Sounds logical. So why is it, I always wondered, that whenever I get plants from the garden centre there is never, but never a layer of drainage material. There may be perlite of sand or something mixed into the potting compost, but at the base nothing. And yet the plants are healthy, happy and show no sign of being waterlogged at all.
So I was intrigued to find this article in the Horticultural Myths section of Linda Chalker-Scott's website. Chalker-Scott, who is an Associate Professor of Horticulture at Washington State University, argues that far from aiding drainage, the broken potsherds, gravel etc usually used will actually prevent it, and leave the soil more rather than less waterlogged.
According to Chalker-Scott this has been proved by a number of studies. Annoyingly she doesn't provide references, so there's no way of checking. But I thought it would be fun to try an experiment and see what happened.
So today we have the great Balcony Garden Drainage Material Experiment. No claims to being scientific, but here's how it goes. Two identical 10cm pots, one with old crocks and large clay granules in the base, the other without. Both are then filled with the same potting compost, and the same amount of water is added - enough to more than waterlog the soil.
The liquid that drains off immediately and after half an hour is measured and, lo and behold is exactly the same.
However, as the whole point is to see how this affects the plants growing in the pots, there's a second stage. Two marigold plants, as nearly as possible identical, are added - one in each pot. I've called them Bill and Ben. Bill is in the pot with crocks and Ben in the pot with no crocks. Over the next few weeks they'll be kept in the same position, receive an equal (though generous) amount of water, and be treated in every way identically. And we'll see what happens.
Who'll grow strongest? Will it be Bill or will it be Ben?