A layer of gravel or pot shards at the bottom of a container improves drainage.
The old "never put gravel in the bottom of a pot" rule is overstated. A 2025 controlled test found a bottom layer usually drained soil-less mixes better and almost never made things worse, though in loam-based compost it mostly made no difference. It's one study and it grew no plants, so treat a layer as an option. It isn't a fix. If you use one, a thicker layer of coarse sand did best.
What is actually known
This one is contested. The popular "it's a myth" answer rests on the perched water table. Water held in the fine mix by capillarity was predicted not to cross into a coarse layer, so a gravel bottom would raise the saturated zone and hold more water. A 2025 controlled experiment, using the potting mixes and drainage materials home gardeners actually use, found the opposite. A drainage layer almost always reduced or did not change the water a container held, and almost never increased it. In soil-less coir mixes every layer drained the mix; in loam-based compost most made no difference. Thicker layers did more, and 60 mm of coarse sand worked best across every mix. The study grew no plants, so whether a given plant benefits is still open.
The evidence
Evidence: checked against 1 source in the research-based literature.
The source
Drainage layers were almost universally found to either decrease or have no effect on the water-holding capacity of containers
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Milpa Gardens. “A layer of gravel or pot shards at the bottom of a container improves drainage.” Evidence grade: Contested. https://milpa.garden/beliefs/gravel-pots-drainage-layer/
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- Well established
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- Plausible by its mechanism, but field studies are thin, so treat it as a lean.
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