Guide
Terrarium layers, bottom to top
By Paul Reaney · Updated 5 August 2026
A terrarium has no drainage holes, so the layers do the job a plant pot's hole normally would. Here is the short version: clay pebbles at the bottom to catch excess water, a thin layer of activated charcoal to keep that water fresh, substrate for the roots, and moss on top. Get these right and the build mostly looks after itself.
| # | Layer | How deep | What it does | Can you skip it? |
|---|---|---|---|---|
| 5 | Top dressing | A scatter | Moss, pebbles or leaf litter. Finishes the scene, holds surface moisture and gives springtails somewhere to live. | Optional — but moss is what makes it look like a forest floor. |
| 4 | Substrate | 4–8 cm (1.5–3 in) | The growing medium roots live in. Coco-fibre mixes for tropical builds; gritty, free-draining mixes for succulents. | Never — this is the layer plants actually grow in. |
| 3 | Activated charcoal | 0.5–1 cm (a thin scatter) | Filters the water that collects below and keeps a sealed system smelling fresh instead of swampy. | Skippable in open builds; strongly recommended in closed ones. |
| 2 | Mesh barrier | One sheet | A piece of fibreglass window screen that stops the substrate washing down into the drainage layer and clogging it. | Optional, but it keeps the layers crisp for years. |
| 1 | Drainage layer | 2–4 cm (1–1.5 in) | LECA (clay pebbles) forming a false bottom. Terrariums have no drainage holes, so excess water collects here, safely below the roots. | Never in a closed build; only very shallow open dishes get away without it. |
Numbered from the bottom of the vessel — build in order 1 → 5.
1. The drainage layer (false bottom)
Because a terrarium can't drain, every drop you add stays in the vessel. A 2–4 cm bed of lightweight expanded clay pebbles (LECA) creates a reservoir at the bottom where excess water collects — safely below the roots instead of around them. Roots that sit in water rot, and root rot is the number-one killer of new terrariums. Gravel works too, but LECA is far lighter and holds a little humidity-boosting moisture in its pores.
2. The mesh barrier
A single sheet of fibreglass window screen (or fine plastic mesh) laid over the pebbles stops the substrate gradually washing down into the drainage layer and clogging it. It's the least glamorous layer and entirely optional — but it's also the difference between crisp, visible strata and a muddy blur a year from now.
3. Activated charcoal
A thin scatter of horticultural activated charcoal sits on or just above the mesh. In a closed terrarium the same water cycles endlessly between substrate, air and glass; charcoal filters it each time it passes through, binding the compounds that would otherwise turn a sealed jar stagnant and sour-smelling. Open builds can skip it — airflow and fresh watering do the same job.
4. Substrate — match it to the build
This is the layer plants actually live in, and it must match your terrarium type. For a closed, tropical build, use a coco-fibre-based mix — it holds humidity without compacting, and it's inert, so it brings no pests in with it. For an open succulent bowl, do the opposite: a gritty cactus mix (roughly half mineral — sand, perlite, fine gravel) that dries out fast. And never garden soil in either — it compacts, drains badly and imports pests into a jar they'll never leave. Aim for 4–8 cm, sloped deeper at the back for a sense of depth.
5. Top dressing
The finishing layer: sheet or cushion moss for a forest floor, pebbles or grit for a desert scene, a few leaves of oak litter if you're going bioactive (springtails love it). Beyond looks, moss buffers surface moisture and hides the soil line where substrate meets glass.
How much does the whole stack take up?
All the layers together should fill roughly the bottom quarter to third of the vessel — enough to work properly, little enough to leave the glass to the plants. If your container is very shallow, shrink the drainage layer before you shrink the substrate; roots need the depth more than the reservoir does.
Ready to build? The kit page lists a researched, well-reviewed option for every layer above — or a complete boxed kit if you'd rather start with everything at once.
Not sure which type you're building? Start with closed vs open.
FAQ
What are the layers of a terrarium, in order?+
From the bottom up: a drainage layer of clay pebbles (LECA), an optional mesh barrier, a thin layer of activated charcoal, the substrate the plants root into, and a top dressing of moss or pebbles. The drainage layer catches excess water, the charcoal keeps it fresh, and the substrate does the growing.
Do you really need activated charcoal in a terrarium?+
In a closed terrarium, yes — the same water recycles endlessly, and a thin charcoal layer filters it and prevents the stagnant, swampy smell sealed jars can develop. In an open terrarium it is optional, because fresh air and regular watering do that job.
Can you use garden soil in a terrarium?+
No. Garden soil compacts into an airless sludge in glass, drains badly, and carries pests, weed seeds and fungus into a sealed environment they will never leave. Use a coco-fibre-based terrarium mix for tropical builds or a gritty cactus mix for open succulent bowls.
How deep should terrarium substrate be?+
Aim for 4–8 cm (1.5–3 in) — enough for roots to anchor, sloped deeper towards the back if you want visual depth. Add a 2–4 cm drainage layer beneath it, and remember the finished layers look best filling roughly the bottom quarter to third of the vessel.
What substrate do carnivorous plants need?+
Carnivorous plants (Venus flytraps, pitcher plants, sundews) need a nutrient-poor mix — classically peat or sphagnum moss with perlite or sand — and never standard potting compost or fertilised mixes, which will kill them. Water only with rain or distilled water.