Posted: 17/06/2012 at 09:13
Ours flower from about the middle of May until the middle of June in a normal season. The ones in shade last a bit longer than the ones in full sun. I leave the heads on as they are quite decorative, but then you do get self seeded ones around and they are not always as good as the original, but still nice.
If you do cut out the old flowering stems, then you may get another flush of flowers later on in the season.