Maybe just a bit of additional text along the lines of "If you know the plants names and can include them, it would be helpful to members."
I agree making it mandatory isn't a good option. There are many times people just don't know the names or the varieties, but post the combinations because they are so pretty. I would also think if anyone wants to recreate a combination, they could ask what the plants are, or post the question if the uploader doesn't know.
You have to remember this isn't a strictly scientific site. It's for all levels of gardeners who treat it more as a hobby and not necessarily a scientific study. Some members do really get into the science of it and that's fine, but most members will be considered casual and just won't have the knowledge to always post full names.
