The Whisperbloom Sapling location near where the Superbloom starts seems to only be spawning either once or twice an hour, starting roughly 10 minutes before the Superbloom. While I can understand limiting this location a bit in order to not have one spawn during the Superbloom to avoid forcing players to make a tough decision, you have an entire extra 30 minutes of the window outside Superblooms not being used to allow this spawn for completion of the “I Dream of Seeds” achievement.
I was waiting on this spawn just now and had to go feed my dog; then someone dropped in as I missed the spawn and planted an epic seed which I don’t have any copies of, so I was unable to get credit. Sigh.