I have 2 cacti (hee hee) in my backyard but they're only 5 feet tall and I don't know the specific species. I think there semaphores. My BF's mom's cactus is flippin' jinormous, at least 8-9 feet. She just lets it sit there and do it's thing, it's amazing!
bmccasland, have you tried potting it first or planting in a really really sunny, dry spot? Or, try a different cactus (if you haven't already). Semaphore's grow well in sticky and humidity filled environments (which is why they do so well in Florida, Virgin Islands, and Jaimaca. I hope the cactus you does well, they're so nice.