You've likely read me hint before about my disdain for the current craze in succulent plants, and succulent terrariums in particular. Don't get me wrong: I'm glad to see an interest in plants in general, and I have nothing against the plants themselves, really; there are a lot of interesting plants that are classified as succulents, displaying a huge variety of shape, colour, and texture, which I suppose is the appeal.They have a reputation of being hard to kill, which I'll admit is partially true; they grow easily enough when conditions are favourable, and if they are not, most of them are at least slow to decline if pretty much left alone, though improper care can hasten their end, especially via overwatering. They are inxepensive, though (particularly the cute young plants that florists and the like get in), and so are easy to replace when one knocks off.
But are people satisfied with this? Isn't the whole thing about keeping plants to try and keep them alive and doing well? I'll readily admit to having killed a plant or two in my time (not usually for lack of trying), but one of the first things I think about when I find a plant I like the look of is whether it's suited to my growing conditions, and, for me at least, most succulents are patently not. They require (for the most part) very bright light, which is an immediate turn off for me personally, and should be for most people I meet who profess to be 'really into succulents right now', most of whom don't have a window with decent exposure, let alone the solarium or greenhouse that would really allow these plants to do their thing.
These plants end up being stuck in a seashell, a shot glass, or some other kitschy place, or worse yet, a terrarium, where conditions are not exactly optimal; these plants are adapted to arid regions, and most require very bright light, free-draining soil and good air movement to do well, and without special considerations these can be very difficult to achieve in a terrarium environment, which are by design retentive of moisture and humidity.
Seeing as how one might have a glass vessel around after succulent disasters of one kind or another (if not, they're readily available from many stores now in various shapes and sizes, though of course any glass or otherwise transparent container will do, really), why not give one a go again with plants that are perfectly at home therein? The really nice ones (such as the orchid to the right) can take a little digging to rustle up, but there are many fine species which can be found without too much trouble and with which one can have some rewarding success with terrarium gardening. Because the terrarium should really be a less intensive and more problem-free way to keep plants indoors; soil moisture is held much longer, so issues with forgetting a watering will not be as severe, and most plants will benefit from the extra humidity and reward the grower with larger, vibrant-looking foliage while at the same time avoiding the many foliar maladies that come with dry indoor air.
This was the original purpose of the terrarium, after all; Wardian cases have been around since the Victorian age as a way of keeping exotic tropical plants alive indoors, which was the style at the time. Ferns and the like were and are appreciative of the shelter provided by these glass enclosures, and we now have better technology which allows us to have seamless (and leadless!) glass enclosures for these plants, which makes them all the more attractive while still being quite functional.
What are the prerequisites for a good terrarium plant? First and foremost the plant needs to be adapted to the conditions approximated by a terrarium, which typically consist of lower light, consistently moist soil, higher humidity, lower air movement and limited space, though all of these variables can be controlled to some degree. Fortunately there are many plants who are right at home in such an environment; think of the tropical forest understory, where the light is dim and the air and soil are often very damp indeed. Many species can be found either growing directly on the ground or else epiphytically on the lower portions of trees, and these are often the best suited for terrarium life.
There are plenty of blogs and articles out there that list suitable species, so I won't bother with one here. Be wary, though, as some of these lists are not entirely well thought out and some of the listed species may eventually grow quite large (relatively speaking); when starting out, working with a larger container will help keep things manageable without needing too much maintenance to keep things from being cramped. Do a little research and find out the maximum size of any prospective species and it will save you trouble in the long run. There are a great many true miniature plants which will stay small for their entire lives, but they are often not available through most retailers, who instead often sell juvenile plants in small pots because they're cute and inexpensive. An alternative might be creeping or stoloniferous plants, for example, like Pellionia, Pilea, Episcia and others, which are pretty easy to find; their size is very easily managed, for they stay low to the ground and need only be trimmed when they start climbing the walls of the terrarium. There are many upright plants which will stay reasonably short in stature that pass through the garden centres and florists' often enough, too, so familiarize yourself with a few of the plants from the terrarium-friendly plant lists beforehand and keep your eyes peeled.
There are in fact whole enterprises geared towards selling terrarium-friendly plants (though not so much in Canada, unfortunately); in the USA, Black Jungle, Josh's Frogs and Glass Box Tropicals are three that are very popular amongst dart frog hobbyists, who build elaborate natural planted enclosures for the species they keep in captivity. Not all of us have access to these places, of course, but their stock lists do serve as a handy reference for plants amenable to the terrarium environment.
A couple tips on setting up and maintaining a terrarium: first, use good quality soil for your terrarium and I guarantee better success than using the dirt from the garden centre (I'd written a little bit about terrarium soil near the bottom of this post): one that allows better aeration and increased longevity will make plants much happier over the long term. And keep up on your watering, but notice that a little water will go a long way; one of the benefits of a clear container is that you can watch the water you apply migrate and spread out into the soil. Do this once or twice while paying attention and you'll get a sense of how much water it will take to keep things just right. And if you're a heavy-handed waterer by nature, do not despair; there are plenty of plants that don't mind wet feet. Plants sold for aquariums, for example, are often riparian species that will do marvelously planted in consistently moist to wet soil and kept humid, and the selection in the trade now is better than ever.
Second, keep that thing out of direct sunlight, especially if it's without any ventilation holes; if you've ever been in a greenhouse on a sunny day you've experienced the greenhouse effect, where heat is trapped and accumulates within. This effect works within the terrarium, too, and plants inside can easily be cooked if temperatures are allowed to become extreme. If you've picked the right plants, bright indirect light should be the most you'll need, and many will tolerate deep shade fairly well.
Third, do your homework! If you learn a thing or two about a particular plant's origins and habitat you'll be able to make an educated decision on whether it's a good candidate for the terrarium or not; a simple search online with the species name appended with the word 'habitat' will bring up a fair amount of information in most cases. Even cultural tips such as 'enjoys high humidity', 'keep out of direct sunlight', and 'keep moist but not wet' are all valuable clues to the suitability of the prospective species. Knowing your plants a little better also increases the richness of keeping them as a hobby, I feel, so don't be afraid to nerd out a little.
If you are hellbent on succulents and not having much luck with them inside glass containers, perhaps a dish garden may be a better direction to go for your diminutive landscapes; this will allow for better air movement and the option for higher light in order to achieve greater success with these plants. And of course all of this is not to say that a succulent terrarium is impossible; I'm sure there are people out there who can pull it off (long-term, I mean; anyone can stuff some plants in a jar and put a photo of it on the internet). But the terrarium environment is really best suited to tropical species which will respond positively to those conditions, and if it's the particular form of succulents that appeals to you, there are many other plants which are succulent-looking in nature but which do hail from tropical forests. Many Peperomia are quite succulent, and most are very well-suited indeed; P. prostrata above has to be one of my all-time favourites. There are some caudex-forming epiphytes such as Mymecodia and Hydnophytum (which are profoundly interesting in their own right due to their mutualistic relationship with ants in nature; read more about that here) which are delightfully rotund at the base and often a little spiny, Cryptanthus and other bromeliads certainly conjure the rosette forms of Echeveria or Haworthia, and the list goes on and on, so give something new a try and have another look at the terrarium; there's so much out there waiting for you.