Opuntia subulata (Austrocylidropuntia subulata) – a tree cholla from the Andes, also known as Eve’s Needles, grows over 25 ft. tall in the Bay Area, although I hear it tops out at 12 ft. in the Andes.
This here is a piece of fruit rooted into the soil and newly branched. They generally don’t go to seed and instead the sterile fruit drop and roll and root. A new tree ensues.
Never water. We get enough rain here that they are the fastest growing cactus we carry. If you also water they’ll grow too fast, won’t form a strong woody core, and will flop over. What with those very long spines (hence the common name) a falling branch doesn’t seem like a good thing.