How to Make a Cactus Terrarium — In-depth Guide

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Terrariums are open or closed glass containers that make up independent growing environments. Terrariums are an increasingly popular way to grow small plants and tend to be used as decorative or ornamental features.

As a rule, terrariums are usually better for humidity-loving plants because they have little to no air circulation and trap damp air.

Cacti famously do not like excessively humid conditions, so growing them in a terrarium is tricky and requires a well-founded understanding of how to care for the chosen cactus varieties. 

 

How to make a cactus terrarium

Cacti require an open glass terrarium with no cover. Choose a container with a large hole or low sides. Keep your cactus terrarium in bright light but away from too much full sun. Make sure there is enough space for the cacti to grow and plant compatible varieties.

 

Give your cacti enough room

Because they are such a popular design feature, many cactus and succulent terrariums are created purely for short-term aesthetic purposes and never stand much of a chance of being a real growing environment.

If cacti are planted in a terrarium without enough forethought and horticultural knowledge, they will probably die. 

It is crucial that cacti are not planted in closed terrariums, because they will not survive. Cacti generally require hot, dry conditions. Terrariums are infamously humid because their glass walls trap moisture. 

Cacti need enough space to grow. While it may look cute to place several different cactus varieties right next to each other in a small glass container, this is not sustainable in the long term as your cacti grow. 

Even though most cacti do not grow very fast, make sure your cacti have enough space to grow over years. If using a smaller container, take care not to plant too many plants in it. 

 

Choose the right cacti for your terrarium 

Another common error made when creating cactus terrariums is to plant a mixture of plants that require contradictory environmental conditions. 

People are often tempted to choose the plants for their terrariums purely based on aesthetics and like to include a wide variety of plant textures. Unfortunately, this rarely results in happy plants, because you will not be able to give all of them the care they really need.

If you are planting several different plants in a terrarium, make sure they like the same things! What kind of soil do they like? What are their ideal day and night-time temperatures? How much sunlight do they need? 

For example, you could not grow a Pachycereus Marginatus, which hardly requires any water at all during its winter dormancy, detests all humidity and loves full sun, in the same terrarium as a Christmas cactus, which originates in humid Brazil, doesn’t like direct sun and needs regular hydration throughout the year. Choose compatible plants for best long-term results!

You might also consider choosing cactus varieties that don’t mind partial shade, or opt for succulents that are not cacti. 

Gasterias and Haworthias are two succulent varieties that don’t need the same amount of full sun as desert cacti and will therefore be happies in a terrarium that can’t be kept in direct sunlight.

If you notice that your any of your cacti are dying in their terrarium, there is no shame in removing them from the terrarium, planting them in a pot with a drainage hole and trying to revive your dying cactus in a new home that is better suited to its needs. 

 

Create good drainage in your cactus terrarium 

There are many articles out there that suggest putting down layers of pebbles and activated charcoal on the bottom of your terrarium under your soil to create good drainage conditions. 

However, these “drainage layers” do very little. The roots of your cacti or other plants will reach down into the water anyway, or the water will be absorbed up into the soil. 

The only way to ensure cacti in terrariums do not sit in too much water and develop root rot as a result is to give them just the right amount of water––not so much that water sits around at the bottom, but enough that your cacti do not dry out. 

Overwatering also causes your terrarium to be more humid––something you want to avoid at all costs when growing cacti. 

Instead of trying to create specific layers, mix activated charcoal, sand and a fast-draining cactus potting mix with pebbles and some larger stones from your garden. This varied organic matter will suit most cacti. 

 

Avoid the greenhouse effect in cactus terrariums

If a glass terrarium is exposed to direct sunlight, it will act like a magnifying glass and intensify the heat of the sun, which can cause any plants living inside to burn. Plants living inside the terrarium will also be exposed to extreme heat. 

While most cacti are native to regions with very hot temperatures, these tend to be areas with a dry climate and plenty of air circulation.

In a terrarium, the greenhouse effect means that not only are temperatures sky high, but because their glass walls trap moisture and prevent air circulation, they are also humid. These two factors combine to create the perfect conditions for fungi and rot to thrive. 

Be picky about your container! Don’t choose a bottle, or some other glass container that is almost entirely closed in. Instead, opt for a glass container that is largely open and has more of a bowl vibe. 

Choosing the right container can ensure greater air circulation and minimize the greenhouse effect.

 

Give your cactus terrarium enough sunlight

While it is true that you want to avoid placing your terrarium in direct sunlight to avoid the greenhouse effect, it is also true that most cacti require direct sunlight and will fade or suffer if they do not get enough of it. 

So, how can you keep cacti in a terrarium happy? As with watering, it is a matter of finesse. Try placing your terrarium in direct sunlight during the early morning but keeping it in shade during the midday hours. 

Keeping your cactus terrarium by an east-facing window is generally a good solution, because it will get some full sun in the morning but will be in bright, indirect light for most of the rest of the day.

To familiarize yourself with the light requirements of different cactus varieties, read this essential guide to houseplant light levels. 

The ideal solution to keeping a cactus terrarium happy by making sure it gets enough UV rays without the extreme heat of the sun is to purchase an LED grow light.