solutions.
For this reason, it is used in emergency rooms as an evidence-based treatment for poisoning and overdoses. It is thought it could also be useful in horticulture to hold water, air and nutrients, the three things plant roots need, and that it could even provide a for beneficial soil microbes.
Trials to test this hypothesis have yielded in agricultural soils, but there is a lack of experimental data to support the claims of charcoal’s apparent, very specific benefits in terrariums. Despite this, proponents claim it is essential to absorbing toxins and preventing fungus outbreaks that occur in the humid, sealed environment in these tiny glass vessels.
But are these benefits worth the cost? With tiny tubs of the stuff often selling for eye-watering prices – gram for gram around five times that of semi-precious stones like rose quartz and tiger eyes from some suppliers – I would expect to see benefits bordering on divine intervention to fork out this much. This is particularly the case when you consider that mould outbreaks and the build-up of toxins in terrariums are largely caused by the other main substrate the same people who recommend using charcoal often advise including: compost.
Compost is made up of partially rotted plant fibres, which, unsurprisingly, continue to decompose in the constantly warm, wet environments of these tiny greenhouses, resulting in unbalanced ecosystems that inhibit plant growth. Simply swapping your potting mix for a mineral-based alternative such as ground pumice or aerated clay pellets, which hold air, water and nutrients just as well as compost but without rotting over time, will remove the need for charcoal and solve the most common terrarium issues – all at a fraction of the cost. No second mortgage needed.
James Wong is a botanist and science writer, with a particular interest in food crops, conservation and the environment. Trained at the Royal Botanic Gardens, Kew, in London, he shares his tiny flat with more than 500 houseplants. You can follow him on X and Instagram @botanygeek
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