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Prune large-flowered cultivars, small-flowered species and their cultivars, and herbaceous species (all of these flower in late summer on the current season's growth).
Cut back all growth to a pair of strong buds 6-7 inches above soil level, before growth begins in early spring.
At the same time check that the roots of the plant are shaded. If not place some stones on the soil around the stems. Clematis like 'their heads in the sun, but feet in the shade'. This also helps prevent clematis wilt.