Many orchids grow in the wild as epiphytes, perched on tree branches with their roots in the air, taking water from rain and humidity rather than soil. Sphagnum moss suits that life well, which is why it is a staple of orchid growing, especially for the popular Phalaenopsis.
Why sphagnum works
Orchid roots want two things that seem to pull against each other: moisture and air. Long-fibre sphagnum delivers both, holding water while staying open and springy enough for air to reach the roots. It is also low in nutrients and gently acidic, which suits plants adapted to a sparse, airy perch rather than rich earth. Packed loosely around the roots, it keeps them evenly damp without the suffocating wetness of ordinary compost.
Moss or bark
The two common orchid media are sphagnum and bark, and the choice shapes how you water. Moss holds far more water and dries slowly, so it suits drier homes and growers who water less often, but it is less forgiving of a heavy hand. Bark drains fast and dries quickly, suiting those who tend to overwater. Many growers use moss for young plants and seedlings, which like steadier moisture, and move to bark as the plant matures.
Mounting and the watering balance
For the epiphytic look, an orchid can be mounted on a slab of cork or wood with a pad of sphagnum tucked under its roots to hold moisture; this needs frequent misting but mimics the wild beautifully. Whether mounted or potted, the key is the watering balance: sphagnum should be moist, never sodden, and should be allowed to approach dryness before rewetting. Constant saturation is the usual cause of root rot, so err towards letting it breathe between waterings, and use rainwater or low-mineral water where you can.
When to refresh it
Sphagnum breaks down in the pot over a year or two, growing dense and sour and holding too much water as it does. Repot into fresh moss when it starts to look matted and stays wet, usually every one to two years, checking the roots over as you go. The wider uses of the moss are gathered in sphagnum moss and its many uses.