Usually I will just tell the sim to talk to the plant and when he quits talking on his own, I figure, maybe that was enough. I know in sims2, when the plants reached the last level, the sim would automatically stop talking. I only have my sim talk to the plants once every cycle. When I harvest a plant, I will have them talk to it and then that is it. My sim's garden is too big now, so I have quit talking to them, besides, when all of the plants are perfect, I see no need anymore.