"If it blooms before June, don't prune."
It may seem intimidating, but pruning the various types of clematis is really a matter of common sense. One bit of vintage clematis pruning advice is, "If it blooms before June, don’t prune” (until after it blooms).
Pruning groups include the early-blooming varieties (Group 1), the repeat bloomers (Group 2), and the vines that usually bloom in summer or later (Group 3).
Blooms in spring on last year’s growth. After blooming, prune lightly to shape vine if needed.
Repeat bloomers, first blooming on last year’s growth, then reblooming on new growth. If needed, lightly trim in early spring when buds swell. After the first bloom, again lightly prune to increase later flowers.
Blooms in summer or later, on new growth. Prune severely in early spring when new buds begin to swell, cutting stems back to 12–14” from the base of the vine to produce good growth and encourage abundant blooming.