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 a great deal of 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. The trick is in the packing: firm enough to hold the plant steady, loose enough that the fibres never pack down into a sodden plug.
Choosing and preparing it
Quality matters more here than in most uses. Long, clean, whole strands hold their springy structure far longer than the short, broken material sold cheaply, which collapses and turns wet within months. Dried sphagnum is what most growers use, soaked before potting and then squeezed out so it is thoroughly damp but not dripping. Live sphagnum, kept growing on the surface, is prized by some for staying sweet and airy, though it needs bright, humid conditions to thrive and will die back in a dry room.
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. Neither is right or wrong; the better medium is the one that matches how you actually water and how dry your rooms run through the heating season.
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 and shows the plant off as it really grows. 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, since sphagnum is slow to shed the salts that hard tap water leaves behind.
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 and trimming away anything soft or blackened. Do it when the plant is not in active flower if you can, and settle it gently back so the fresh moss sits snug but never tight around the healthy roots. The wider uses of the moss are gathered in sphagnum moss and its many uses.