Hack it right down! What I also do is lift them, and divide them up with a knife into little pieces aiming for the base of stems with a shoot or two plus a few roots attached.
It tends to be a fairly short lived perennial where I live in Scotland and I’ve found it doesn’t like hard pruning here. Because of this I cut it back to about nine inches and only after it’s started making new growth. I get a second, sometimes a third year out of it that way. It’s a prolific self seeder though, so I’m never short of new plants.
i tend to cut the heads off in the autumn for plants grown on my patio area because it self seeds in between the paving blocks and can be a pest to remove.
Some of my VB's died off earlier this year - they became brittle and snapped. However luckily I collected plenty of seed last Autumn from these lovely purple flowered plants which are now 2" seedlings - germination is easy - even I can manage to do it!
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Wisdom is not putting it in a fruit salad.
i tend to cut the heads off in the autumn for plants grown on my patio area because it self seeds in between the paving blocks and can be a pest to remove.