Growing up, there were certain myths my mother loved to perpetuate. Because she was my mother I believed them to be true. Things like, if you swallow chewing gum it will take seven years to leave your system, or, if you don't put on a jacket when you go outside you'll catch a cold. Sorry, mom, not true.
The other day I was driving to a small gathering with a few friends. Crammed into the back seat beside me was a beautiful and rather large glazed garden pot. I admired it from close range and my friend Janna waxed on about the perfect plant she had that would eventually grace the pot outside her new house.
This pot was large, about 36-inch T, and as usual, there was one tiny hole in the base of it for drainage. This tends to happen a lot with pots made for the garden. When you consider that container-grown plants and trees need excellent drainage to thrive, it makes you wonder: Why the one tiny hole?
I recommended she find a small drill or dremel and make a few more holes before she planted it up. She then asked if she should put a layer of gravel along the bottom for drainage. It's a question many people ask and is probably the No. 1 garden myth.
Unfortunately, this is one of those garden myths that just keeps getting perpetuated by well-meaning individuals. Gardeners in the know realize there is concrete scientific research to dispute this.
Like every good myth, you will find the instructions to place coarse material at the bottom of containers for drainage in tons of books and websites. The usual recommended culprits are sand, pebbles, gravel and even shards of broken pots!
The reality of this myth is that soil scientists have shown that water does not move easily from layers of fine-textured materials to layers of coarse-textured materials. There are many studies that have proven this to be true.
It is also known that more moisture is retained in soil which has gravel underneath it than soil that has a layer of sand under it. Science shows that the coarser and thicker the underlying material, the more impossible it is for water to travel across the interface.
I remember the wonderful Lynda Chalker Scott talking about this issue and quipping, "Imagine what happens when a container is lined with pot shards!"
Soil has an air-water interface which can get in the way of water movement. Gravity will not move water from a fine soil to a coarse soil until the fine soil is completely soaked. Since the point of adding a layer of "drainage " material is to aid drainage, it is ironic that adding that type of material will produce the very situation you are trying to avoid.
When planting containers this summer, remember to use rich, well-draining soil in all the vessels you decide to use and skip the "drainage " layer. Your plants will thank you for it.
