Building a Simple Terrarium for Your Home

Why a Terrarium Makes Sense in a British Home

Most British homes are dry for half the year. Once the heating goes on, indoor humidity can drop below 40 per cent, which is roughly the level of a desert morning as far as ferns and mosses are concerned. Add grey winter light and windowsills that are either shaded by a neighbouring wall or blasted by a south-facing sun, and many lovely houseplants quietly give up by February.

A terrarium sidesteps most of that. Inside a glass vessel, water evaporates from the compost and leaves, condenses on the glass, and drips back into the soil. The plants sit in their own small, humid climate, protected from draughts and radiators. You get a miniature green landscape that needs watering a few times a year rather than a few times a week, and it fits neatly on a shelf, a desk or a narrow windowsill in a flat where floor space is precious.

Choosing the Glass and Gathering Your Kit

The container decides everything else, so choose it first. A wide opening makes planting straightforward: a large preserving jar, a glass fishbowl, a sweet-shop jar or an open-topped cloche all work beautifully. Narrow-necked bottles look wonderful but demand long tweezers and a good deal of patience, so save those for your second attempt. Whatever you use, wash it with warm soapy water, rinse thoroughly and let it dry, so the glass is clean and free of bacteria.

• A glass container of roughly 2–5 litres, with an opening wide enough for your hand or a pair of tweezers
• Rinsed pea gravel or small pebbles, about 2–3 cm deep
• Horticultural charcoal, roughly 1 cm deep
• Sphagnum moss or a small square of weed-suppressing fabric
• Peat-free houseplant compost mixed with a handful of perlite and some coir
• A spray bottle, a spoon, a chopstick and long tweezers

The Four Layers That Keep It Balanced

Drainage. Spoon in the gravel and level it gently. This reservoir catches any excess water so the roots never sit in a swamp, which is the single most common cause of a terrarium turning sour.

Charcoal. A thin layer of horticultural charcoal sits on top of the gravel. It is not a filter in the laboratory sense, but it does help keep the enclosed air and water sweet, and it prevents the faint stagnant smell that develops in neglected jars.

Barrier. Add a thin layer of damp sphagnum moss, or cut a piece of fabric to fit. This stops compost trickling down into the drainage layer, which would ruin the effect and undo the work of the gravel.

Compost. Use a peat-free mix lightened with perlite and coir, and add it to a depth of about 5–7 cm. Slope it upwards at the back if you want a sense of landscape, and press it down lightly with the back of a spoon. Moisten it with a few sprays before planting so it holds together.

Plants That Love a Humid, Enclosed Space

Choose plants that enjoy moisture and low-to-moderate light, and keep them small. A closed terrarium suits ferns and creeping foliage far better than anything succulent.

• Fittonia (nerve plant) – vivid veined leaves, happy in shade, quick to sulk when dry
• Asplenium (bird's nest fern) – a neat rosette that stays compact for years in a jar
• Selaginella – a low, fern-like creeper that knits itself across the compost
• Soleirolia soleirolii – tiny leaves that form a soft green carpet between larger plants
• Pellaea or small ivy – for a little height and a trailing edge
• Moss – gathered from a shady wall or bought as a small pot, pressed onto the surface to soften the joins

Avoid cacti, succulents and anything with grey, woolly leaves. They rot quickly in permanently damp air.

Planting It Up, Step by Step

• Plan your layout on a sheet of newspaper, arranging the plants in their pots until the shapes please you.
• Plant the largest first. Make a hollow with the chopstick, tease away excess compost from the roots, and settle the plant in with the tweezers. Firm gently.
• Fill in with the smaller creepers around the edges, leaving the front slightly lower so you can see into the jar.
• Add moss to cover bare compost, pressing it down so it makes contact.
• Mist lightly with the spray bottle. You want damp, not drenched, and no standing water in the gravel.
• Leave it open for 24 hours so any bruised leaves dry off, then close the lid or leave it open-topped if you prefer an open design.

Watering, Light and Ongoing Care

Place the terrarium in bright but indirect light: an east or north-facing windowsill, or a metre back from a sunny south window. Rotate it a quarter turn every fortnight so growth stays even. Keep it away from radiators and draughts, and aim for a steady 15–24°C.

A sealed terrarium needs very little water indeed. Check the glass: a light misting in the morning is normal. If condensation is so thick you cannot see through it, take the lid off for a few hours. Water only when the compost feels dry about a centimetre down, which might be every six weeks or longer. Use filtered or rainwater if you live in a hard-water area, as tap water leaves chalky marks and can build up minerals in the soil.

Trim anything that grows into the glass, and remove yellowing leaves promptly so they do not encourage mould. Feed almost never; a single diluted houseplant feed once or twice a year is plenty, since growth is slow in an enclosed space. Keep the whole arrangement out of reach of small children and pets, as several of these plants are not safe to chew.

When Something Looks Wrong

White fluffy mould means too much moisture and too little air. Remove the affected leaves, open the lid for a couple of days and ease off the spraying.

Yellow, mushy lower leaves point to overwatering. Stop watering entirely and let the compost dry a little.

Pale, stretched growth tells you the light is too weak. Move the jar somewhere brighter, but out of direct midday sun, which will cook the contents through the glass.

Brown, crisp edges on ferns usually mean a sudden dry spell or a spell beside a radiator. Trim the fronds, mist gently and relocate.

Look after it, and a single afternoon's work will give you a little green world to enjoy for years.