What are the steps for trimming an Organ Pipe cactus?
What Are Guidelines for Trimming Organ Pipe Cactus
This cactus is very slow growing, but can reach a 10 foot diameter in 150 years. If it has been planted too close to it's neighbor, someone didn't take later growth into consideration.
To trim an arm, (branch) on this cactus, you should use a very sharp saw and cut as close to the base of the plant as possible.
If you are asking about trimming the entire plant, this is what I would suggest. If the plant is too tall, and there is a subspecies that grows to only 10' tall, take out the tallest arms. DO NOT saw off the arms to shorten the plant. The plant will initiate new growth at the injury site. The new growth will be too heavy and weigh the arm down, causing more injury, or an unsightly appearance.