Potting Tip

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Why putting broken pottery in your plant pot won’t help with drainage

Putting a layer of “crocks”, or broken pottery, in the base of flower pots won’t improve drainage – and may actually worsen it, says James Wong

Take a look at essentially any segment on container gardening in the media, and you will encounter the claim that placing a layer of broken pottery, or “crocks”, in the base of containers is a key step to ensure adequate drainage. This is despite the fact that soil scientists have shown for nearly a century that water passes more slowly through layers of material with different particle sizes than through a homogenous mix.

Air pockets in the coarser material below impede the gravitational pull of the water through the finer material, meaning water tends to only percolate down across the interface when the above layer is fully saturated. This means crocking is likely to have the exact opposite of its desired effect, potentially impeding drainage rather than improving it.

But is it possible that water behaves differently in domestic flowerpots than on the vast scale of agricultural fields? Perhaps, but to date there seems to be only one study tackling this. Carried out by consumer magazine Which?, it compared the performance of Calibrachoa plants (which are highly susceptible to root rot in soils with poor drainage) grown in a range of pots, including those lined with a layer of crocks and those without. They found adding the crocks made no measurable difference to plant growth.

Bottom line: if you are breaking perfectly good pots just to make crocks (which loads of people do), and find the whole business a bit of a faff, remember that all the scientific evidence we have to date says the practice is at best unnecessary, and at worst may even give you poorer results.

9 COMMENTS

  1. So therefore, if we are to ‘Remember that all the scientific evidence we have to date says the practice is at best unnecessary, and at worst may even give you poorer results’ in respect of plants, why doesn’t the same principle apply to a certain vaccine?

    Or is it ‘different’ when large amounts of money (And POLITICIANS) are involved?

    (Asking for a friend)

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  2. I do this when there is drainage holes in the pots. And all it does is slow down the soil loss through the drainage holes. Anyone doing it for any other reason is just wasting their time. I suspect people just made dumb assumptions about why other people were doing it and repeated it as gospel. A certain well known NZ gardener had something published in a NZ gardening magazine some years ago as a fact that I knew was absolutely rubbish. Assumptions are the mother of all mistakes.

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  3. I put a thin layer of pumice at the bottom should the pots drainage holes be too large. And I sprinkle treated pumice impregnated with Mycorrhizal bacteria throughout the mix. Seems to work fine for me. I grow papaya, and as anyone knows who grows these trees, one mistake with watering and drainage and you have a dead tree.

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