I hesitated to use firbark when it became necessary to repot my larger cymbidiums, which are, of course, terrestrial orchids. I noticed that they had originally been potted by the grower in a medium with considerably more substance to it. Therefore I tried a mixture of equal parts of sand, firbark, and shredded osmunda fiber, and apparently hit upon a good combination for my older plants thrive and bloom freely in this.
Need for repotting will usually be quite apparent. Roots will push out of drainage holes or extend over the tops of pots. Once plants are established in fairly large pots, say 6's or 8's, it will not be necessary to repot for two or three years. Then large plants should be divided.
Mature orchids with pseudobulbs cattleyas and laelias should be split with a sharp knife. Cut through the rhizome leaving at least three and preferably four pseudobulbs to each section. Get rid of any dead roots and worn-out potting material. If one or two roots have grown very long, cut them back; you will not injure the plant. Put the sections in individual pots with the heel of the pseudobulb about half an inch from the wall of the pot. Use pots large enough to allow room for the development of two to three leads before the next repotting. For repotting, use firbark or my firbark-sand-osmunda fiber mixture.
Handle cypripediums a little differently. Cut the plant into pieces, each with at least one bit of growth. Repot in a mixture of equal parts of osmunda fiber, sand, and leafmold, for these orchids are of the terrestrial type.
Propagate the evergreen dendrobiums by offsets, those tiny plants that develop their own root systems while still a part of the parent plant. With the deciduous dendrobiums whose pseudobulbs bloom only once and can now be cut out of the plant make 3- to 5-inch sections each containing a node or two. Spread these out in moist sand in a warm, humid place, where small plants will form in a few weeks. When they have developed a good root system, cut them apart and plant individually in small pots. The thin wiry root systems do not need large pots, even when plants are mature.