As a means of providing a controlled and independent mini-environment in which to house exotic plants terrariums have become quite popular, and they make to it possible to cultivate ferns in homes without a conservatory or greenhouse. Especially in countries such as our own, with strongly contrasting summer and winter seasons, terrariums open a way for us to enjoy growing ferns throughout the entire year, as well as making it possible to grow some of the tropical and semi-tropical ferns that normally call for a conservatory or heated green house

Siting a terrarium
A terrarium can of course be placed anywhere in a house, but in terms of lighting and heating the conditions prevailing where it is placed (the ambient conditions) will inevitably affect it. In the winter a centrally-heated room will generally be at a high enough temperature for the terrarium to require no additional heating of its own; conversely, a south-facing windowsill in summer may prove far too hot and bright. Some thought and careful monitoring of the ambient conditions will clearly be necessary. More will be said about this below.

Obtaining a terrarium of the right size
Terrariums come in all sorts of shapes and sizes. Beautifully crafted but highly priced miniature terrariums, that might be suitable for cultivating a few small, delicate varieties such as filmy-ferns are easily found with an internet search. Pet stores often sell vivariums designed for keeping reptiles or amphibians, and providing these have a leak-proof base, they will certainly function as terrariums. But probably for most of us our terrarium will simply be a customised aquarium. Aquariums, including good second-hand ones, are widely available and can be found with a variety of dimensions. Many come complete with the addition of a lighting system fitted within the hood. What size and proportions the terrarium has will have consequences for what ferns you will be able to grow. If, for example, you want to grow epiphytes or long-fronded species you will need a taller terrarium. Whatever is decided upon, it is important to make sure you will have easy access to the interior for watering and other activities such as planting, re-positioning or removing plants, cleaning the glass, etc.; doing such things can be very awkward if the terrarium is tall but too narrow.

Preparing the interior prior to planting
A bark-based compost covering the bottom to a depth of 10 – 12 cm will suit most terrestrial ferns, and may be improved and made more attractive by incorporating some sphagnum moss, bark chips and fragments of tree-fern fibre. Pieces of (dead) tree-fern trunk or gnarled logs positioned upright or horizontally may be added to create interest, and these will also serve for attaching ferns having running rhizomes. Covering the back and side panels with sheets of cork bark, compressed cork panels and/or slabs of tree-fern fibre provides a perfect medium for the growth of epiphytes, and also functions in blocking excess external light, as ferns are generally shade-lovers. You may wish to add a suggestion of ‘jungle’ to the walls with a collage of pieces of interlaced ‘networks’ of ivy stem or interestingly twisted branches, often found as driftwood (see photo)¹. In a short time, if kept moist, such walls will become green as dormant fern and moss spore lying in them germinate. All the interior as just described should then be thoroughly soaked with water and left for a few days, and the process should be repeated if it appears not fully saturated.

Planting the terrarium
Terrestrial ferns are planted directly into the compost. Making sure some of them have their crowns pushed partly under logs so that growth emerges at an angle will make for a natural look. Epiphytes will need to be attached to vertical or upward inclined surfaces, and this is where fibrous logs and slabs of tree-fern prove so useful. Some epiphytes have long, thin, running rhizomes, and these are best attached with a thread binding them to the surface at several places. To prevent the as-yet unrooted rhizome from drying out, it should be wrapped in a thin bundle of sphagnum moss strands before attaching it with the thread. Eventually the rhizome will root into the substratum. Other epiphytes have creeping rhizomes that are much thicker, while some even grow from a crown. For all these a hole may be scooped out in the substratum. This is especially easy to do in fibrous tree-fern logs or slab. The hole is partly filled with compost and sphagnum moss and the rhizome or crown is inserted and held in place with a layer of moss over and around it. This being then held in place with thread.

After-care
It is most important to keep the plants watered, and this is best done with a spray gun. Spraying is essential in the case of epiphytes, where water applied in any other way will run off. Because the passage of air into and out of the terrarium will be restricted , the amount of moisture lost by evaporation will be less, and the air in the terrarium will usually be humid. In general, ferns thrive in a humid, though not stagnant, atmosphere; however, too much water in the compost may cause bottom-planted rhizomes to rot. You may find that spraying even as little as once a week leads to an excess of moisture – observable as a constant film of condensation on the glass. There is no simple rule of thumb that can be offered with respect to frequency of watering, so it will be necessary to monitor the situation carefully in your terrarium in order to achieve the right balance.

Ferns in a terrarium will need nutrition, and the addition of a few plant-feed pellets around bottom- planted ferns will probably last for many months, but will eventually need replenishing according to their term of viability. It is not usually practicable to use pellets with climbing epiphytes, and for these nutrition should be supplied as a liquid in their spray. There are some liquid orchid fertilizers that are very suitable for this purpose. For both pellets and liquid feed instructions concerning their application will usually be found on the product, but, as is the case in the cultivation of ferns in general, the amount of fertilizers applied should be approximately half what is recommended for other plants.

Training and removing old and unsightly fronds or training the direction of growth of a plant must of course be carried out from the top with the cover removed. For tall terrariums it might prove useful to invest in some forceps, and other tools enabling an extended reach.

Additional heat and/or light
For satisfactory growth most plants in a terrarium will require temperatures ranging between 15 and 25C. The ambient temperature range of the place where the terrarium stands may meet that requirement, but if housing it is in a place where the external temperature range is insufficient (as for example in a cellar), extra heat will be needed. Tubular bulbs that emit a specified heat can be fitted overhead in a covering hood. Such bulbs, known as ‘grow-lights’ or ‘grow-bulbs’ can be obtained from horticultural supply manufacturers as well as from pet stores that sell vivariums. Such bulbs typically provide light as well as heat, and since any alternation of lighting should follow a regular day : night rhythm, their operation should be controlled with a timer. Alternatively, heating may be generated by electric heating cables or ‘reptile heat-mats’ placed under the compost or under the Vivarium as the manufacturers recommend. These may be controlled either with a timer or a thermostat. Without some controlling device however, it is all-too easy to overheat the terrarium.

If heat requirements are adequate, but better lighting is needed, this is best provided nowadays with LED light strips. Those used for aquarium lighting with blue as well as white light are excellent, and plants seem to flourish under them.

The air in very tall terrariums often becomes stagnant. Small electric fans, such as those in found in some computers, are ideal for creating air movement that counters such stagnancy. Short bursts at intervals throughout the day are easily programmed with a timer. Positioning them usefully in the terrarium however without undoing the attractiveness of the whole may present a challenge.

Pests
However scrupulously items that go into the terrarium, including the ferns themselves, are inspected pests are bound to turn up. The commonest are slugs and woodlice, and the latter are not as innocuous as they are often said to be. Removal by hand during repeated night-time searches with a torch is probably still the best way to eventually get rid of them. How best to deal with scale and fern aphid are described in the pamphlet Ferns for indoor conditions.

Ferns for a terrarium
Ferns suitable for a terrarium will almost certainly need to be relatively small-sized. The following list is far from exhaustive, but lists some species that would not only be suitable, but that can also actually be obtained from specialist suppliers. There are two sections: one listing ferns best grown as epiphytes; the other listing species best suited to bottom growth. But it should be borne in mind that the growth habits of some ferns overlap these categories. Terrarium culture is not a necessity or even desirable for most of the ferns listed, and they could as well be grown in any suitable indoor space (see Ferns for indoor conditions). When a species is considered as especially suited for terrarium culture it is marked with an asterisk.

Best grown as epiphytes

Best grown rooted in bottom compost

In addition to the above it may be interesting to include young specimens of species that will ultimately grow too large and will need to be removed and planted elsewhere. The least-sized Platycerium (P. ellisii) could be a suitable candidate for this.

Anything added in in this way will almost certainly harbour slugs, woodlice and other potential pests, and although these may not be readily visible, they will eventually cause problems. This can be prevented by soaking the pieces in an insecticidal solution overnight followed by a thorough soaking in clean cold water to remove traces of the pesticide