tree fern
Tree fern, also known by its Hawaiian name "hapuu", is the fibrous trunk of any of various species of tropical tree ferns. It is available as logs, slabs, or other solid shapes that may be used to mount orchids directly on. It is also available as shredded tree fern in several grades of coarseness that may be used for potting. Chunks may also be found at times, and are useful for larger pots and specimen plants. Fine tree fern may be used by itself, or mixed with other ingredients for compots and seedlings.

Tree fern has excellent properties as an orchid potting medium; it is easy to use and provides good drainage and aeration to roots. It is thought to provide minimal nutrition to plants and lasts about two years under normal growing conditions. Occasionally tree fern is mixed with additives such as redwood bark, which is thought to discourage snow mold. Coarse aggregates can also be mixed in to "stretch" the tree fern and improve drainage. Orchids grown in tree fern are best fertilized with a balanced fertilizer, i.e. 20-20-20.

Dicksoniaceae and Cyatheaceae  (tree fern) species are listed on Appendix II of CITES. They are endangered in many of their native habitats. Other alternatives may be easier to find and more ecologically sound.  

on the left is medium tree fern mixed with redwood bark and on the right is coarse tree fern