Are gardeners wrong to put 'crocks' in plant pots?

Man puts terracotta pieces into pot Image copyright Alamy

For centuries, gardeners have put shards of pottery - "crocks" - at the bottom of plant pots to increase drainage. But a new study has debunked the tradition, writes Tom de Castella.

Crocks in pots are an article of faith for gardeners. A piece of crockery in the bottom of the pot aids drainage. It allows water to run off more quickly than soil would and prevents compost from clogging up the holes at the bottom of the pot. That at least is the age-old wisdom handed down from one generation of green-fingered sages to the next. Gardeners' Question Time, Monty Don and Alan Titchmarsh have all endorsed the tradition.

But a study by consumer magazine Which? suggests it's a myth. Researchers planted 40 pots each with five "Million bells trailing yellow" - a flowering plant prone to root rot in saturated soils. Permutations involved plastic pot, and terracotta pot, and with either saucers or no saucers. Half got crocks, half did not. The plants were recorded for "vigour and flowering impact". The magazine found that the crocks "made no difference to how well our plants did".

You might think that the bigger gaps where soil meets bits of broken crockery would allow more water to filter through. But this turns out not to be the case, some argue. Guy Barter, chief horticultural adviser at the RHS, says a crock is actually likely to worsen drainage by creating a block. It's better to have a layer of sand underneath soil that will allow water to drain into it and later be sucked up by roots if needed. The only minor points in favour of crocks are that they might block drainage in hot weather, and also save money on compost. In most cases, people would be better off omitting them, he says.

Like many gardening "tips" it probably owes more to Victorian fashion than practicality, says Matthew Biggs, a panel member on Gardeners' Question Time. There was something comforting about re-used shards of terracotta performing magic inside the pot. "It's always sad when a tradition gets blown away by modern science." But many will probably ignore the science, and carry on with this "intuitive ritual" that is part of gardening heritage, he says.

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