Gardeners use Canadian sphagnum peat moss as a stand-alone product or as a component of commercially produced potting soil. The sphagnum peat moss we buy in stores is a lightweight, fluffy, brown material that has been milled and harvested. Typically, you will find it bagged in compressed bales. Peat moss grows in several places around the world, however, Canada is one of the top producers of horticultural sphagnum peat moss.
The Sphagnum peat moss used in horticulture is partially decayed or decomposed sphagnum moss. Sphagnum moss (Sphagnum spp.) grows in small, dense patches in wet and boggy areas. According to the State of Washington's Ecology Department, "peat moss is unique in that it can hold large quantities of water inside its cells. Some species can hold up to 20 times their dry weight in water which is why peat moss is commonly sold as a soil amendment." A guide produced by Brandeis University notes that the moss absorbs water in a manner similar to a sponge, through its capillary tubes, and you can squeeze the water out and it will reabsorb more water.
Gardeners use sphagnum peat moss to aerate clay soil, lighten soils in containers and as a soil-less growing medium for seeds and seedlings. Many of the uses stem from peat moss' ability to absorb water. Sphagnum peat moss does not have many nutrients of its own to contribute to plants; it does have the ability to absorb soluble nutrients and release them back to the plants. Manufacturers also press peat moss into forms that gardeners use as planters.
The Canadian Sphagnum Peat Moss Association is a trade association composed of 95 percent of Canada's peat moss producers. To harvest peat moss, harvesters drain the low side of the bog through a drainage ditch. They remove all of the living vegetation growing at the top of the bog. Sun and wind dry the exposed layers of peat, and the harvesters then use large vacuums to collect the peat.
The gardening community is engaged in an ongoing debate about whether peat moss is a renewable resource. The Canadian Sphagnum Peat Moss Association provides the most information suggesting sustainability. In the 1990s, the group created a Preservation and Reclamation Policy for its members that promises to restore bogs to peatland after harvesting. The information suggesting that peat moss is not harvested in a sustainable fashion stems from disparate sources. Writing for the University of Minnesota's journal, "Restoration and Reclamation Review," Matt McCue notes that "sphagnum moss regeneration is very limited." "The peat remaining in the bog after harvesting is more decomposed and is subject to shrinkage, oxidation and compression due to exposure from anaerobic to aerobic conditions." Sphagnum moss needs water, but there is very little water for the moss because the small pore size of the decomposed peat layers left behind after harvesting does not provide enough water. The vacuum harvest method creates a flat topography, that makes it easier for wind and water to erode the field. All parties in the debate will need to wait many decades for answers while the harvested fields begin to grow new vegetation.
Alternative soil conditioners are decomposed leaf materials, organic compost and commercially formulated products. Coir is a by-product of coconut processing. According to the Oregon State University Extension program, "Coir has proven to hold moisture well, wet more easily than peat, drain well, decompose more slowly and withstand compression better than peat. Plus coir dust does not have the small sticks and possible seeds that peat has."