When dendrobiums send out roots high up on the cane, it means that the plant is not happy with something in it's culture. Dendrobiums need lots of bright light, almost as much as catts, and do not like lots of nitrogen. In the spring and summer they need lots of water and fertilizer, and then in the fall and winter they like to be dryer and need less fertilizer. What type of dendrobium do you have? A den-phal type or a nobile?