Terrarium Vessel Size Reference

Every “drainage material” column above assumes the classic build: a layer of LECA, gravel, or lava rock at the very bottom, under a thin charcoal filter, under the substrate. This is the most widely taught terrarium layering method, and it isn’t wrong — but the mechanism behind it is more debated than most care guides let on, and it’s worth understanding both sides before you build.

The argument for it is straightforward: a terrarium has no drainage hole, so overwatering has nowhere to go. A layer of coarse material below the substrate gives excess water somewhere to sit that isn’t directly against the roots, buying you a margin of error.

The argument against treating it as settled science comes from soil physics: when a fine material (substrate) sits directly on top of a coarse one (gravel or LECA), water doesn’t move freely between them the way gravity alone would suggest. It gets held at the boundary by capillary action — something soil scientists call a “perched water table” — until the fine layer directly above the boundary is genuinely saturated. In a normal potted plant with a drainage hole, that perched layer eventually overflows the pot and the excess actually leaves the system. In a sealed terrarium with no hole, there’s nowhere for it to overflow to — the water just sits at that boundary indefinitely, which means the bottom few centimetres of substrate directly above the gravel can stay wetter for longer than the ratio-based depth in the table above would suggest on its own.

Both of these things are true at the same time, which is why this isn’t a settled “drainage layers work” or “drainage layers are a myth” argument. The layer does do real work: it keeps standing water away from direct contact with root systems and gives a terrarium with no other outlet somewhere for a watering mistake to go. What it doesn’t do is drain the way the word usually implies — it’s a reservoir, not plumbing. Practically, that means the depths in this table matter less than watching the actual signs: condensation patterns, substrate that stays dark and heavy at the surface, and any sour smell when you open the lid, all covered in more depth on how much to water a closed terrarium.

A mesh or landscape-fabric barrier between the charcoal and substrate layers doesn’t change this mechanism — it stops fine soil particles from washing down and clogging the drainage material over time, which is a separate, unrelated problem from the perched-water-table question.