How to Build a Moss Terrarium, Step by Step

A closed terrarium is a small world that waters itself, and building one is easier than it looks, so this is the way I build them, step by step, with the reasons behind each layer so you can adapt it to whatever jar you have.

What you’ll need

A glass vessel with a lid, drainage rocks or scoria, horticultural charcoal, a handful of sphagnum moss, a terrarium substrate (my own mix is in our care guide), live moss, a small tropical plant such as a fern or fittonia, some decorative sand if you like the look, and a spray bottle of rainwater or demineralised water.

1. The drainage layer

Start with the drainage, pouring the rocks into the base of the glass, since this layer holds any excess water away from the roots so nothing sits in a puddle and rots.

2. The charcoal filter

Sprinkle the charcoal over the rocks without worrying about being neat, because nature isn’t, and it does the quiet work of filtering the water and absorbing the smells that build up in a sealed jar over time.

3. The sphagnum barrier

Soak the sphagnum, squeeze it out and tease it apart until it’s loose again, then press it in an even layer over the charcoal right to the edges of the glass, so the soil above stays put and doesn’t wash down into your clean drainage.

4. The substrate

Add the substrate and level it lightly, shaping hills or slopes if you want a landscape, then tamp it down with a spoon or your knuckles to push out the air pockets that would let plants shift later.

5. Preparing the roots

Take the plant out of its pot and loosen the root ball, teasing away as much of the old soil as you can, and if it won’t come off, dip the roots in a bowl of water and shake gently until it does, trimming any very long roots with clean scissors so they sit comfortably in the substrate.

6. Planting

Make a hole where you want the plant with a chopstick, press it in until the base sits flush with the soil line, and move it around until it feels right, since there’s no wrong layout, and if it’s too tall for the jar you can prune the longer leaves.

7. Mossing

Then the moss, which you can tear or cut to suit your design, since it handles being cut or torn without complaint, pressing each piece firmly onto the damp substrate because it needs full contact to take, and leaving gaps wherever you want sand to show through. Fern and feather moss, the moss I forage, both suit a closed jar, and if you’d like some when I’m next out in the forest, join the forage waitlist.

8. Sand

Pour the sand around the moss for a clean finish.

9. Sealing it

Mist everything lightly, put the lid on, and the water cycle begins, the moisture rising, beading on the glass and falling back on the plants like rain.

Over the first week, watch the glass. If it’s so foggy you can’t see in, or water pools at the bottom, take the lid off for a few hours to let it breathe, and if no condensation appears at all over several days, give it a few more sprays.

Keeping it alive

Never use tap water, which is too harsh for moss, so use rainwater, demineralised water, or tap water boiled for ten minutes and cooled. Keep the jar out of direct sun, which heats the glass and scorches the moss, and find it a cool spot with bright indirect light. There’s no need for fertiliser, and above all don’t overwater, since a terrarium that’s too dry is far easier to save than one that’s drowning.

If you’d like a clean up crew, a few springtails will keep mould down and the soil turning over, and our springtail guide explains how they work. If you get stuck along the way, write to me, I’m always happy to help.

Emrah