I am dumb for even answering you on this subject...so different are our climates. Where I live in Zone 9, I prune my few Hybrid Perpetuals almost like Hybrid Teas. I take out old canes that are tired to rejuvenate the plant and trigger new basal breaks and remove dead wood. I don't prune anything as drastically as 6" though. I don't have that many HPs: Grandmother's Hat, Yolanda D'Aragon and Jacques Cartier. I took Grandmother's hat down to about 4.5 feet, Jacques Cartier to about mid-thigh height and Yolanda down to waist height...3.5 feet? Something like that. All three look very happy. All three are own-root plants.

Hope this is helpful. I don't grow Mrs. John Laing. None of my HPs are what I would call "twiggy".

Jerome

Dublinbay, I prune my HP's taller, 3-5 feet depending on the variety, like shrubs, dead wood, crossing branches, etc just as you say. I would only cut them back as severely as a HT if I were wanting some show-quality larger blooms.

As far as the different Mrs. Laings are concerned, just let them do their own thing...the one with fewer canes should eventually spread a little and catch up; assuming both are own roots...otherwise you may have a situation where the graft on one has rooted while the other is still dependent on the graft.

Here is a link that might be useful: Garden Musings blog

Thank you, jerome and professor, for your advice. Now I feel a little more confident about tackling my HPs.

Kate