Because the cost of potting bark gets higher every year, growers are looking for new less expensive mediums for their orchids. Potting bark has been washed, sterilized, treated for weeds, and graded for size. The large size bark is good for large-rooted orchids such as Vandas and large Cattleyas. The medium size bark is used for medium sized plants such as miniature Cattleyas, Odontoglossums, and Phalaenopsis. The fine size bark can be used for your fine rooted orchids like the Oncidiums and the ones that like to be moist like Paphiopedilums.
Many people like to mix charcoal and perlite with the bark.
It is also a good idea to soak the bark for a day before you use it to help it absorb water and to remove any of the fine dust particles. If you live in an area where sphagnum moss grows, you are very lucky. If you are able to collect it live, it should survive in your orchid pots and your orchids will also like it. Do not pack it in the pot, leave it loose around the roots. This type of moss is typically used as a base for mounting orchids onto pieces of wood, or other mounting material.