I'm reporting this so others know it's possible: I just got Ferru by breeding two reeds. Not stickreed, regular reed. It was in a desert biome at sea level, I don't think I even fertilized or hydrated it; if I did it was only once at the start. There was one iron ore and one dirt block below that.
I'd been breeding reed hundreds of times without fertilizing or hydrating, just trying to get whatever new plants I could. So I guess it was brute force.