Do I need crocks or gravel in the bottom of a pot?

Why a layer of gravel or broken pot in the base makes drainage worse rather than better, and what actually controls how a container drains.

Difficulty: beginner
Time: 10 min
Read: 3 min

Short answer

No. Water does not move from fine compost into coarse gravel until the compost above is saturated, so a gravel layer creates a wet zone higher up the pot than you would otherwise have. Fill with compost to the base and control drainage with the compost mix, an open hole and pot feet.

This is one of the most persistent pieces of received garden advice, and it is the opposite of what happens physically. The effect is called a perched water table, and it is well demonstrated: the coarser the lower layer, the higher the saturated zone sits.

Why crocks in the bottom of a pot make drainage worse

Step by step

Fill the pot with compost to the base.No crocks, no gravel, no polystyrene as a drainage layer. Compost all the way to the drainage hole gives the deepest possible drained zone.
Choose a compost that drains.This is the actual control. Adding grit, perlite or bark to a multipurpose compost improves drainage far more than anything you put underneath it.
Keep the drainage hole open.One piece of broken crock laid concave-side down over the hole, or a square of mesh, stops compost washing out without forming a layer. That is a different thing from a gravel bed.
Raise the pot off the ground.Pot feet, tiles or a plinth. A pot standing flat on a paved surface effectively seals its own drainage hole, and in winter that is what kills plants and cracks terracotta.
Empty the saucer in winter.A saucer of standing water in October to March is a root rot machine. Remove saucers for the winter or tip them out after rain.
Use bulk fill only to save weight, and knowingly.Filling the bottom third of an enormous planter with something inert reduces compost cost and weight, and it does raise the wet zone. That is an acceptable trade in a very deep planter for shallow-rooted plants; it is not a drainage improvement.
Check the hole is not blocked.Roots, compacted compost and slugs all block drainage holes. A pot that stays wet for days after watering usually has a blocked base, not the wrong compost.

Tips

One crock over the hole to keep compost in is fine. A layer of gravel is not.
Pot feet do more for drainage than anything you can put inside the pot.
A pot standing in a full saucer through winter kills more plants than frost does.

Common mistakes

Putting 50mm of gravel in the base 'for drainage' — It raises the saturated zone in the pot rather than lowering it, and it takes up root space. The physics runs the other way from the intuition.
Standing pots directly on paving — Surface tension seals the drainage hole against the slab, so the pot holds water. Feet, tiles or a plinth cost almost nothing and solve it.

If it doesn't work

Pot stays soggy for days

Cause: Blocked hole, no feet, or compost too fine — Fix: Check and clear the hole, raise the pot, and repot into a grittier mix.

Plant rotting despite gravel in the base

Cause: The gravel is part of the problem, not the solution — Fix: Repot with compost throughout, into a pot standing on feet.

Compost washing out of the base

Cause: Hole uncovered — Fix: Lay one crock concave-side down over the hole, or a piece of mesh.

Very large planter costing a fortune to fill

Cause: Genuinely large volume — Fix: Bulk-fill the bottom third with inert material if the plants are shallow-rooted, accepting the slightly wetter zone.

Questions people ask

Why does everyone say to use crocks then?

It is long-established advice from an era of heavy loam composts and unglazed pots, and it survives because it sounds sensible. Modern testing consistently shows the layered pot holds more water, not less.

What about polystyrene in a big planter?

As a weight and cost saver in a very deep planter, it is reasonable. As drainage, it does the same thing gravel does — raises the wet zone. Know which you are doing it for.

So how do I improve drainage in a pot?

Add grit, perlite or composted bark to the compost, use a pot with an adequate hole, and stand it on feet so water can actually leave.