When I have a slender, columnar cactus like a Cleistocactus (pictured below), and it gets too tall, How can I prune it, to get it back to a manageable height?
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Wait till the next growing season if it isn't already. Then do the following:
Make a clean cut and remove the top 2/3rds of the plant.
Let it sit in a dry place for a few days to callous over.
Pot it in fresh cactus mix in a small pot. Support if necessary. Leave a couple inches between the pot bottom and the end of the cuttings.
It should catch on in a couple weeks. Keep the mixture constantly but slightly damp.
By week 6, treat as an established plant.
This method is better than cutting back and keeping the original plant, because while the original plant will probably regrow some more stems near the cut, that cut will turn into a conspicuous scar.