- Author: Ed Perry
Container gardening is popular both for growing annual and perennial flowers, herbs and certain vegetables. When discussing the topic with gardeners, nearly everyone agrees that the best growing media to place in the container are the mostly organic potting soils readily available at retail nurseries and garden stores. However, some gardeners still insist on adding a layer of gravel or other coarse material in the bottom of the container in the belief that drainage will be improved. I recently came across an article entitled "The Myth of Drainage Material in Container Plantings," by Linda Chalker-Scott, Ph.D., Extension Horticulturist and Associate Professor at Washington State University. According to Chalker-Scott, "This is just one of those myths that refuses to die, regardless of solid scientific evidence to the contrary."
Chalker-Scott writes that nearly every book or web site on container gardening recommends placing coarse material at the bottom of containers for drainage. The materials most often recommended for this practice are sand, gravel, pebbles, and pot shards. Some of these recommendations are quite specific and scientific sounding. To illustrate her point she refers to this piece of advice from a 1960's book on container plants: “Adequate drainage is secured by covering the hole in the bottom of the pot with a piece of broken flowerpot, concave side down; this in turn is covered with a layer (1/2" to 1" deep) of flowerpot chips. On top of this, a 1/4" to 3/8" layer of coarse organic material, such as flaky leaf mold, is placed.” The advice seems to make perfect sense, says Chalker-Scott, and it's presented so precisely. After all, we know that plants need good drainage, so their roots receive adequate oxygen, and we also know that water passes through coarsely textured material faster than it does fine material So, what's not to like?
Chalker-Scott then explains that nearly 100 years ago, soil scientists demonstrated that water does not move easily from layers of fine textured materials to layers of coarser textured materials. Since then, similar studies have produced the same results. Additionally, one study found that more moisture was retained in the soil underlain by gravel than that underlain by sand. Therefore, the coarser the underlying material, the more difficult it is for water to move across the interface. Imagine what happens in a container lined with pot shards! Chalker-Scott states that, despite popular belief, gravitational water will not move from a fine textured soil into a coarser soil until the finer soil is saturated. Roots suffocate in saturated soil, because water replaces oxygen in the soil pore spaces, and roots require oxygen. Concludes Chalker-Scott, since the stated goal for using coarse material in the bottoms of containers is to "keep soil from getting water logged," it is ironic that adding such material will do just the opposite. The bottom line: fill the container from bottom to top with a well-drained potting soil, and don't block the drain hole.
Ed Perry is the emeritus Environmental Horticultural Advisor for University of California Cooperative Extension (UCCE) in Stanislaus County where he worked for over 30 years.