I am sure each cactus species is different, but my Spanish Dagger flower pods are trimmed with tree pruning shears as deep in the plant as I can reach. I do not believe that is the source of next years flowers. To trim the plant itself you must cut off whole branches at a time. You may cut one off and two more will grow out of the ground or out of a branch. The damaged or dying spines are simply cut off with the tree shears as far in the plant as I can reach. This will help keep the appearance nice and fresh.