Why grow a begonia in a terrarium?
The question might be: where do you grow your begonias? Possible answers: in my flat, in an unheated conservatory, in a “spare” room of the house, in a greenhouse. To grow begonias in good conditions, as close as possible to their natural living conditions – and this is also true for hybrids – you need to give them:
- High humidity
- Warmth
- Light
You cannot grow your beloved begonias properly in a flat (too dry), in the house (too dark) or in a conservatory (too cold in winter if unheated). If you do not have a heated, humid, well-lit greenhouse, the only way to give your plants all three elements – warmth, humidity and light – is to grow them in “mini-greenhouses” or terrariums, where it is much easier and cheaper to create and maintain these three basic conditions.
Indeed, once these conditions are in place, growing in a terrarium keeps them stable for several weeks without any intervention. As you will see, a terrarium is easy to set up, growing plants in it is really very simple, and you can even go away and leave your plants for several weeks!
Which container?
Any transparent and “large enough” container will do, whether glass or plastic: being transparent, it lets light through; being airtight, it keeps the humidity stable; and it can easily be placed (and moved) somewhere practical, suitable, warm and bright.
The photos show that there are many possible models. Aquariums are particularly suitable for growing small begonias, whether “bowls” or classic rectangular tanks. You can use a glass bottle if it is large enough, a plastic one to start cuttings, a mini-greenhouse from a garden centre, a glass microwave dish covered with a cloche, a saucer with holes also covered with a cloche, a freezer container, etc., etc. Our American friends can buy handy terrariums specially designed for growing plants. If you know how to cut and glue glass, you can also make “made-to-measure” terrariums yourself, in particular ones big enough for large plants.
One important point: you must be able to close and open the terrarium easily : the best option is a sheet of glass (cut to size, with smoothed edges) that closes it without blocking the light and can be slid aside for ventilation when needed, or a piece of cling film with a few holes in it.
Humidity, warmth and light
To get satisfactory humidity in the terrarium, the substrate must be moistened at the start – I did say “moistened”, and this is done before the begonia is planted (if you push a finger into the substrate, the hole should not fill with water!). Condensation on the sides is a sign of good humidity, which is then close to 90%. There is no need to water the plant (except when planting it, so that the roots are in contact with the substrate): as the terrarium stays closed, the humidity will last for quite a while. You can also get good humidity by growing the plant on living Sphagnum moss: besides its antiseptic effect, this moss “self-maintains” the humidity in the terrarium.
Providing warmth in winter is important: begonias cope badly with cold, especially in a very humid environment, which is the case in a terrarium. So place it near a source of heat; this is easy in a house or flat, less so in an unheated conservatory. You can also lay an electric heating cable in the bottom of the substrate, if the terrarium is large enough and can be drilled to pass the wires through. These cables are easy to find in garden centres or aquarium shops. Beware: these cables do not heat the terrarium, they heat the substrate, and this form of heating tends to dry it out! In any case, it is much easier and much cheaper to heat a terrarium than a conservatory or greenhouse!
In nature, even in the relatively dark rainforest, begonias receive a certain amount of light which, through photosynthesis, allows the plant to feed. In the tropics where they live, day length varies from about 10 to 14 hours. In France the variations are greater: in summer the day lasts about 16 hours, but only 8 hours in winter. Artificial light can make up the shortfall in winter. While light is sufficient in a conservatory or greenhouse, this is not the case in a house or flat; you can then keep the terrariums – usually in the house or flat – under artificial light all day, i.e. for about 14 hours. (Several articles have been written about which lamps to use: see the Bégofil.) Most begonias grow well under artificial light.
The substrate
What is true for growing begonias in pots also applies to terrariums: the substrate must be light and airy and free-draining. Unfortunately, it is hard to find a compost suitable for begonias in garden centres; you can buy it from suppliers to professional growers: it is relatively cheaper and better suited (it is a non-composted, peat-based compost), but it comes in 70-litre bags. You can club together to share the cost.
How do you aerate or lighten the compost ? By adding perlite (expanded volcanic rock in the form of white granules). Usually about one third perlite is added, but you can go up to two thirds for saxicolous plants (those that grow on rocks). On the other hand, avoid vermiculite, which holds too much water (it can, however, be used for cuttings). Perlite gives a better-draining substrate, because it is more airy. You can also put a layer of a few centimetres of perlite or clay balls in the bottom of the terrarium, before the compost. This will ensure good drainage and stop the plants’ roots from suffocating at the bottom of the terrarium, where a little water can collect.
Add a little charcoal to keep the compost from going stale, which in the long run causes rot that is obviously harmful to the plants. The charcoal used in aquariums works well.
How deep should the substrate be in the aquarium? If you plant the begonia “in the ground”, it needs enough substrate – I would say about a quarter of the height of the terrarium. If your plant is in a pot, on a cork slab or on a piece of travertine (tufa), less is enough, but some is absolutely necessary to maintain humidity.
You can “plant” living Sphagnum moss in the terrarium. In fact, a few strands of this moss laid on the substrate take root and then grow. As I said above, this bog moss has a remarkable antiseptic effect and keeps the terrarium permanently humid. In general, begonias love Sphagnum (not all of them, because of the acidity it creates), especially for cuttings: a begonia leaf, a rhizome or a piece of stem laid on living Sphagnum in a terrarium roots very quickly. You can also use dried Sphagnum (usually from Chile, sold in some garden centres), cut into small pieces and mixed into the substrate. Its antiseptic and humidifying effect is weaker.
Some begonias grow on limestone rocks and so need “calcium”. The simplest solution is to mix a little coral sand into the substrate (available from garden centres with an aquarium section).
Growing in a terrarium: looking after the plant and the terrarium
Setting up a plant is simple: after adding the substrate, make a round hole in the middle of the surface to take the root ball of the plant; firm it a little, and barely water it ; put the lid on and place the terrarium in an “ideal” spot… That’s all! Your plant can now last a few weeks, or even a few months, without any further care.
- Keep an eye on the humidity (the condensation that should form on the sides), and if it seems too low, mist with rainwater. If you think everything is too wet, ventilate.
- Keep an eye on the leaves: those that rot (this is normal, it always happens) drop off and cause the leaves they touch to rot too.
- As the plant grows, its leaves may touch the sides of the terrarium: if so, your terrarium was too small for the plant you chose!
- Likewise, when the plant flowers, the stalks sometimes rise above the leaves and touch the glass, which often makes them drop! Remove the lid and plan a larger terrarium next time…
- Beware! If a fruit forms, ventilate the terrarium. Remember that in nature, begonias produce their fruit and then their seeds at the start of the dry season, when the plant’s energy is no longer devoted to growing but to ensuring its survival by making seeds. If you keep a fruit in too damp an environment, you are almost certain to lose it. So open the terrarium or, better still, leave an opening (in the “paper lid” if you used cling film) through which the stalk bearing the precious fruit can pass.
- Use fertiliser very sparingly; if the leaves turn yellow, the plant may be short of food – in that case, mist with water very lightly enriched with fertiliser. Only do this on healthy, mature plants.
- After a few months, the compost in the terrarium may dry out; if you water, the water no longer moistens the substrate (this is especially true of peat-based compost) and runs straight down the sides to the bottom of the terrarium: the plant stops growing and its roots may end up in water and suffocate! Sorry, but you will have to redo the terrarium! But as you will have taken a cutting of the treasure you were growing, you can replace the tired original plant with a more vigorous young one!
- Another common problem with terrariums: after a few months, green algae form on the glass or plastic sides. They are hard to remove – some always remain, and they come back stronger than ever! Time to redo the terrarium… Throw the compost away; do not keep it for repotting… it is “tired”
- Moss also often forms on the surface of the substrate, and it is hard to get rid of; note that having sphagnum – itself a moss – in the terrarium often prevents other mosses from forming…
- In short, if you use terrariums, plan to redo them completely after a year, or two at most. Use bleach to clean the containers.
Which begonias to grow in a terrarium?
All the small begonias from humid tropical regions can be grown in a terrarium; match the size of the container to the size of the adult plant. On the other hand, avoid growing thick-leaved begonias in a terrarium (B. conchifolia for example), as these plants tolerate dry periods. Here is a non-exhaustive list of begonias to grow in a terrarium:
- Central America: bowerae / hydrocotylifolia / squarrosa / strigillosa / aridicaulis / ludicra / violifolia, etc…
- All the small African rhizomatous begonias, all the small tuberous begonias from Madagascar.
- Southeast Asia: all the small begonias from the Philippines, Malaysia, Sumatra, Borneo and New Guinea: decora / nigritarum / nurii / sudjanae / rajah / luzonensis / rhombicarpa / ionophylla / amphioxus, etc …
- Mainland Asia: China, Vietnam: leprosa / pseudodryadis / augustinei / versicolor / pedatifida / handelii / chuniana, etc …
- South America: crispula / lanceolata / olsoniae / depauperata / herbacea etc …
Small tuberous begonias, especially the species from Madagascar, are easy to grow in a terrarium, and this method has several advantages for these plants. The main difficulty in growing them is keeping the tuber – sometimes tiny (no more than 5 mm across) – alive when the plant goes dormant. In a terrarium under artificial light, these plants hardly go dormant at all and, better still, flower continuously! So the problem of keeping them is solved; however, it is wise to keep a few back-up cuttings, as these begonias do not rebuild their reserves as tuberous begonias usually do during dormancy, and they weaken and so live less long. On the other hand, if they do go dormant even in the good conditions described above, leave the tuber in the terrarium with the lid slightly open: it will neither rot nor dry out!
Plan larger terrariums for species such as: chlorosticta / bipinnatifida / augustae / goegoensis / erythrogyna / pendula / borneensis / isoptera / polilloensis / malipoensis / hemsleyana etc., etc…
Terrariums are very useful for rooting begonia cuttings (in fact, a fallen leaf often roots spontaneously in a terrarium). A mineral water bottle cut in half is a very easy and effective way to root cuttings (see Bégofil no. 79)
Likewise, I often put a weak or sick plant in a terrarium for a few weeks, and this stay in “intensive care” usually does the plant a lot of good.
