Pine bark is undoubtedly the most common ingredient of orchid potting mixes, especially for cymbidiums, Australian native dendrobiums and sarcochilus but Sphagnum moss is gaining popularity as a potting mix for many other genera.
Sphagnum moss grows in swamps or bogs, where it may form layers many feet deep. It is widely distributed in the northern hemisphere but grows only in sub-alpine zones in Australia and New Zealand. The most common species, and the only one harvested commercially, is Sphagnum cristatum. It is found in mountain bogs in Victoria, New South Wales, the ACT and Tasmania. The growth rate varies between 4 and 42 mm per annum in Tasmania.
Dehydrated Sphagnum moss can absorb 10-20 times its weight of water. The moss is relatively sterile and for this reason it was used as a dressing for burns and other wounds during the first World War. It’s believed to contain anti-fungal substances, and it is therefore a useful medium for newly de-flasked orchid seedlings. It dries out much more slowly than other potting media, and consequently doesn’t need watering as often as other potting mixes.
The moss is usually supplied commercially either as dehydrated material or as living material straight from the bog. Dehydrated moss takes several days to reconstitute. To avoid breaking the strands, squirt the moss with water occasionally over a day or two before unravelling them. The moss is best stored in foam boxes kept in good light, and watered occasionally, preferably with rainwater. Material straight from the bog is ready to use immediately.
Any orchid that requires a fairly constant level of moisture seems to do well planted in moss. Masdevallias, odontoglossums and Colombian miltonias (Miltoniopsis species and their hybrids) grow better for me in potting mixes containing moss than they formerly did in pine bark. Disas, especially, grow better in moss than in other mixes.
Many growers prefer to use a mix containing both Sphagnum moss and crumbed polystyrene foam, generally in the ratio 2:1 or 1:1 by volume. It leads to improved drainage and also saves money. It’s usual to cut the moss into 10 mm lengths before mixing it with the foam. I use ‘straight’ uncut moss for my disas but prefer a moss/foam mix for most other orchids. Squeeze as much moisture as possible from the moss before use. Then it doesn’t need to be packed so tightly in the pot, because it swells when the plant is subsequently watered.
Moss is an effective potting medium only when alive. It’s important not to apply high concentrations of liquid fertilisers, as these will kill moss. I suggest no more than a quarter of the manufacturer’s recommendation (150-200 ppm). Heavily chlorinated water and water containing high levels of salt also kill Sphagnum moss. Melbourne tap water is satisfactory but rainwater is better. For best results plants should be re-potted annually. Otherwise the lower levels of moss gradually die and become soggy, causing the mix to drain too slowly.
Sphagnum moss is an effective medium for striking cymbidium back-bulbs and those of many other orchids. It can also be used to encourage root growth in Australian native dendrobium keikis. Another use is to revive sick orchids. Plants that have lost their roots should be repotted in moss in as small a pot as possible. After watering, the pot can be sealed in a plastic bag to conserve moisture, in which case no further watering is necessary. This treatment often leads to the growth of new roots. But not always – Sphagnum moss is a great potting medium but it can’t work miracles!