| Don't know about cholla. Is that a cactus type of plant? It needs to shed water, and not absorb much. Water retention will facilitate substrate breakdown (rot). That said, if the mount looks good, you may want to go ahead regardless. It would probably last a few years at the least and then you can always remount the plant.
Might as well orient the plant upwards; it's going to go that way eventually. The roots are more likely to wander all about.
Hope this goes well for you. I was reluctant to get involved with mounted orchids, but now it is my preference. If you can meet the water/misting/humidity requirements, many other concerns simply disappear.
jeff |