Definitely cut them off. If you do it as soon as all the flowers are spent,
they will often rebloom. True, the second blooming may not be as good as the
first, but it's worth the effort. Cutting off the spent flower heads also
keeps them from setting seeds, which you don't really want because the
seedlings from these can often be undesirable in color and size. Once in a
while you can get something worthwhile, but most often the seedlings are not
as good as the parent plant.
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