The "flower balls" are one of the many galls on willows, and induced by an insect: a gall midge, most likely. Of course, they are not real flowers at all; they are the willow's growth reaction to the bug infections. I you were to look at them now (months later), they would probably be dried up, and depending on the type, look like a wooden flower bud or open flower. Nice additions for a dried flower arrangement.
