Why drainage holes matter

Roots need air as well as water. Between the particles of a potting mix are tiny spaces. After watering, gravity pulls the extra water out through the bottom of the pot and air moves back into those spaces. Without a hole, the water has nowhere to go. The spaces stay flooded and, as the RHS puts it, the roots can drown.

So the first rule of pots is simple: the pot the plant actually grows in needs drainage holes. Many beautiful ceramic pots have none. They are fine as decorative outer pots, sometimes called cachepots: keep the plant in a plastic pot with holes inside, and lift it out to water, as you practised in the last unit.

You may read that a layer of gravel at the bottom of a hole-less pot fixes drainage. It does not replace a hole: water still collects at the bottom with nowhere to go, and you cannot see it. Some sources suggest covering the hole with a shard of pot, a coffee filter or mesh so the mix does not wash out while water still drains; that is fine.