if the stems are hollow fold them over.
otherwise you can snip them off.
i like onion flowers so i tend to leave
them on more often than not and then i
cut the heads off before they start dropping
too many seeds all over the place.
this spring i buried a few hundred thousand
old onion seeds (down pretty deep). eventually
the worms and/or fungi will get them digested.
i don't think many of them were viable anyways.
still didn't want to risk it.
i was going to feed them to the birds through
the winter by scattering them on the snow in
the grassy areas that we mow, but we had so
very little snow i never did that. probably a
good thing. i really don't need onions spread
all over the place... garlic is plenty enough
to keep after.