As gardeners, we probably all hoard broken pots for later use as drainage when potting new plants, but is it fact or fable as regards doing the job? Elizabeth McCorquodale investigates:
When I set out to undertake this trial, I wondered if many people still put crocks in their pots, so I conducted a quick straw poll among friends and family to test the waters, so to speak. Of the twenty people I asked, eleven said they always used crocks, four said they felt they should but usually didn’t, and five said no, never… so I set out to see who was right.
Putting crocks or gravel in the bottom of pots is supposed to improve drainage in the pot because excess water flows out of the denser soil material into the large cavities between the crocks. They are also supposed to plug the drainage hole and stop soil from being washed out of the pot or, conversely (and depending on who you talk to), they cover the drainage hole, thereby stopping it becoming clogged with soil.
To test out the theory I took fifteen 1-litre plastic pots and filled five of them with a 2.5cm layer of terracotta shards, another five with a 2.5cm layer of coarse gravel, and then I filled them all with a well-known, soil-based potting medium. The final five pots had no drainage material and were completely filled with the potting medium. Then I set them all on a wire screen and placed a saucer under the screen beneath each pot so that I could accurately measure the run-off from each one. I then watered each pot with 0.75 litres of water, waited half an hour and then measured the amount of water that had drained out of each pot.
I also conducted an experiment that I used to carry out with gardening club students when discussing what size and shape of pot to choose; I took a large rectangular sponge, saturated it and laid it down horizontally on a saucer filled with coarse gravel, then I waited until no more water dripped out. When it stopped dripping, I turned it so that the sponge was standing upright on its short edge and observed the result.
First off, there was no discernible difference between any of the pots with regard to the amount of soil that was washed out of the drainage holes. As regards drainage, the amount of water that drained away was, on average, the same for all the pots. At first glance this doesn’t seem to make sense: there is, after all, less soil in the crocked pots to hold on to the water because some of the room is taken up with crocks, so it seems sensible that more water should drain from them.
The trouble is, it all comes down not to common sense, but rather to physics. Gravity and capillary action govern the way water moves around the pot, and it can be a little odd. To see this clearly we can go back to the experiment with the sponge – positioning it horizontally, I allowed the sponge to drain until no more water dripped out, then I turned it on end, and, lo and behold, as soon as I did, more water dripped from it. When the sponge was lying flat it was able to hold more water than when upright. It obviously had the same volume, but the weight of the water combined with the pull of gravity dictated that more water was drawn out of the sponge when vertical.
Coupled with this is the physical problem with water movement between areas of finely textured materials (the soil or the sponge) and coarsely textured materials (the crocks or gravel below the sponge). Water will not flow from the finely textured soil into the space below until it has exceeded saturation point in the soil, and as there is no capillary connection between the soil and the coarser material, it cannot be drawn down, either.
In addition, the shape of the space the soil occupies in the crocked pots is shallower than it would be without the crocks, so it will drain differently; just like the horizontal sponge, the soil in the crocked pots is able to hold on to more water than a similar amount of soil in a more upright shape.
The truth is that crocks (or any other drainage material, for that matter) in the bottom of pots do not aid drainage, and could even result in the remaining soil in the pot being wetter for longer than if no crocks were used.
A pot partly filled with crocks is simply a pot with less room for valuable potting medium.
It is matching the species of plant to the correct potting medium, the size of plant to the correct size of pot, the amount and frequency of watering and whether or not the pot is left sitting in a saucer of water for long periods, that determines if a pot becomes waterlogged or not, rather than the presence or absence of crocks!