The stalks and old flower heads look good in frosts and provide shelter for insects over winter so leave them until the first teeny green shoots appear at the base next spring and then cut all the old stems down to about 6" above ground and before the new shoots get that high or you'll lose their lovely points.
Vendée - 20kms from Atlantic coast. "We don't stop playing because we grow old; we grow old because we stop playing." - George Bernard Shaw
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"We don't stop playing because we grow old; we grow old because we stop playing." - George Bernard Shaw