A layer of gravel or pot shards at the bottom of a container improves drainage.
Contested
The old "never put gravel in the bottom of a pot" rule turns out to be overstated — a recent controlled test found a coarse bottom layer usually helps drainage a little or does nothing, not that it hurts. It's also no cure: a good potting mix and a real drainage hole matter far more. Layer or don't; just don't count on it to fix a soggy mix.
What is actually known
This one is genuinely unsettled, and the popular "it's a myth, gravel makes drainage worse" answer overstates the case. The theory behind that answer is the perched water table: water is held in the fine mix by capillarity and does not cross freely into a coarse layer, so a gravel bottom was predicted to push the saturated zone up into the root ball. But a 2025 controlled experiment on real containers found the opposite of the prediction — a coarse bottom layer almost always reduced or did not change water retention, and essentially never increased it, with a coarse-sand layer draining best. So a drainage layer does not reliably harm drainage and may modestly help. It is also not the fix it is sometimes sold as: a well-drained potting mix and an open drainage hole matter far more than anything layered at the bottom.
Evidence: not yet confirmed against its 1 source.
what this rests on - one pointer
PLoS ONE (2025), 'Effect of drainage layers on water retention of potting media in containers' (read 2026-07-14 via PMC PMC11856292): 'Drainage layers were almost universally found to either decrease or have no effect on the water-holding capacity of containers'; 'any drainage layer was likely to reduce water retention of any medium, and almost never increased it' — a controlled result that contradicts the perched-water-table prediction that a coarse bottom layer worsens drainage. Grade demoted F -> D 2026-07-14 on this evidence.
When would that grow at your place? →
Cite this page
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/
A garden-planning corpus: every claim states its evidence grade and links its sources on the page. Cite it with the grade, and add the date you read it.