How to make a terrarium: closed vs open jars and the layers that go inside
TUREFF• •0
A terrarium is a small garden under glass, and the good news is that building one takes an afternoon and very little money. The part people get wrong is not the planting but the choice made before it: whether the container will be closed or open. That decision sets which plants will survive, how much you water and what the layers underneath have to do.
Closed or open: decide first
- Closed terrariums have a lid, cork or stopper. Water evaporates from the soil and leaves, condenses on the glass and runs back down, so the jar recycles most of its own moisture. The air inside stays humid, which suits ferns, mosses and small tropical plants. A well-balanced closed jar may need water only occasionally.
- Open terrariums have no lid, or a wide mouth. Moisture escapes, so the air inside is close to room air. These suit plants that want to dry out between waterings, such as succulents and cacti, and they are easier to reach into for pruning.
The two are not interchangeable. Desert plants sealed in a humid jar tend to rot, and humidity lovers in an open bowl tend to crisp at the edges.
Choosing a container
Almost any clear glass works: sweet jars, carboys, fish bowls, kitchen storage jars with a seal. Clear glass is better than tinted, which cuts light. A wide opening makes planting far easier for a first build; narrow bottles look striking but need long tools. Wash the container well and dry it, as leftover residue can feed mould later.
The layers, from the bottom up
- Drainage layer: a few centimetres of gravel, small pebbles or expanded clay balls. Terrariums have no drainage hole, so this gives excess water somewhere to sit below the roots rather than around them.
- Barrier layer: a thin sheet of mesh, a piece of fine fabric or a layer of moss laid over the gravel. Its job is to stop soil washing down into the drainage layer and clogging it.
- Charcoal (optional): many builders add a thin scattering of horticultural charcoal, said to help keep things fresh. Opinions differ on how much it does; it is not essential, but it does no harm.
- Substrate: the growing layer, usually the deepest. For a closed jar, use a mix that holds moisture but still lets air in, such as a peat-free potting compost lightened with bark, coir or perlite. For an open succulent bowl, use a gritty, fast-draining mix sold for cacti, or ordinary compost cut with plenty of grit.
- Plants: make planting holes with a spoon or chopstick, tease the old soil from the roots and firm them in. Leave space between plants; they grow.
- Moss and top dressing: in a closed jar, cushions or sheets of moss cover bare soil and keep the surface evenly damp. In an open bowl, fine gravel or sand keeps succulent stems out of wet soil.
Keep the substrate and plants in proportion to the jar. As a rough guide, the layers should take up no more than about a third of the height, leaving room for leaves and air.
Building it step by step
- Lay out your plants beside the jar and plan the arrangement, taller plants towards the back or centre.
- Add the drainage layer, then the barrier, then the substrate, smoothing each one.
- Plant from the largest to the smallest.
- Press moss or top dressing into place.
- Clean the inside of the glass with a soft brush or a paper towel on a stick.
- Water lightly. A spray bottle is easier to control than a jug; a closed jar needs only enough to darken the soil, not to pool in the gravel.
The first few weeks
Put a new closed jar in bright light out of direct sun and watch it. Heavy condensation that never clears means too much water, so leave the lid off for a day. No condensation at all means it is too dry. Remove any leaf that yellows or rots before it can feed mould.
Share a photo of your first build, or ask about a container you are unsure of, in the comments below.