“Do I cut the flower spike when the plant stops blooming?”
Questions about cutting the flower spike abound, and like most orchid-related questions, the answer to “do I do it or not” is a resounding “It depends…”
For the most part, once a plant has dropped all of its blossoms, it is safe to cut it off as close to the plant as possible and remove it, but some plants do re-bloom from old inflorescences, so we think the preferred method is to wait and see. If the inflorescence remains green, it may rebloom. If it starts to yellow, it’s a goner and can be removed.
Phalaenopsis are sort of a special case. Some will advise cutting the old spike just below the point that the first (lowest) blossom formed, as that will sometimes “force” the plant to form a branch and bloom again in the short term. We recommend against doing that, as the subsequent blossoms tend to be smaller and fewer, and such forcing can substantially weaken the plant, so much so that it may skip its next blooming cycle or even die.
We still subscribe to the “wait and see” approach for phals. If they are strong enough to rebloom, they will. If not, they won’t. If you simply do not like the looks of the bare spike, feel free to remove it, as the plant will grow another when it’s good and ready.