Conditions & terms
Pot-bound
Also known as: root-bound
A plant whose roots have filled its pot and started circling the inside wall, so it dries out fast and struggles to keep growing.
When a plant outgrows its container, its roots run out of fresh compost to explore. They reach the wall, can't go any further, and turn to follow it round, until the pot holds more root than compost. That plant is pot-bound (gardeners also say root-bound). It isn't dead, or even always unhappy at first, but it has very little compost left to hold water, so it wilts quickly and stops putting on much new growth.
The signs are easy to spot. Roots poke out of the drainage holes, water runs straight through without soaking in, and the plant needs watering far more often than it used to. Tip it gently out of its pot and a pot-bound plant comes out as a solid block, with roots wound round the outside and little compost showing.
The fix is to pot on into a container a size or two bigger. Tease the outer roots loose with your fingers first, so they grow out into the new compost instead of carrying on in circles, then water it in well.
Prevention is mostly about pot size. Hungry, thirsty crops need plenty of compost: a tomato wants a pot holding at least 20 litres per plant, and much less than that leaves it short of root room by midsummer. The tomato planter shows this happening: pick the 10-litre pot and watch the roots reach the wall and start to circle. For other crops, see our guide to choosing pots and container sizes.
In a UK garden
It shows up most in a UK July, when a pot-bound tomato or courgette in a small pot can need watering several times on a hot day.
Example
A tomato tipped out of its pot in May comes out as a solid block, with a mat of white roots wound round the outside of the compost. It should have been potted on a week or two earlier.