Improving Container Drainage with Gravel
Based on an intern project by Rachel Vecchio, Certified Master Gardener
THE MYTH: Put gravel or rocks in the bottom of containers to improve drainage.
THE REALITY: This long-held gardening belief is based on the idea that gravel or rocks at the base of a container will allow water to move more freely from top to bottom, thus preventing the plants’ roots from becoming waterlogged. Ironically, the exact opposite occurs.
Soil scientists have proven that water travels through and over different textures at different rates. Texture, in this case, is defined as the surface area of the medium. Container soil or planting medium has a much finer, or smaller, texture than gravel or rocks, which have a coarse, or larger, texture. Water moves at a slower rate over and through a fine texture than over a coarse texture, and it does not move easily across the interface between layers of different textures.
According to Dr. Linda Chalker-Scott, Extension Horticulturist and associate professor at Washington State University, “Gravitational water will not move from a finely textured soil into a coarser material until the finer soil is saturated.” In other words, the soil in the container behaves like a sponge. It fills with water, but the water won’t drip through until the soil reaches maximum capacity. As a result, a person may unintentionally overwater a container plant, believing that the water is moving through the soil and draining out over the gravel, when, in fact, it is not. Consistently overwatering a plant will most likely lead to root rot, which generally causes plant death.
So how does one improve drainage in a container?
- All planting containers should have drainage holes at the bottom which allow for root aeration.
- If the container is large or deep, place empty, sealed plastic beverage bottles at the bottom to take up space. This will reduce the amount of planting medium required. Avoid Styrofoam, though, as roots can penetrate it, and a plant in Styrofoam can float out of the container.
- Use a high-quality potting mix and combine small amounts of leaf mold, compost, and topsoil throughout the mix.
- If using a heavier garden soil or topsoil, add perlite to create a lighter mix that will improve drainage.
Chalker Scott, L. (n.d.). The Myth of Drainage Material in Container Plantings [PDF]. Puyallup: Washington State University
Gillman, J., & Maynard, M. (2012). Decoding gardening advice: the science behind the 100 most common recommendations. Portland, Or.: Timber Press.
Olsen, E. (2016, March 10). Hydrological Discontinuity. Retrieved February 28, 2018, from https://blogs.ext.vt.edu/henrico-hort/2016/03/10/hydrological-discontinuity/
Successful Container Gardens. https://extension.illinois.edu/containergardening/choosing_drainage.cfm