Not until all risk of frost has gone, as the old flower heads give some protection to the new shoots. Then take each stem down to a healthy pair of shoots and cut just above them. If they're mature shrubs, you can take them down to about 18" high, or eeven lower! Newer shrubs - not so hard this year.

Thank you !

If it is a mature bush, it is recommended that you take out completely a third of the shoots each year, at pruning time....this prevents the bush getting too congested and unwieldy........