I personally would let them die back on their own and enjoy them til late autumn, also their seeds will re-seed and you will have more plants. This is the way I garden but other folk may have other ideas.

Penstemons can be tender - you may lose them in a hard winter, same applies to Coreopsis. Verbena bonariensis occasionally make it through a winter if its mild, but otherwise will get killed but hopefully may have seeded itself - which you won't know till Spring, so the trick is to recognise the seedlings when they pop up. I too, wouldn't cut any of these down at the end of the season, other than to deadhead if you want to. The more topgrowth there is left, the more likely it'll trap other leaves and be more insulated from the winter weather.

Bamboo, I knew Coreopsis was tender and at the garden centre they advised I kept it in a pot, but would Verbena survive if I potted it for the winter or should I just mulch and hope for the best?

thanks for some new information.

I would not cut back a butterfly garden until mid spring, let stuff pupate.

Actually, Pimpernel, it sounds like a garden to feed adult butterflies, rather than many species of larvae.

Grannyb, I leave the Coreopsis in the ground to take its chances, with all the dead topgrowth present so it traps fallen leaves and stuff to add protection. As for the Verbena, if you mean bonariensis, I've had it survive in the ground, even 2 that came through last winter grew again from below ground, but I lost a lot last year too. You could certainly try potting it if you've got somewhere bright, cool but frost free to keep it. If you pot the coreopsis, that'll need a similar position too.

I leave most of these until spring. The verbena bon is useful to blue-tits in winter. They are either eating the seed or searching for tiny insects that have hidden themselves in the dead heads. I noticed the same with my hollyhocks. Both plants self seed quite happily in my garden, the verbena bon often coming up through cracks in the concrete!