Sphagnum is the one moss most growers handle by name, usually dried in a bag. It is the bog moss, the peat-builder, and its trick of holding many times its own weight in water makes it useful in ways no other moss is.
What it is
Sphagnum is a group of mosses adapted to waterlogged, acidic, nutrient-poor bogs, found across the cool, wet parts of the world where it builds the raised bogs and blanket mires of the north. Rather than a single species it is a whole genus, varying in colour from fresh green through gold to deep red, but all of them share the same spongy build and the same way of life. Its role in laying down peat is covered from the bog's side in sphagnum, peat and why peat-free matters.
Why it holds so much water
The secret is in the leaf. Alongside the narrow living green cells, a sphagnum leaf carries large empty cells with pores in their walls, and these fill and hold water like a sponge, which is how a small handful can soak up many times its own weight. The living moss also sours its surroundings, taking up nutrients and giving off acid in exchange, so a sphagnum bog is not merely wet but distinctly acidic and poor in nutrients. That acidity checks the bacteria and fungi of decay, and it is the same property that made sphagnum a genuinely useful wound dressing, absorbent and mildly antiseptic at once, most famously gathered by the sackful in the First World War when cotton ran short. There is more on that in moss through history.
In horticulture
- Orchids: long-fibre sphagnum is a classic potting medium, holding moisture around roots that still want air.
- Carnivorous plants: its acidity and low nutrients suit sundews, pitchers and Venus flytraps, which hate rich compost.
- Kokedama: it binds and holds water around the root ball; see the kokedama guide.
- Moss poles: damp sphagnum packed into a pole gives climbing aroids the wet, grippy surface their aerial roots want; see moss poles.
- Propagation: damp sphagnum is excellent for air-layering and rooting cuttings.
Live, dried and milled
Live sphagnum can be grown on the surface of carnivorous plant pots and in bog gardens, where it carries on soaking and souring exactly as it does in the wild. Most sold, though, is dried long-fibre moss, which rehydrates in minutes for the uses above and stores almost indefinitely in the meantime. Milled sphagnum, ground fine, is used as a seed-sowing medium because its mild acidity discourages the fungus that causes damping-off, the collapse that fells seedlings at the stem. Between these forms almost every use a grower has for the moss is covered, from a whole pot packed with the live plant to a light dusting of the milled kind over a tray of seed.
Source it with care
Sphagnum and the peat it forms come from bogs that are slow to build and important to leave intact, so look for sustainably harvested or cultivated sources rather than moss stripped from sensitive ground. Some growers keep a tray or pot of live sphagnum going and simply crop their own, which sidesteps the question and gives a steady supply of fresh moss. A little goes a long way once it is rehydrated, so a single bag lasts most households a long time.