Sarrs. are fall ripening. I leave the flower stalks alone until the
seed capsule/ovary dries and turns brown. In many cases the style will
have already been lost earlier in the summer. Once brown, I usually
leave it alone until I begin to see it start to split, then I remove it
and store the seeds in the frig. If you are expecting bad weather or
will be away for a few weeks and don't want to lose any seed you can go
ahead and cut the flower head off before it begins to split.
Tom in Fl