Question: One of the gardening podcasts I listened to recently said I shouldn’t put gravel in containers. I have always done this; I was told it promotes drainage. Can you explain further?
Answer: You are far from alone in believing gravel, potsherds and the like are crucial for drainage in pots, but in fact, it is a myth.
Dr. Linda Chalker-Scott, a horticulture professor and extension agent at Washington State University, and one of the contributors to The Garden Professors, explains that water does not move easily from a fine-textured material to a coarser material, even with the aid of gravity.
Soil scientists have proven this time and time again. Putting coarse material, be it sand or stones, beneath potting soil will actually make the soil more likely to become waterlogged. No more gravel in containers. Chalker-Scott recommends using good soil throughout the container and, of course, making sure the pot has drainage holes.
Gravel in containers has also been cited as a source of support for potted plants’ roots, but Chalker-Scott says there’s no research behind this claim. You may hear advice to fill the bottom of a large pot with Styrofoam or the like to reduce the weight of the planted container and to use less potting media, especially with shallow-rooted annuals.
Chalker-Scott thinks there’s no harm in this, but she adds, “I’d probably put the plants in a smaller pot on top of the foam materials in the larger pot. Otherwise, the potting media would wash through and the roots would become more exposed over time.”
Myth busted! Gravel in the bottom of containers is not smart gardening.