First of all, I agree with Jdonlop's answer on the level that there will not be any surface life on a rogue planet because anything on the surface would be a subarctic wasteland with liquid or gaseous water only ever existing very temporarily following major eruptions. This water just could not exist long enough for life to evolve there.
That said you may still get something that sort of resembles seasons based on the formation and death of hydrothermal vents. While some of these vents last as long as 10,000 years, many last much less than that. The life cycle of vents can vary a lot though based on how deep underwater they are, how much pressure is built up in the magma chamber feeding it, and how quickly your plate tectonics are drifting apart.
In general, your plant like organisms would colonize the chimneys of hydrothermal vents where the water is cool enough for macroscopic organisms to survive, but still putting out enough chemicals to feed them. Your organisms will slowly bloom and become more numerous the taller the chimney becomes as thermally survivable surface area increases. Eventually, the chimney would either fall over or simply lose pressure killing off any life that is there
There is no guarantee that you organisms will flower and fruit at all since strictly speaking, they are not really "plants"; so, I would personally expect a life cycle more similar to coral polyps, but if you choose to make them flower and fruit it could be related to their ability to "feel out" the impending death of their vent. As the chimney gets too tall, it will become more seismically unstable letting the plants on it know it's time to colonize. At this point they could flower and fruit so that "fish" or whatever motile organisms you choose to consume your fruit can consume the seeds and carry them to other nearby vents where their excreations can deposit them.