Cactus Terrariums: Easy on the Eye But Sooo Hard to Manage

Cacti and other succulents are currently very fashionable. They are seen more and more in upscale boutiques, often in costly imported pots or hanging in kokedamas (Japanese moss  balls), but also in containers with glass walls. In other words, in terrariums. But this last use almost always leads to the death of the succulents it contains.

They die most rapidly in closed terrariums, that is those completely sealed with a glass cover, but even in open terrariums, ones with glass sides but an open top, the plants usually end up dead.

What’s going on?

The Wrong Environment

To really succeed with any plant, you have to give it an environment to which it is adapted. Well, most succulents, including cacti, are full sun plants and also like their potting soil to dry out between waterings. Plus they’re from climates where the atmosphere is on the dry side, not humid and stagnant. In other words, pretty much the exact opposite of terrarium conditions.

Cultivating succulents in a terrarium is like trying to raise goldfish in a sandbox, the only difference being that goldfish die instantly when taken out of their watery comfort zone while succulents usually die very, very slowly.

Beware of the Greenhouse Effect

If you place a terrarium, which, by definition, has glass sides, in full sun, the heat inside will explode. Expect temperatures to reach 100 to 120?F (40 to 50?C) in only minutes. This increase in temperature inside a glass container is called the greenhouse effect and is caused by heat radiating upward and not being able to readily escape. And if there is any moisture in the soil, it will start to evaporate, pushing the humidity through the roof (it can easily reach 100% in a sealed terrarium).

Although many succulents do come from hot climates where midday temperatures also soar, at least there is good air circulation and low atmospheric humidity, plus the temperature drops at night, giving the plants a respite. Not so a terrarium.

Sellers of succulent terrariums know their arrangements can’t take full sun, so instead recommend placing them in a spot “with good light, but sheltered from direct sunlight”. That does help avoid overheating, but it also starves the plants inside. You see, light is a plant’s only source of energy and, again, most succulents are full-sun plants, yet you’ve cut back on the light they receive, generally to the point where they are no longer getting enough light to grow normally.

When a succulent lacks light, it has the interesting habit (for the plant seller) of showing no sign of its distress at first. It simply stops growing, as if waiting for conditions to improve. Thus its owner thinks that all is well and is quite surprised when, 6 months, 9 months, or even 1 year later, the plant begins to rot or to etiolate.  There are a few succulents that can grow well in partial shade (gasterias, rhipsalis and some haworthias, for example) and these survive longer. Desert cacti, on the other hand, never do well in a terrarium: they really need their full dose of sunlight.

Success Under Lights

You can solve the dilemma of succulent terrariums needing intense light but being unable to stand the greenhouse effect that accompanies their placement in full sun… by setting them under truly intense artificial light of a type that generates little heat, say a bank of 4 fluorescent tubes (2 tubes would suffice for succulents not needing full sun). Unfortunately such intense lighting is rarely welcome in the average home décor, yet most terrarium owners want to put their mini-garden on display, not hide it away in a basement or closet or behind a screen.

Still, artificial lighting is essentially the only way to really succeed long term with cactus and succulent terrariums. And lighting alone doesn’t solve all the problems succulent terrariums face.

Too Much Moisture… In the Soil and the Air

Correctly watering a terrarium is difficult under the best of circumstances. There is no hole in the bottom of the container to let excess water drain out, so one spoonful of water too many leaves the plants soaking in it. And succulents don’t like their roots soaking in stagnant water.

But the stylists who prepare commercial terrariums (I would hardly call them horticulturists!) inevitably include a drainage layer of gravel at the bottom of the terrarium. Won’t that solve the problem? Won’t the excess water will go there?

The problem is that putting a drainage layer at the bottom of a terrarium has never worked. Real terrarium experts (by that I mean people who keep their terrariums going for years, not stylists who create beautiful arrangements for sale, but who never see the long-term results of their lack of horticultural knowledge) never put in a drainage layer: they know that it’s simply a waste of space. That’s because there is little space for roots in the shallow soil of the typical terrarium, so the roots of the plants, succulent or not, eventually work their way to the bottom of the container and end up soaking in water.

At any rate, when there is water sitting at the bottom of a layer of gravel, it will simply move up into the soil above by capillarity, thus assuring the soil is always moist, much to the dislike of plants that like dry conditions, like succulents.

In a container with open sides and no cover, like a typical flowerpot, this kind of surplus water would soon evaporate, but in a terrarium there is little air circulation, so the air remains moist and stagnant at all times. Evaporation thus moves at a snail’s pace in an open terrarium. It doesn’t occur at all in a closed one.

The Compatibility Question

So, if overheating, excess groundwater and extreme air humidity are the main culprits in the demise of succulent terrariums, you still have to add to this the fact that the stylists who prepare commercial terrariums never seem to take into account how compatible the plants are.

They often place plants that have very different needs in the same terrarium, like a cactus that prefers a cold winter and very dry conditions with a succulent that likes its winters warm and is too not disturbed by bit of soil moisture. Or a plant that has a very specific dormancy period with a plant that grows year-round. Often I see succulent terrariums decorated with living moss… a plant that requires exactly the opposite conditions of succulents: low light and extreme humidity!

There is also a question of compatibility of the plants with the space available. Many succulents added to terrariums will simply become too large over time, crowding out their neighbors or colliding with the containers sides or top. Again, stylists don’t even take that factor into consideration: they just assemble their arrangements based on what looks good for the moment. And why would they be concerned about how big the plants could get? They almost never survive long enough to grow to any degree!

One-Plant Terrariums

A mini-terrarium containing a single plant is often easier to manage than a group planting. At least, when you know that plant’s specific needs, you can try and find a spot that meets them, rather than searching for intermediate conditions for a group of disparate plants, a compromise that makes none of them happy.

Even with mini-terrariums, though, it’s still difficult finding a spot where the average succulent can get the intense light it needs to flourish without dying of heat stroke. I’d suggest sticking to those few succulents that do grow well in partial shade like gasterias, rhipsalis and some haworthias.

And excess water remains a problem in mini-terrariums too. They have to be watered with great care, keeping them always a bit on the dry side, if you want to see them succeed long term.

Summing Up

The succulent terrariums sold commercially are designed for their ability to please the eye and remain attractive for at least several months, until they are sold. The long-term viability of the arrangement is rarely taken into consideration. If you just want to decorate your home for a few months, go ahead and buy one. When the plants begin to die, the seller will usually be willing to replant the terrarium for another 6-month-or-so display… if you’re willing to pay for the service, of course. But still, it’s sad to see all those poor plants end up in the compost.

I’ve been seeing more and more terrariums that contain plastic succulents. With these, no maintenance other than dusting is required, plus you can place them anywhere, both in full sun and in shady corners! I usually hate to recommend the use of fake plants, but if you’ll only be killing living ones over and over again, isn’t that the more logical choice?

But for the real gardeners reading this blog, the ones who like to grow plants under the best possible conditions so they not only live, but flourish, remember terrariums simply aren’t ideal for cacti and succulents. You’d do better to grow them in individual pots or in a “cactus garden”, a large pot without glass walls and equipped with drainage holes, making sure, of course, you plant together only plants that share the same needs.