Love the dual shades of blue too. I have dual yellow ones. They spread by rhizome rather than seeds so I just cut the whole stem and hope the leaves give enough energy.
I remove the individual dead heads as they die and leave the stems to die off before removing them. In September I cut the foliage down to about 4/5 ins. with a diagonal cut, leaving a fan shaped stump.
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When you don't even know who's in the team
S.Yorkshire/Derbyshire border