Photo: ohsummossum.com

Terrariums have been in vogue for a couple of years now and just about every garden center and florist shop has them on display. These glass containers, with or without a transparent cover, are usually decorated with mixed plantings of small plants, creating the effect of a miniature garden. They’re really quite attractive and probably most people buy one simply for its beauty, but did you know that a well-designed terrarium is easier to maintain than any other indoor planting?

That’s because the humidity level in a terrarium is always high. In a closed terrarium, it can even approach 100%. Compare that to the relative humidity found in a normal house which struggles to reach 30% at certain times of the year. Since most houseplants prefer a humidity of 70% or more, a terrarium seems like paradise to them, especially when compared to conditions outside the container.

This extreme humidity greatly reduces watering needs. After all, when a plant grows in open air, most of the water it absorbs is lost to transpiration, so you have to water again and again to replace it. In a terrarium, though, the transpiration rate falls dramatically. Therefore, the plant uses most of the water it receives for its growth rather than losing most of its moisture to transpiration and the frequency of watering drops like a stone. An open terrarium (one without a lid) with high sides rarely requires watering more than twice a month. And you may only need to water a closed terrarium (one with a lid) once a year and even then, one or two spoonfuls of water will probably be enough! It’s the low-maintenance indoor garden par excellence … at least for plants that like a humid atmosphere!

But You Have to Find Suitable Plants

Given their druthers, the vast majority of tropical plants we grow as houseplants would rather grow in a terrarium than in the open … but the problem is then, under the extraordinarily good conditions offered by the terrarium, their growth rate explodes and they rapidly outgrow the container, if indeed they weren’t already too big to begin with. Unless you have a huge terrarium, say walk-in size, it is therefore better to choose plants that are naturally small or that you can keep small with the occasional pruning.

Also, when you place a high-sided or closed terrarium in a sunny spot, the temperature inside rises dramatically (think “greenhouse effect”) to the point where the plants will literally cook. That means your terrarium will have to be placed in indirect light. It is therefore logical to avoid plants that need full sun and to choose instead those that tolerate low to medium light levels.

Here are a few smaller plants that adapt to medium to low light and will thrive under terrarium conditions:

1. Asparagus fern (Asparagus spp.), seedlings only
2. Babies tears (Soleirolia soleirolii)
3. Bird’s nest fern (Asplenium nidus), young specimens
4. Creeping fig (Ficus pumila)
5. Dwarf palm (Chamaedorea elegans, syn. Neanthe bella), seedlings only
6. Earth star (Cryptanthus cvs)
7. English ivy (Hedera helix cvs)
8. Episcia (Episcia cvs)
9. Fittonia (Fittonia cvs)
10. Maidenhair fern (Adiantum cvs)
11. Miniature African violet (Saintpaulia cvs)
12. Miniature orchids (various species and cvs)
13. Miniature sinningia (Sinningia pusilla and others)
14. Moss (various species)
15. Philodendron (Philodendron cvs), smaller varieties
16. Pilea (Pilea depressa, P. microphylla, etc.)
17. Polka dot plant (Hypoestes phyllostachya)
18. Pothos (Epipremnum aureum)
19. Rhizomatous begonia (Begonia spp.)
20. Round-leaved fern (Pellaea rotundifolia)
21. Spikemoss (Selaginella cvs)
22. Syngonium (Syngonium cvs), dwarf varieties
23. Weeping fig (Ficus benjamina), dwarf varieties like ‘Too Little’

Carnivorous Plants

Carnivorous plants also love terrarium conditions and in fact, most will only do will indoors inside a terrarium. However if you want to grow them alongside the terrarium plants described above, you’ll need to choose tropical carnivorous plants, such as tropical sundews (Drosera capensis and others), tropical butterworts (Pinguicula spp.) and tropical pitcher plants (Nepenthes spp.). Varieties that require cold winters, like Venus flytraps (Dionaea muscipula), hardy pitcher plants (Sarracenia spp.) and temperate-climate sundews (Drosera rotundifolia and others) require their own separate terrarium, one where the winter temperature can drop to about 40˚ F (5˚ C).

Also, most carnivorous plants require very acid soil (usually sphagnum moss is used rather than true soil) as well as water devoid of any minerals, so you’ll need rain water or distilled water.

For those reasons, you may find it easiest to grow even tropical carnivorous plants in their own terrarium where they can receive the special attention they deserve.

Avoid Cactus and Succulents 

The worst mistake beginners make is to fill their terrarium with cactus and succulents. Yet cactus and succulent terrariums are what you most often see in retail stores. If garden centers sell cactus and succulent terrariums, how can that be wrong?

You have to understand that terrariums offered in commercial enterprises are made to be sold, not to be viable. The “designers” (I could hardly call them horticulturists!) who create commercial terrariums love cacti and succulents not because they grow well in terrariums, but because they don’t grow, or only very, very slowly. That means a cactus and succulent terrarium is like a still-life portrait: it will remain exactly the same for months on end, a major asset for sales. Eventually, though, the plants start to rot and the terrarium will be removed from display. The seller hopes you’ll buy the cactus and succulent terrarium early on, while it still looks good. By the time it starts to collapse, it will probably have been in your home for months and you’ll assume the decline was your fault. In fact, though, it was planned obsolescence, horticulture style.

What went wrong? The environment inside a terrarium is simply not conducive to healthy cactus and succulent growth.

First of all, the high humidity prevailing in a terrarium is too much for plants from an arid environment to take, plus there is very little air circulation, both of which tend to lead to rot. Futhermore, the soil remains damp far too long, not something cactus and succulent adapt well to. And to make things worse, it’s difficult to find a spot in the average home that suits the light needs of a cactus and succulent terrarium. These plants prefer full sun or nearly full sun, but if you place a high-sided or, worse yet, closed terrarium in the sun, the temperature inside will cook the plants. Yes, there is a limit to the heat even desert plants can stand!

If you insist on growing succulents in a terrarium, at least use an open terrarium with low sides, something closer to a big dish than a bottle. That will allow air to circulate more freely, removing excess moisture. And temperatures will be lower. Also prefer succulents other than cacti, such as crassulas, haworthias, gasterias, sedums, and echeverias, as true cacti are generally more susceptible to rot than many other succulents. Living stones (Lithops and their ilk) likewise tend to rot in terrarium, even low-sided ones.

If you really want to work with cacti and succulents, might I instead suggest a miniature “cactus garden*” in a pot (with one or more drainage holes!) rather than a terrarium, because without glass walls to trap humid air, air circulation will be good and heat will not build up excessively. You’ll therefore be able to place such your cactus garden in the sunny spot it prefers. Plus, excess water will drain out into the saucer underneath rather than accumulating in the pot and causing rot.

*Although called a “cactus garden,” such decorative miniature gardens often contain succulents other than cactus and may not even contain cactus at all!

Gather the Materials

Preparing a terrarium is a breeze, making it a great project for families and for schools as well. You can easily assemble it in half an hour! But first you have to assemble the materials necessary and that can take some time.

First, nearly any glass or transparent container, whatever its form, will make a suitable terrarium: a brandy snifter, a wide-mouth bottle, an aquarium, etc. You can choose an open terrarium (without a cover) or a closed one (covered with a lid or a piece of glass or Plexiglas). You can even make a terrarium in a bottle with a narrow opening … but that’s more complicated and I won’t address that technique here. It is easier to use a container with an opening through which you can easily insert your hand.

Collect all the decorative elements too: driftwood, pebbles, pieces of bark, branches, moss, or others. Wash them well before use. You will find all sorts of decorations in nature (pebble beaches and forests are ideal places to look). If not, a pet store that specializes in vivariums will likely offer many objects of interest … for a price!

Moss can be very useful in covering your terrarium’s soil. If you harvest moss from the wild, though, it would be wise to plunge it underwater for an hour to drown any unwanted intruders. You will also find dried moss in garden centers. However, the moss sold in garden centers is not simply a dried but living moss that will start to grow again in the terrarium, as many people think. It is a “preserved” moss: treated with glycerine in order to kill it, yet allow it to keep its green color for years. Preserved mosses will not come back to life.

You’ll also need potting soil. Any commercial potting soil for houseplants will do. You do not need to add activated charcoal, despite of what you see on a lot of Internet sites offering advice on terrariums (all apparently written by people who have never tried actually maintaining a terrarium!). Try two terrariums, one with and one without charcoal, and you’ll see: its use makes absolutely no difference, either in the immediate or years later. And activated charcoal is expensive.

Finally, you’ll need to go shopping for terrarium plants. Some of the better garden centers even have a special terrarium plant section, making your search all that much easier!

How to Plant a Terrarium

It’s best to work with damp soil. So pour the potting mix into a bowl, add a little warm water and stir to make so the water penetrates evenly. You’ll want the soil to be barely damp, certainly not soggy.

Pour the soil into the terrarium. The depth will vary depending on the size of the terrarium and the effect you want to create: at least 2 inches (5 cm), but 4 inches (10 cm) is much better. You can use rocks or other objects to create different gradients, especially in a large terrarium.

Note that you do not need a “drainage layer” (again, lots of “terrarium” websites say the contrary, a good sign they are not good sources of information). There is no possible drainage in a terrarium, period, because glass containers don’t have a drainage hole through which excess water can drain. The roots of your plants will plunge to the bottom of the container whether you add a layer of gravel at the bottom or not. Thus, this false drainage layer simply wastes valuable space that could be devoted to potting soil, a product that at least your terrarium plants will appreciate.

Next, unpot the plants. If the root ball is too high for your needs (and it often is), break it up and spread the roots out in all directions to reduce its height.

Dig a planting hole, insert the plant, and fill in with potting soil. Repeat with the other plants. It couldn’t be easier!

To complete the installation, add moss and other decorative elements. Let your imagination run wild: terrariums offer endless design possibilities!

When finished, spray lightly with tepid water to settle the soil and place the container in a brightly lit spot, but away from full sun, at normal room temperatures. If you ever see condensation forming on a covered terrarium, remove the top for a few days to allow to excess water to evaporate. And if ever the soil seems downright soggy, absorb any excess water with a cloth or paper towel.

If you ignored the advice above and put a drainage layer in the bottom of your terrarium where water has accumulated, which is, after all, the point of a drainage layer, you have a huge problem. You’ll soon be losing plants to rot. You don’t ever want water accumulating in the bottom of a terrarium! To drain the excess moisture, tray inserting a turkey baster through the soil into the bottom of the terrarium and sucking out as much water as you can.

As for Maintenance…

A properly planned terrarium requires very little care. Other than the occasional watering, the only maintenance needed is to pinch or prune plants to control their growth … and sometimes to replace a plant that is really getting too big. You won’t need to fertilize for a least a year, and even afterwards, do so only very sparingly. In a terrarium setting, the last thing you want is to stimulate rapid growth!

Some terrariums thrive with only modest changes for 15–20 years.

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Given its minimal care needs, a well-planned terrarium truly is the ideal indoor garden for a laidback gardener!