A true terrarium is a sealed glass container, where the

The amount of water vapour in the atmosphere. Different plants require different levels of humidity. Houseplants that need high humidity are best grown in a steamy bathroom, misted regularly or the pot placed in a saucer of damp pebbles. In a greenhouse, humidity can be raised in hot weather by damping down (wetting) the floor, overhead watering or misting. However, high humidity can cause fungal problems, in which case open vents to improve ventilation.

Many houseplant retailers sell plants specifically for terrariums and bottle gardens – these are generally small, slow-growing foliage plants that enjoy high humidity. Check the eventual height and spread on the label, as many standard tropical houseplants, even young ones, will be too vigorous for terrariums in the long term. It’s also best to avoid flowering plants in a closed terrarium, because each time you open it to remove faded blooms, the environment/balance will be disturbed.

Suitable plants for closed terrariums include:

• peperomias – such as Peperomia argyreia AGM, P. caperata cultivars, P. griseoargentea AGM

• fittonias – such as Fittonia albivenis (Argyroneura Group) ‘Nana’

• hypoestes – such as Hypoestes phyllostachya AGM

• pileas – such as Pilea cadierei AGM, P. involucrata AGM, P. microphylla

• small-leaved ferns – such as Nephrolepsis exaltata ‘Fluffy Ruffles’ AGM, Pellaea rotundifolia AGM

• other low, compact plants – such as Cryptanthus species, Elatostema repens AGM, Selaginella kraussiana AGM 

Moss for terrariums
Moss is often used to cover the surface of the potting compost. It holds onto moisture, as well as looking attractive. Choose cushion moss or fresh, sustainably sourced sphagnum moss, available from online retailers and aquarium specialists. You can also use moss from your garden, but make sure there are no insects in it. Never take moss from the countryside, unless you have the land-owner’s permission.

More specialist terrarium plants
Aquarium specialists also sell plants for terrariums. Some, known as rheophytes (including Anubias and Bucephalandra), grow in the wild beside tropical rivers, attached to stones. In a terrarium they don’t need to be in compost and their rhizomes must never be covered. Some are very slow growing and ideal for small terrariums. Before buying, check they were propagated in a laboratory, to ensure protection of wild plants.

Plants for open terrariums
You can also make a terrarium that has a large, open top – known as an open terrarium. These can be used for growing small houseplants that don’t need high humidity. Succulents are particularly suitable for open terrariums and require very little maintenance. See our guide to growing succulents for more advice.

Any clear glass container can be used as a terrarium, from a large jar to an aquarium. For a closed terrarium, it should have a sealable lid, ideally also made of glass. Alternatively, you can buy purpose-made, decorative terrariums in various shapes and sizes. 

The larger the container, the easier it will be to maintain the right balance of moisture, potting compost and plants. About 30cm (12in) wide and tall is the optimum size. Smaller ones are fine, but the number and choice of plants you can grow will be more limited. If possible, avoid opaque lids, as they reduce the light levels, and metal screw-tops, which can rust.

You could also use a wide-topped bottle, or carboy, which are often available online or from car-boot sales. Regulating the moisture in these may be easier without a stopper. The humidity will still be high enough, but the wider the neck, the greater the water loss.

For an open terrarium, a large open-topped bottle or similar glass container would be ideal. There are also purpose-made options available from houseplant retailers.

To create your closed terrarium:

• Place a drainage layer of expanded clay (Leca or Hydroleca) or washed gravel in the base

• Add enough potting compost to accommodate the rootballs of your chosen plants. Use a mix of peat-free multi-purpose compost and a drainage material, such as perlite or horticultural grit, at a ratio of about 4:1

• Position any small rocks or pieces of wood, if you wish. Take care to clean them first, to avoid introducing insects or diseases

• To plant, make a small well in the compost, insert the plant, fill in around it with compost and firm in. If your container has a narrow neck, you may need to use improvised tools to help with planting, such as chopsticks, a spoon tied to a cane or barbecue tongs 

• Place moss on the surface of the compost​

• Water lightly with a mister, then close the lid

• Position the terrarium in bright but indirect light

​Also see our quick video guide:

A terrarium needs little maintenance once set up, especially if it has a sealed lid. But make sure you position it in a suitable spot and water when needed, keeping an eye on the amount of condensation. Also, look out for any dead leaves or other plant debris and remove promptly, to avoid mould.

Position and light
Terrarium plants need bright, indirect light for at least six hours a day, so a position near a north- or east-facing window is ideal, and a south- or west-facing window in winter. Keep terrariums out of direct summer sun and away from heat sources.

See our video guide to positioning and light for houseplants:

• Water sparingly, especially if the compost is already moist

• Use a mister to apply the water and allow it to drain down the inside of the glass. This will clean the glass and wash any compost off the plant leaves

• A couple of days later, check to see whether you need to add more water. If there’s some light condensation, this shows there is just enough water. If condensation is excessive, remove some with a cloth or leave the lid open for a while

Terrariums are usually trouble free and low maintenance – as long as you use appropriate plants, place it in a suitable location and water sparingly. But a few issues to look out for include:

• Yellowing/dying leaves – usually due to overwatering. Always remove any dying or dead plant material promptly to avoid rotting and mould

• Excess condensation – this will cause plants to rot, so leave the lid open for a while or wipe away with a cloth