How to Layer a Terrarium: Drainage, Substrate, and Moss

A planted terrarium has no drainage hole, so the layers you build inside it have to do that job instead. Get the order and proportions right and the container quietly manages its own moisture for months. Get it wrong and you end up with either a swamp at the bottom or substrate that dries out faster than it should. The good news is that the stack is the same four layers every time, just scaled to the size of your jar — and the ratios scale predictably enough that you can work them out from your container's height alone, rather than guessing.

Layer one: drainage

At the very bottom goes a layer of something water can pool in without touching the substrate above it — expanded clay pebbles (LECA), aquarium gravel, or lava rock are the three most common choices. This is the layer that stands in for the hole a normal plant pot would have. Any excess water that the substrate can't hold works its way down here instead of sitting against the roots.

LECA is the most forgiving of the three for a beginner: it's lightweight, rinses clean easily, and its round pellets leave large, consistent air gaps for water to sit in. Aquarium gravel is cheap and widely available but packs more densely, so you typically need a slightly deeper layer of it to hold the same volume of water. Lava rock is the lightest option by weight and looks striking left visible through clear glass, but its jagged surface can be sharp enough to nick fingers during planting, so it's worth handling carefully.

How deep this layer needs to be depends on two things: how tall the whole container is, and whether it's open or closed. A closed terrarium recycles more of its own moisture through condensation, so this site's own layer ratios give it about 15% of the container's height for drainage, against about 10% for an open build of the same height — a real, if modest, difference that adds up on a tall jar. Whatever the ratio suggests, most calculators (including the one on this site) also enforce a practical floor: roughly 2.5cm for a sealed container with no hole at all, and about 1cm if the vessel does have drainage holes, since a layer thinner than that stops functioning as a reservoir no matter what the math says.

Layer two: activated charcoal

A thin layer of horticultural activated charcoal sits directly on top of the drainage material, before any substrate goes in. It's a small layer — often less than a centimetre, and typically only a few percent of the container's total height — but it does real work in a sealed environment: it helps keep the water sitting in the drainage layer from turning stagnant and souring, which matters far more in a jar with a lid than in an open pot where air moves freely.

If you're building something very shallow, like a wide, low dish that gets opened and refreshed often, it's reasonable to skip this layer entirely. The charcoal is doing the most good in taller, sealed containers where water sits for longer between refreshes.

Layer three: the substrate

This is where roots actually live, and it's usually the thickest layer in the stack by a wide margin — commonly seventy to eighty percent of the container's total height once drainage, charcoal, and top dressing are accounted for. A terrarium-specific mix, or a well-draining potting soil blended with extra perlite or orchid bark, holds enough moisture for humidity-loving plants without staying waterlogged. Avoid a soil that's been pre-loaded with slow-release fertiliser pellets meant for outdoor beds or vegetable pots; terrarium plants are usually chosen for slow, controlled growth, and a heavy feed schedule can push them to outgrow a sealed glass footprint faster than you'd like.

Some builders lay a thin sheet of mesh or landscape fabric between the charcoal and the substrate first, so fine soil particles don't wash down and clog the drainage layer over time — a small step that saves a lot of hassle later. It's worth noting this mesh barrier solves a completely different problem from the drainage layer itself; more on that distinction below.

Depth here should leave enough room for a plant's root ball to sit comfortably below the surface, with a little extra for growth. Shallow-rooted mosses and small ferns need less than a fern with a larger root system, so it's worth thinking about your specific plant list before settling on a final substrate depth.

Layer four: the top dressing

The visible surface layer is mostly about finishing the look and helping retain moisture right where the plants meet the air — a moss carpet, fine decorative gravel, or sand, depending on the style you're going for. It's a thin layer, usually around a tenth of the container's height, and it isn't structural, so don't expect it to compensate for a substrate or drainage layer that's under-built.

A worked example: a 20cm-wide, 30cm-tall cylinder jar

Numbers make this easier to picture than percentages alone. A straight-sided cylinder jar 20cm across and 30cm tall holds a little over 9 litres total (about 9.4L, or roughly 2.5 US gallons), with somewhere around 5.6L of that realistically usable for layers and plants once you leave headspace for airflow and growth.

Built as a closed terrarium, that 30cm of height splits out to roughly 4.5cm of drainage material, under 1cm of charcoal, about 21.6cm of substrate, and 3cm of top dressing. Because the drainage layer sits under 21.6cm of substrate rather than a shallower amount, the material calculator puts the actual LECA or gravel needed at around 3.9cm deep across that jar's floor — a little over a litre of material, roughly 5 cups.

Built open at the same 30cm height, the split shifts slightly: about 3cm of drainage, under 1cm of charcoal, 23.1cm of substrate (a bit more room for roots, since open builds don't need as deep a reservoir), and 3cm of top dressing. The drainage material itself works out to about 4.2cm deep, a touch more than the closed version, because it's sitting under a slightly deeper substrate layer even though the ratio itself is smaller.

If your own container is a different size or shape, the substrate layer ratio calculator and drainage layer calculator on this site run the same arithmetic against your exact dimensions, and the vessel size reference lays out these numbers side by side for eight common jar, globe, bell jar, and tank sizes if you'd rather compare a few options before you measure your own.

Does the drainage layer actually drain? The honest answer

It's worth being upfront about something most terrarium guides gloss over: the classic gravel-under-substrate layer is widely taught, but whether it actually "drains" the way people assume is genuinely debated among growers and soil scientists, not settled fact.

Here's the mechanism. When a fine material like potting substrate sits directly on top of a coarse one like gravel or LECA, water doesn't move freely across that boundary just because gravity is pulling it down. Capillary action holds moisture in the fine substrate against gravity, right at the point where it meets the coarse layer, until the substrate immediately above that boundary is genuinely saturated — a phenomenon soil scientists call a perched water table. In an outdoor pot with a real drainage hole, that perched layer eventually overflows the pot and the excess genuinely leaves the system. In a sealed terrarium with no hole anywhere, it never does: the water just sits at that boundary, which means the bottom few centimetres of substrate directly above the gravel layer can stay noticeably wetter for longer than the ratio-based depth above would suggest on its own.

That doesn't make the layer pointless. It's still doing real work: it's a reservoir that keeps standing water below the root zone rather than directly against it, and it gives a sealed container with nowhere else for excess water to go somewhere to put a watering mistake. What it isn't is plumbing — it doesn't remove water from the system the way a hole in the bottom of a pot does. Practically, that means watching the actual signs of your specific terrarium (how the condensation on the glass behaves, whether the substrate surface looks and feels right) matters more than trusting the layer depths alone to keep things balanced.

Putting it together

Working from the bottom up in the order above — drainage, charcoal, substrate, top dressing — keeps water moving the direction it's supposed to: down and away from the roots, with just enough held in the substrate to keep plants comfortable between waterings. Press each layer down gently as you go so there aren't large air gaps, but don't compact the substrate so hard that roots struggle to spread through it.

Once the layers are in, plant from the tallest or most structural piece first and work outward, so you're not reaching over newly placed smaller plants and disturbing them. A quick water-in after planting helps the substrate settle around the roots and closes any pockets that were left by the trowel.

A few common layering mistakes

Skipping the charcoal layer in a tall, fully sealed jar is the most common shortcut that causes trouble later — it's easy to leave out when you're planting and hard to add back in once the substrate is down. Using a very fine aquarium gravel in place of coarser LECA or lava rock is another: it packs down tightly enough that it stops functioning as an open reservoir and starts behaving more like a second substrate layer. And piling the top dressing on thick enough to bury the base of a plant's stem, rather than just dressing the visible soil around it, can trap moisture against the stem and encourage rot at exactly the point where the plant is most vulnerable.

If you'd rather not eyeball the split, our substrate layer ratio calculator will work out the drainage, charcoal, substrate, and top-dressing depths for your exact container height and type, and the drainage layer calculator converts that substrate depth into an actual volume of LECA, gravel, or lava rock to buy.