I have two beautiful tall big cacti. Should they be trimmed? Repotted? They are in smallish pots and now are nearly 6 foot tall. They are very nice and still growing strongly.
Cactus are low maintenance plants that thrive with benevolent neglect. All cacti prefer to be slightly pot-bound, and although the temptation might be to put a tall cactus into a much larger pot, it is often better to leave them in their smaller containers, and if they are top-heavy, place this container into a larger pot and anchor this pot with heavy stones or other decorative materials. Repotting a columnar cactus isn’t necessary until it is evident that the roots are protruding from the bottom of the pot. It is important to repot in a new container that is only one size larger, rather than moving the cactus into a much larger container. Large containers retain more moisture than smaller pots, and cactus roots, which are tender and easily damaged, are susceptible to excessive moisture. Over-watering is one of the most frequent reasons cacti fail to thrive.
Pruning a columnar cactus is possible, but if the cactus is thriving and healthy, and not too tall for its space, it isn’t necessary. If it is spindly or too tall, it may be cut off to the desired height using a saw whose blades are clean to give a clear cut and help prevent disease. From the cut, depending upon the variety of cactus, the plant will either branch out or will remain at that height.
The Toronto Cactus Club is a good resource for cactus fanciers: