Well, the thing with names is--IMO--at heart they're all kind of nonsensical.
Or I should say, they're much more dependent on culturally-specific attitudes than on anything resembling universal logic.
For literal names--like you could use that word in an ordinary sentence--I think a lot of them "work" better for girls than boys in the US. Like if a family is trying to do all plant names or gem names or virtue names, it's a lot easier to find ones that fit girls better. My impression is, flowers are considered to be feminine culturally, so most flower names would be considered female. Trees, on the other hand, have a cultural image more related to masculinity, so more of them would be considered to make good boys' names.
Lily, Rose, Iris, Violet, Jasmine (flowers) are all well-established as female names here. If they have a brother, the parents would probably move into tree names like Ash, Oak, or Pine, which are still kind of uncommon names for kids period. Or they'd go more general, like Leaf or Forest. And there's always exceptions anyway--Willow, for example, is a tree, but also a well-established name for girls. Same with Acacia, Laurel, Olive, Hazel. Why is a laurel tree considered "feminine" while a pine tree is "masculine"? No idea, really.
The website 20,000 Names is very interesting in this regard. If you go to their list of "flower names," there are 18 they declare unisex, 60 for boys, and 481 for girls. I don't know how accurate their derivations are, of course, or how close the names are to the language anymore. For example, they say a boy name is Ren, Japanese, "water lily." When someone talks about water lilies in Japanese, do they use the word "ren"? Is there something about a water lily that makes it seem culturally appropriate for a boy, and not so much for a girl? I dunno. I think it's neat to think about, though.
