For generations, "gravel in the bottom of the pot" was advice nobody had tested. In 2025 someone finally did.
For as long as I've been potting up plants, one piece of gardening wisdom has been as unshakeable as the smell of potting mix on my hands:
"Always put a layer of gravel at the bottom of the pot. Helps drainage. Keeps the roots healthy. Stops rot."
You hear it from parents, neighbours, TV gardeners and well-meaning plant shop staff. It feels right. It sounds right. It must be right.
Soil scientists have been side-eyeing it for decades. Their argument: water doesn't fall from a fine material into a coarse one until the fine layer is fully saturated, so a coarse layer at the base can create a perched water table — a saturated zone sitting on top of the gravel, higher in the pot than it would otherwise be. On that theory, your drainage layer makes the roots wetter, not drier.
The trouble is that this was theory and modelling, not measurement. Nobody had put mix in pots, added layers, and weighed them. Gardeners, reasonably, kept doing what they'd always done.
Then in February 2025, Avery Rowe published the first controlled experiment on the question in the open-access journal PLOS ONE. So — should you still be tipping gravel into your pots?
About the study
Effect of drainage layers on water retention of potting media in containers. Avery Rowe, PLOS ONE, 25 February 2025.
The question was simple: does adding a drainage layer actually reduce how much water a container holds? Rowe tested:
- Two layer depths — 30mm and 60mm
- Four materials — coarse sand, fine gravel, coarse gravel, and scoria (a lightweight volcanic rock)
- Three potting media — a loamless organic mix (coir, peat, bark — the kind most indoor mixes are), a loam-based mix, and a sandier horticultural loam
Each setup was saturated, allowed to drain freely, then weighed to measure what remained. Two calculation methods were used as a cross-check: water retained in the whole container, and water retained in the potting mix alone, excluding the layer's own volume.
What it found
Loamless organic mix — the clearest result
This is the one that matters for indoor plants, because it's what most bagged indoor mixes are. Every drainage layer reduced water retention compared to no layer at all, both calculation methods agreed, and the deeper 60mm layer worked better than 30mm.
Loam-based mix — murkier
Measured across the whole container, most layers made no significant difference. Measured in the mix alone, finer layers like coarse sand sometimes reduced retention. The two methods didn't consistently agree, so the effect here is possible but not established.
Sandy loam — barely anything
Some layers made a small difference, some none. In a mix that already drains freely, a coarse base has little left to improve.
Across everything
Despite decades of warnings, the study rarely found a layer increasing water retention. Where it changed anything, it went down. And the most consistently effective option wasn't gravel at all — it was a 60mm layer of coarse sand, which outperformed fine gravel, coarse gravel and scoria across all three media.
So both camps were partly right. The perched-water-table theory doesn't show up as wetter pots in practice. But the gravel everyone's been using isn't the material that helps, either.
What we'd actually do with this
The study tells you a drainage layer isn't the villain the theory made it out to be. It doesn't tell you it's a big lever, and it isn't. The pot's drainage holes and the structure of the mix decide almost everything about how water behaves in a container; a layer at the base adjusts it at the margin.
So, in order: drainage holes first — nothing in the bottom of a pot helps if water can't leave it. Drainage holes for indoor pots covers the options. Mix structure second — a chunky, open mix holds air and water at the same time, which is what a layer is trying to approximate from below. If your mix stays wet regardless, why is my potting mix staying wet? works through the causes. How you water third — watering to runoff and letting the pot drain does more for the root zone than anything you put under it.
Then, if you want the layer: skip the gravel, use coarse sand, and go 60mm deep if the pot allows. It's most likely to help in a loamless organic mix — the kind of indoor potting mix most of us use — and least likely to matter in something already gritty, like a cactus and succulent mix. Test it on one or two pots before you overhaul the collection.
Other things that change the answer
Pot depth. Shallower pots have less vertical space, so any shift in where the water sits has a bigger effect on the roots.
The plant. Succulents, many orchids and a lot of natives want drier roots and are the plants most likely to benefit. Ferns, calathea and most tropicals would rather have the moisture.
Climate. In dry heat, holding water can be worth more than shedding it. In a muggy coastal summer, or a cool room where pots dry slowly, drainage matters more.
Don't stack layers. The study tested single layers of single materials. Sand plus gravel plus charcoal plus pot shards is a different experiment, and there's no reason to assume it behaves the same.
Why one study matters
It's the first real data on a claim that's been repeated for generations. It isn't the last word — the author notes the need for field trials outdoors, more plant types, and long-term work on whether layers affect root growth. But it gives gardeners something they've never had: evidence, rather than a potting-shed argument.
The short version
In a loamless indoor mix, a 60mm coarse-sand layer reduces water retention. Gravel does little. Neither replaces drainage holes and a well-structured mix. The "crushed terracotta in the bottom" debate has finally moved beyond the pub — and the answer is that it was never the thing that mattered most.
Leave a comment