How to Make a Cactus Terrarium – The Prickly Details

The cactus terrarium.

A mainstay on Pinterest year-round, but sadly these spiky wonders often don’t last long in the home.

It’s all too easy to make the mistake in thinking they’ll work in any old terrarium (and of course they look great in those Instagrammable enclosed hanging terrariums) but as you might expect, desert plants and tropical conditions are not a good match.

To make sure your cacti go the distance, you’re going to have to put their needs first.

But, with the right setup and substrate you can do it – read on to find out how!

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I love terrariums for all their diversity, but cacti and closed containers are a recipe for disaster. After all, the great qualities of a true terrarium that allow us to grow tropical plants, work against us when we try to grow arid plants.

• Humidity – As you might expect from a desert dwelling plant, cacti are built to thrive in dry environments and they cannot tolerate humidity. Whereas, closed terrariums are literal humidity generators as they trap moisture in the air within the vessel.

• Water Retention – Part of creating a healthy water cycle in a terrarium involves retaining an adequate level of moisture in the substrate or foundation. Cacti on the other hand – though they do actually like water – will quickly rot in a soggy substrate.

• Drainage – To counter excess water retention in the substrate, cacti need excellent drainage. A drainage layer is a big help here, but then a drainage layer full of water can cause problems by wicking up into the substrate, or just through passively generating more humidity.

• Airflow – Part of keeping cacti dry is the constant airflow they’re subjected to in their natural environment. Most terrarium containers block airflow to a large degree, and so this can be a big problem when combined with the added humidity.

So, is a cactus terrarium a good idea? Well, probably not in the strict definition of a terrarium, but with an open terrarium and the right approach we can absolutely make a gorgeous and functional piece of natural art.

Ideal Containers

You’ll want a wide open container like a dish or shallow bowl.

These help to maintain airflow around the plants and they minimise the amount of humidity the container can generate.

• See glass bowls on Etsy.

Stay away from any sealed containers, a cactus in a jar or any other type of closed cactus terrarium is a no-no. Even the likes of long open-topped vases or those with a hole cut out will still trap too much humidity.

Cactus Terrarium Layers

Any old substrate isn’t going to cut it here.

Cacti need more drainage and root aeration than most, and your substrate choice is essential in providing that.

“Cactus mixes” seem to come in two main varieties: gritty and soil-based.

The gritty mixes are primarily comprised of volcanic rocks and clay pebbles, with a relatively small amount of typical substrate bases (e.g. coco fiber).

The rocky mix provides lots of space for root aeration, and it both drains and dries out in record time, so there’s not much opportunity for root rot to develop. On the flip side, you’re going to need to water your cactus terrarium a lot more often.

If you’re looking for an easy all-in-one solution, a gritty mix could be a good shout – see the range of mixes on Etsy.

On the other hand, soil mixes are what you’ll likely come across if you buy a pre-packaged bag from a garden center. They’re often a typical mix of terrarium substrate ingredients, but with a higher percentage of supplements that increase drainage e.g. perlite, sand or orchid bark.

Ratios and ingredients can vary dramatically, always check the label as some of these can be really poorly suited in my experience. I’d particularly advise you avoid ones that use sphagnum moss or peat moss because they retain far too much water for my liking.

I’d also recommend you create a false bottom with cacti terrariums, as it’s absolutely essential that the water is not able to wick up to the roots (LECA is a good option here).

To be honest, cacti don’t really have a distinctive list of terrarium suitable species.

That being said, if you’re looking for a great small cactus for terrariums that’s going to stay small, here’s some true miniature varieties.

• Mammillaria rhodantha (Rainbow Pincushion)
• Opuntia rufida ‘Minima’ (Cinnamon Cactus)
• Echinopsis chamaecereus (Peanut Cactus)
• Mammillaria gracilis (Thimble Mammillaria)
• Echinopsis subdenudata (Domino Cactus)

On the other hand, on a whole I’ve found cacti to grow quite slowly. Plus, unlike closed terrarium ecosystems, it’s very easy to access and alter an open cactus terrarium.

So, you can likely get away with planting a variety of (potentially larger) species, provided you’re prepared to re-pot them at some point over the years.

Here’s a lovely 2″ Cactus Variety Pack by ThachlyDesigns on Etsy which can get you started off on the right foot!

Caring for a cactus terrarium is very different to a tropical one.

On one hand, you have no delicate ecosystem to balance so it’s a lot more straightforward, but you also have no self-watering mechanism either.

It’s much more akin to watering your potted plants than a typical terrarium.

How to Water a Cactus Terrarium

The aim of the game is simply to provide sufficient water whilst never allowing the cacti to sit in saturated substrate.

After all, cacti do like regular water.

They’re just used to it draining and evaporating off super fast as it would in the desert. The better you can replicate that, the better they’ll do.

Try a light watering a few times per month and adjust accordingly. Less is more. Think of them as just a little top up, never enough to completely saturate the substrate or fill the false bottom.

Have you managed to keep a cactus terrarium healthy and happy for a long time?

Let us know your secrets in the comments!

Or, if you’re in the mood for planting more spiky things, see my guide on How to Grow a Pineapple Plant.