There is a particular disappointment in building a terrarium, getting it looking exactly right, then watching the moss turn brown within a fortnight. The moss was healthy in the shop, healthy on the wall you lifted it from, and then it sits in a jar on a shelf and quietly gives up.

The cause is almost always a mismatch. Moss is not one plant but hundreds of species with very different tastes in light, moisture and air movement. A bog species in an open bowl on a sunny windowsill will crisp. A moss from a drystone wall in a sealed jar will rot. Match the species to the container and moss becomes one of the most forgiving things you can grow indoors.

Closed or open? Decide this first

A closed terrarium — a jar or bottle with a lid — holds humidity close to saturation. Water evaporates from the substrate, condenses on the glass and runs back down, so the system recycles its own moisture. You might water it twice a year. The trade-off is stagnant air and no drainage, which suits damp woodland mosses and quickly kills anything that likes to dry out.

An open terrarium — a bowl, a rimless vase, an open glass cube — behaves much more like a houseplant on a shelf. It dries out fast, needs misting several times a week, and offers plenty of airflow. This is the right home for mosses that evolved on walls, paving and lawns, where they get soaked and then dry out again.

A useful rule: if you found the moss in deep shade on ground that stays damp all year, it belongs in a closed jar. If you found it on a wall, in a pavement crack or in a dryish lawn, it will do better in the open.

Mosses that thrive in closed terrariums

These species come from woodland floors, stream banks and boggy ground. They want steady moisture and low light, and they resent drying out.

• Hypnum cupressiforme (sheet moss) — the workhorse of terrarium planting. Feathery, rich green, spreads readily across a damp surface and tolerates the occasional mistake.
• Thuidium tamariscinum (common tamarisk moss) — delicate, fern-like fronds in a slightly yellowed green. Beautiful in a jar, but it needs consistently high humidity to keep that fine structure.
• Leucobryum glaucum (cushion moss) — pale, almost grey-green hummocks. Slow-growing and striking as a single focal clump, provided the substrate never dries fully.
• Sphagnum species — the classic bog moss, in greens, reds and russets. Wonderful in a wet, acidic terrarium, but keep it in its own container; its acidity and constant wetness will see off most other mosses.

Liverworts such as Marchantia and Riccia are often sold alongside moss and behave well in closed containers. Bear in mind that Marchantia spreads aggressively and will eventually colonise everything.

Mosses for open terrariums

Open containers suit the opportunists: species that survive drought by going dormant and green up again after rain. They need more light than woodland mosses but far less pampering.

• Bryum argenteum (silver moss) — tiny cushions with silvery tips, often seen between paving stones. Copes with drying out and looks neat in small bowls.
• Ceratodon purpureus (redshank) — bright green shoots on reddish stems, extremely tolerant of neglect and bright light.
• Rhytidiadelphus squarrosus (springy turf moss) — the moss of garden lawns, with star-like shoot tips. Happy in a bright open bowl.
• Tortula muralis (wall screw-moss) — grey-green tufts with wiry white hairs. It will dry to almost nothing between mistings and revive within hours.

What not to buy, and where to collect

Bags of moss sold for floristry and craft work are almost always preserved or dyed. Soft, springy and unnaturally uniform in colour, they are dead and will never grow. Buy live moss from a terrarium or aquatic specialist, or from a nursery that lists species names, and check for green growth at the tips before you plant.

Collecting your own is perfectly reasonable if you do it thoughtfully. Take moss from your own garden, or with the landowner's permission. Never lift it from nature reserves, Sites of Special Scientific Interest or other protected ground — that is an offence, and slow-growing mosses take years to recover. Lift small patches from areas where the moss is abundant, and take a thin sliver of the soil beneath so the mat stays intact. Leave most of it behind.

Planting and watering

Moss has no true roots. It anchors itself with fine filaments and takes in water through its leaves, so the substrate is there to hold moisture and give purchase rather than to feed anything.

1. Add a shallow layer of grit or pebbles for drainage — essential in open containers, useful in closed ones.
2. Top with a thin layer of activated charcoal, which is widely used to keep terrarium air fresh.
3. Add a peat-free substrate mixed with grit and a little chopped sphagnum or coir. It should be damp, not sodden.
4. Press the moss gently onto the surface. Do not bury it; stems below the substrate rot quickly.
5. Mist thoroughly, then leave the whole thing to settle for a day before closing the lid.

Use rainwater wherever you can. Most tap water in the UK is hard and chemically treated, and the dissolved minerals leave pale crusty deposits on moss and scorch the growing tips over time. Bottled distilled or reverse-osmosis water is a good substitute if you have no water butt. Water from a softener is not — the added sodium is worse for moss than hardness.

Keeping it green: light, air and troubleshooting

Aim for bright but indirect light. A north-facing windowsill is ideal, as is a spot a metre back from a south-facing window. Direct summer sun through glass will cook a sealed jar in an afternoon, even in Britain.

Watering is where most terrariums go wrong. In a closed container, water very rarely. If condensation coats the glass all day, it is too wet; take the lid off for a few hours. In an open container, mist two or three times a week and water properly when the moss feels dry and slightly crisp, before it turns brittle.

Let closed terrariums breathe. Remove the lid for ten minutes once a week, or use a jar with a loose-fitting lid. Mould is nearly always a sign of still, wet air. Springtails — tiny jumping bugs sold for terrariums — graze on mould and decaying plant matter and are a genuinely useful addition.

Common symptoms and their usual causes:

• Brown, crispy and brittle: too dry, or too much direct sun.
• Brown but soft and mushy at the base: too wet, poor drainage, rotting.
• White furry patches: mould — increase airflow and remove the affected moss.
• Pale, thin, stretched growth: not enough light.
• Dark green slime or black patches: algae and waterlogged substrate, often from tap water.

Start with one jar

Pick a single forgiving species — Hypnum cupressiforme is hard to beat — and a jar with a lid. Layer grit, charcoal and damp peat-free compost, press the moss on top, mist, close the lid and stand it somewhere bright but out of direct sun. Then leave it alone for a fortnight.

Change one thing at a time. If it browns, move it further from the window or mist more often. If it goes soggy, open the lid. Moss responds slowly, so give any adjustment two or three weeks before deciding whether it worked. Once you can keep one jar green through a winter, you will know exactly what the second one needs.

Photo: Nennieinszweidrei / Pixabay