Mine grow about 1-2 feet a year (zone 9b). They also sprout arms anywhere from the base to near the top. Because they can quickly spread out with those arms, I keep mine trimmed, and don't allow them to grow higher than 7 feet. Whenever a piece is cut back it quickly heals over and most often will generate new, smaller diameter arm buds at the cut end (see the top of cacti in picture). The root system is not very massive, so the cacti do well in general garden areas, and can do well even in pots. I have raised a couple of arms in pots and not only have they thrived over four years without root-trimming, but last summer one of them even flowered.
