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. Its leaves are built with large dead cells that act as reservoirs, which is why it soaks up so much water and why, over millennia, its undecayed remains build peat. See sphagnum, peat and why peat-free matters for that side of the story.
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. Most sold is dried long-fibre moss, which rehydrates for the uses above. Milled sphagnum, ground fine, is used as a seed-sowing medium because its mild acidity discourages the fungus that causes damping-off.
Source it with care
Sphagnum and the peat it forms come from bogs that are slow to form and important to leave intact, so look for sustainably harvested or cultivated sources rather than moss stripped from sensitive ground. Its long history as an absorbent, antiseptic wound dressing, most famously in the First World War, is covered in moss through history.