This cactus lives in a clay pot but it grows in a sub-irrigated planter (SIP aka erroneously as "self-watering") insert (note plastic rim). It was a 2" pot size plant a little over five years ago. It has spent most of its time on a windowsill here in my Brooklyn apartment and more recently on a light garden shelf. The can is a scale figure.
This cactus has been thriving in my Brooklyn apartment expect for one thing. It is a very healthy plant except for the brownish ring about half way up. It was caused by a gross operator error (by me). Contrary to what most people might think, it was not over-watering. Can you guess what caused it?
I'll bring this plant to the Brooklyn Botanic Garden SIP workshop next Wednesday and all will be revealed.
See prior photos of this cactus in this Flickr photo set.