Answer: Named varieties of hybrid clematis bloom in 2 different ways: some bloom on wood grown the previous year and some on wood grown in the same year. Clematis in the Jackman group of hybrids bloom on new growth (wood grown in the same year). If you wish to prune the Jackman clematis, do so in spring as the buds swell or slightly before. You can prune to within 4-6" of the base if you wish. For this reason, I would recommend Clematis jackmanii, which you can prune at the base when it overgrows your pergola. Of course you will need to pull the vines down, but at least you won't have to climb a ladder to prune it back several times a year.
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