When talking about growing plants in containers, I usually emphasize how important it is to use a quality potting mix. Admittedly, this is vague and does not help when trying to decide what potting mix to buy.
In the 1950s, the selection of bagged potting mixes was not difficult, because there were none. Most gardeners planted annual flowers in flower beds, or regular garden soil was used for planting in pots. However, the problem with using soil from the garden was that it typically did not provide adequate drainage and aeration.
In the 1960s, universities like Cornell University researched what materials worked better for growing plants in pots other than plain soil. Their research was prompted by a nursery industry that was finding it difficult to find good topsoil for growing potted plants. They needed a readily available substrate that would enable growth in quality plants, one that was disease and weed seed free, was relatively lightweight, provided good drainage and adequate nutrients, and did not contain residual herbicides.
Cornell’s solution to the problem was a soil-less potting mix called the Cornell Peat-lite Mix. Their basic mix was 50 percent by volume, sphagnum peat moss and 50 percent horticultural-grade vermiculite. Their B Mix was a 50:50 mix of peat moss and horticultural perlite. Fertilizers were added to the mixes to provide nutrients for growth. Around the same time, the University of California developed a basic UC Mix that contained sand and peat moss in equal proportions.
Potting mixes have changed since the ’60s. One reason is the expense of obtaining sphagnum peat moss and environmental concerns over the destruction of peat bogs in Canada and elsewhere. Compost, softwood conifer bark, composted manure and coconut coir (made from coconut husks) have been used to replace some or all of the peat moss in soil-less mixes. Horticultural vermiculite has also fallen out of favor because it will compact if not handled gently, losing its ability to provide aeration and drainage. There also have been concerns about using vermiculite because its ore naturally contains 2 percent to 3 percent asbestos fibers.
My preference in potting mixes is one as close to Cornell’s B Mix as possible, but this is difficult to find. As already noted, many companies substitute other materials for the peat moss component. This substitution works well if the substituted material is fairly stable. Coconut coir and composted pine and fir bark all decompose slowly and serve as adequate peat moss substitutes.
Potting mixes that get a thumbs down from me are those that are predominantly plant-based compost or contain inferior components. These mixes are usually the lower-priced potting mixes. While they may be dark and crumbly, they often do not drain well or provide adequate aeration. Stay away from mixes that contain sedge peat, soil, stones and discernable pieces of sticks and twigs, and ones lacking perlite or vermiculite for drainage. Do not use products labeled “garden soil.” These are not intended for use in containers.
Finally, remember the adage of, “You get what you pay for.” Look for the recommended ingredients on the bags of potting mix and potting soils, and invest in “a good quality potting mix” for your container gardens.
Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.