How To Transplant Saguaro Cactus Arms . While you can root a cactus branch any time of year, rooting while the cactus is actively growing gives the best chance of success. The big storm knocked down a beautiful 25'+ saguaro last night. Stunning Saguaros Ten Fascinating Facts About Saguaro Cacti Desert from blog.desertmuseum.org There is also another permit. The arms won’t appear until the cactus is around 70 years old. After this time, dust the base with a rooting hormone and plant into a pot that is at least 12 inches deep.