If you’re like me, you learned early on to fill the bottom of a container with gravel or pieces of a broken pot to improve drainage. I have since discovered that this practice actually impairs rather than improves drainage.
When you water a plant in a container, a certain amount of soil at the bottom remains saturated. Because most roots won’t grow into saturated soil, where oxygen is unavailable, the growing space in your container is reduced. If your soil mix tends to hold on to a 2-inch depth of water, for example, then an 8-inch-deep pot effectively becomes a 6-inch-deep pot. Adding gravel or “drainage pads” can’t encourage this water to drain away; drainage is a property of the growing mix. A layer of anything other than soil at the bottom of a container further reduces the available well-drained growing medium and increases the chance that the plant will be overwatered.