I have one, and it is quite thick but has not grown very tall (18 inches). Do you have any pruning or maintenance suggestions?
I have not grown one of these but there are a couple interesting threads on the forums that you should read: http://www.botanicalgarden.ubc.ca/forums/showthread.php?t=7100&highlight=pencil+cactus http://www.botanicalgarden.ubc.ca/forums/showthread.php?t=8036&highlight=pencil+cactus Make sure you are cautious with the sap!