Some clematis bloom on old wood and others only bloom on new growth. If you hard prune the first type, most of the early bloom will be lost for that season. The latter type ban be prune,d to within 6-8 inches of the ground. Some times I hard prune and other times do a moch lighter pruning to six feet or so. The plants have always responded well to either approach. I have usually pruned in mid winter.
Eclectic gardening style, drawing from 45 years of interest and experience. Mostly plant in raised beds and containers primarily using intensive gardening techniques.