Why moss suits a jar in a British home

Moss is the most obliging plant in Britain. It grows on damp walls in Manchester, on paving slabs in Bristol, and on the north side of every fence in the country. It asks for no direct sun, no rich soil and no fuss, which makes it perfect for anyone with a windowsill instead of a garden, or a rented flat where you cannot drill into the wall.

A glass jar takes that natural talent and turns it into a tiny, self-contained landscape. Because the jar traps moisture, the moss recycles its own water. You build it once, and for months afterwards you mostly just look at it. It is a wonderfully forgiving first project, and it costs almost nothing if you collect your moss from a legal, sensible spot.

Choosing a jar and gathering your materials

Aim for a jar with a wide mouth, because you will be working inside it with your fingers and a spoon. A large kilner or preserving jar works beautifully, as does a big sweet-shop jar from a charity shop. Anything from about one litre upwards is comfortable to plant. Give it a good wash and rinse, and let it dry completely before you start.

You will need:

• A clear glass jar with a lid, roughly 1–3 litres.
• Horticultural gravel or small pebbles for drainage — about a handful.
• Activated charcoal (the sort sold for aquariums or as filter granules) to keep the air inside sweet.
• Sterilised peat-free compost, or a mix of compost and coir.
• Moss — a few generous handfuls.
• A long spoon, tweezers or chopsticks, a mister, and a clean cloth.
• Optional: a tiny stone, a piece of fallen bark, or a small fern for height.

You can also add a pinch of springtails from a healthy compost heap. These microscopic creatures eat mould before you ever see it, and they turn a jar into a genuinely self-regulating little world.

Building the layers from the bottom up

The secret of a long-lived terrarium is the layer cake at the base. Each layer does one job, and skipping any of them usually ends in a soggy, smelly jar.

• Drainage layer: pour in 2–3 cm of gravel. Any water that drains down through the compost sits here, well away from the roots.
• Charcoal layer: scatter a thin 5–10 mm layer of activated charcoal over the gravel. This absorbs odours and slows the growth of mould.
• Barrier: if you have a scrap of horticultural fleece or fine mesh, lay it over the charcoal to stop compost trickling down. A single layer of dried moss works too.
• Compost: add 4–6 cm of compost, firming it gently with the back of a spoon. Slope it upwards at the back if you want a sense of landscape.

Keep the compost level below the shoulder of the jar. You want a clear band of glass at the top so you can see in and so condensation has somewhere to run.

Collecting and planting your moss

Moss is easy to find, but collect it kindly. Lift small patches from your own garden, a willing friend's wall, or a quiet corner of a park where it is clearly abundant, and never strip a whole surface bare. Take only a little from each spot, and never collect from protected sites, nature reserves or ancient woodland without permission. The cushion-forming species with neat rounded humps are the easiest to handle, while feathery trailing mosses look lovely draped over a stone.

Shake the moss gently to remove soil and any passing insects. Then press each piece firmly onto the compost, pushing the base down so it makes good contact. Leave small gaps between patches — they will knit together within a few weeks — and tuck a stone or a scrap of bark into any bare corner. Mist everything lightly with rainwater so the moss is damp but not soaked.

Sealing, siting and keeping the balance

A lid makes the terrarium almost maintenance-free, but you must not forget about it entirely. Seal the jar and place it somewhere bright but out of direct sun; a north or east-facing windowsill is ideal. Strong sun through glass will cook the moss in an afternoon.

Check it after a day or two. If the glass is lightly misted in the mornings and clear by afternoon, the balance is right. If water streams down the sides constantly, take the lid off for a few hours to let some moisture escape. If the moss looks dry and pale, add a little rainwater and reseal.

Open the jar for ten minutes once a month to let fresh air in, and trim or remove any moss that has gone brown or slimy. Sealed terrariums often need no watering at all for months. Open ones need a mist every week or so, using rainwater rather than tap water, or tap water left to stand overnight so the chlorine disperses.

Troubleshooting and simple variations

Mould appears as white, wispy threads and is nearly always a sign of too much water and too little air. Remove the affected moss, open the lid, and let things dry out for a day. Yellowing moss usually means too much sunlight or a dry spell. Algae on the glass is a sign of standing water in the base — pour off any excess.

Once you have mastered one jar, the variations are endless. A taller jar with a fern and a piece of bark becomes a woodland floor. A wide bowl planted with three different mosses looks like a miniature moorland. Whatever you choose, keep the layers honest, keep the light gentle and let the jar do the rest of the work for you.