I've had these two Cereus peruvianus plants since 2004. These are the ones I had to treat for mealybugs for over a year -- I used rubbing alcohol and went over the whole stem surface with Q-Tips, I sprayed with neem, I repotted, I took them outside for a summer, I added imidacloprid granules to the soil, and finally something worked.
They grew about a foot during the course of the summer (2008) they were outside, too, but then they came back in, they went through the winter, then we moved them to the house, and they've been in the southwest corner of the plant room since the plant room was habitable by plants.
Why the southwest corner? Well, the main reason is that I don't really have any good southern exposure in the house, and that was the brightest spot I could find for them, but it also happened to work out really well, because they were just the right height to fit that corner perfectly. Yes, I knew they'd grow out of the spot at some point, but it worked well enough originally.
So then I waited for a year, and not only did they never outgrow the spot, but they never grew at all. Which was strange, because, after all, these are the same plants that went from 17 to 51 inches (0.4 to 1.3 m) in the first year I had them, and just the summer before we moved here, they'd added another foot. So they're perfectly capable of growing very fast, but they weren't doing it. Why not? Not a mystery I was especially interested in solving. They weren't outgrowing their spot, they weren't dying, they were in a spot that was awkward to get to so it would have been tough to check into the matter, therefore if it ain't broke don't fix it.
So then last Friday, for unknown reasons, I happened to check under the pots, and saw roots coming out. Which eventually led to me repotting these plants, which is why they were outside in the photo (no room to repot something that tall in the house). They turned out to be the most rootbound cacti I've ever seen (I apologize for the lack of a picture), alas, which explains the lack of growth.1
Anyway. So now they're in new soil, in the southeast corner of the plant room. It's not a particularly good spot (it's a little cold and dark for their purposes, though we might be able to fix that, at least partly), but it's the spot in the house with the highest ceiling: 9'5" (2.9 m). The taller of the two cacti is 7'5" (2.3 m) right now, so something else will need to be done before the year is up, but better this than have them continue to live rootbound.
It turns out, in case you're interested, not to be that tough to repot a 7 1/2-foot-tall Cereus peruvianus. The spines are relatively small, and the ribs are positioned such that one can hold on to the plant without getting stuck. I did have to enlist the husband's help to hold the plants upright while I repotted,2 but aside from that it was a piece of cake.
1 Something I hadn't thought of until recently: having this many plants means I should probably be repotting more than I am. If you're supposed to repot once a year, then in any given watering cycle (14 days), I should have to repot 33 plants, on average. I repot a lot, but I know I'm not doing two or three plants a day. So no doubt there are other plants in the house in dire straits as well. I haven't yet started writing down when I repot, because it hadn't seemed necessary, but I'm realizing that I probably ought to start.
2 They don't look upright in the photo, but that's because they're both curved slightly. This is probably the result of me not getting them properly vertical when I repotted them in 2008, plus a long period in the corner of the plant room, during which time I didn't turn them that often because they were hard to reach, and not growing anyway, so why bother. At least with the husband's assistance, I can get the weight balanced, even if the stems don't wind up perfectly vertical.