Even with botania:flower added to all four seasons, I cannot bonemeal Botania mystical flowers in winter to get the double version. As soon as spring hit, I was able to do so.

So, previously only seeds were allowed to be listed in the config file, but I've fixed it so that you can add blocks as well, for growable crops/plants that don't have seeds, such as saplings, or the Botania flowers in this case. Update will be live soon on CurseForge.