Rowan is for a girl.
No, it's not. ;) Just kidding. Really, though, I much prefer it on a boy, though it's not intolerable on a girl (which most unisex names are to me). I'd pair it with something very feminine, easily recognized as a "girl" name so there isn't any confusion as to gender when you see the full name. For instance -
Rowan Isabel would be much better than
Rowan Taylor.