Sphagnum is a genus of between 150-350 species of mosses commonly called peat moss, due to its prevalence in peat bogs and mires. Bogs are dependent on precipitation as their main source of water and nutrients, thus making them a favourable habitat for sphagnum as it can retain water quite well. Members of this genus can hold large quantities of water inside their 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. Sphagnum and the peat formed from it do not decay readily because of the phenolic compounds embedded in the moss's cell walls. Peat moss can also acidify its surroundings by taking up calcium and magnesium and releasing hydrogen ions.
Common sundew in a Sphagnum moss cushionIndividual peat moss plants consist of a main stem, with tightly arranged clusters of branch fascicles usually consisting of two or three spreading branches and two to four hanging branches. The top of the plant, or capitulum, has compact clusters of young branches. Along the stem are scattered leaves of various shape, named stem leaves; the shape varies according to species. The leaves consist of two kinds of cell; small, green, living cells (chlorophyllose cells), and large, clear, structural, dead cells (hyaline cells). The latter have the large water-holding capacity.
Spores are released from specialized black, shiny capsules located at the tips of thin stalks. Sphagnum species also reproduce by fragmentation.
Peat moss can be distinguished from other moss species by its unique branch clusters. The plant and stem color, the shape of the branch and stem leaves, and the shape of the green cells are all characteristics used to identify peat moss to species.