It's impossible to properly discuss watering without considering rooting media. Orchids are commonly potted in one of two media: moss or bark. Both are perfectly good materials, but they require somewhat different care. Moss acts like a sponge, and it takes a lot longer to dry out. Thus, for orchids like Phalaenopsis and Cattleya that need to dry out thoroughly before watering, moss requires a longer wait before watering and is less forgiving of too-frequent watering. Bark, which holds little water, poses less risk for these orchids. The rule of thumb for these orchids is: Water the day before the medium is completely dry.
Lady slipper and nun's orchids, as I pointed out above, enjoy conditions on the moist side and they'll do better if you don't let them go completely dry. Moss is a good choice for them, supplying adequate water for longer intervals between watering. Can these moisture lovers be grown in bark, too? Sure, if it's fine-textured. But be prepared to water more frequently.
Continued on page 5: Repotting and Feeding Orchids