Gardening: ‘Good quality potting mix’ is soilless, sterile and drains well

We see and hear the phrase “Use a good quality potting mix” all the time when we are getting ready to plant houseplants, deck containers or starting seeds. But what the heck is potting mix and why should we use it?

Potting mix is a sterile, soilless potting medium that holds water and stays fluffy so that roots can grow easily, and excess water can drain away quickly. In general, you need to use potting mix when planting any containerized plantings. Being sterile, it doesn’t contain pesky weed seeds, insects or diseases. Regular garden dirt is too fine for containers and packs down too much, impeding root growth and drainage.

It is important to read the label when you are buying potting mix. If the bag doesn’t list its ingredients, pick another product. Quality potting mixes are generally made up of differing blends of ground conifer bark called fines, commercial compost, ground peat moss or coir fiber, perlite or vermiculite and sometimes a slow release fertilizer.

Bark fines are a byproduct of our Northwest forest industry. They add structure, water holding capacity and help to retain nutrients. Commercial compost is made of organic matter from a variety of sources and processed in a controlled way so that it heated enough to kill bacteria, disease and weed seeds.

Peat moss is harvested from peat bogs in Canada and the U.S. while coir fiber is made from coconut husks and is a byproduct of coconut harvesting in the tropics. Both these materials help the mix retain water and an open structure. Perlite and vermiculite are naturally occurring minerals whose larger size also helps create an airier, less dense mix. Some manufacturers will add a slow release fertilizer to the mix as the other mix ingredients are generally low in nutrients.

Peat moss is much more common in soil mixes than coir fiber but it has some serious challenges. First, it tends to be hydrophilic, which means when it dries out it doesn’t easily reabsorb water. This can be a problem in containers when they dry out. You think you are getting the soil wet but in reality, the water is probably running around the root ball and draining away leaving the plant dry.

Secondly, there is a huge debate about whether peat moss is sustainably harvested. It took thousands of years for the peat bogs to form from decaying vegetation and sphagnum moss. The bogs hold a tremendous amount of carbon, which in these times of global warming is important. While many of the peat harvesting companies do have restoration programs to rebuild the bog after harvest, there is debate about whether they should be left alone to hold carbon.

Coir fiber on the other hand is less common in commonly available mixes. It does have two major advantages over peat. First, it is not hydrophilic and readily absorbs water. Secondly, it is a renewable and recycled product left over from coconut harvesting in the tropics. As long as we harvest coconuts there will be coir fiber.