Sometime in the early 90's, Nancy and I attended a seminar at our annual Garden Writers Meeting that dealt with soil and drainage in containers. A professor from some mid-western university gave the talk and very clearly proved to us that placing shards or pebbles in the bottom of any container for growing plants is a bad thing. We all stopped doing that and wrote about the problem in our books and magazine articles. He explained the the space around the shards served to prevent the bottom inch of potting soil from draining completely, potentially causing rot in the roots.
What is frustrating is that Nancy and I have in the past ten years continued to see in magazines, newspapers, books, and the internet the strong advice to always put shards in the bottom of the container before planting. This practice has been proven detrimental to the health of plants. Why does this bad information continue to be published?
I guess because people really don't check their facts any more.
For a technical description of this issue I copied below a piece written by Steve Frowin in the National Gardening Magazine in 1985. Steve is a world wide known horticulturist who knows his stuff.
As Steve Frowine points out, (Healthy Houseplants
Steve Frowine from National Gardening Magazine, Nov. 1985 } more container-grown plants are killed by overwatering and poor drainage than by any other
cultural practice, disease or insect problem.
The practice of placing gravel, pot shards or some other material in the bottom of
the container is commonly recommended to improve drainage; in fact, it has the
Be your plant an African violet, a geranium or a zinnia, whether its container is
indoors or out, under most circumstances its roots will be less healthy and therefore its
top portion less vigorous if its roots are subjected to poor drainage conditions when
grown in a container.
One of the major differences between growing a plant in a container and growing
it directly in the soil outdoors is the way in which water drains through the growing
medium. Outdoors in the garden the soil provides a continuous column of growing
medium that keeps water draining downward away from a plant’s root zone. In a
container, however, the growing medium forms a very limited column; when water gets
to the bottom of the pot it is stopped.
When gravel or pot shards or other coarse materials are put in the bottom of a
container they in effect raise the level at which water drainage stops. They make the
discontinuous column even shorter and decrease rather than increase container
drainage. This negative effect can be either made worse, or lessened, by the type of
growing medium used and by the depth of the container.