Cut it down to about a foot high in early spring when roses are pruned in your area. Jackmannii is a pruning group III, hard prune every year and will bloom better for you if you do hard prune it.

She's right Clematis "Jackmanii" bloom on the new vines produced during the season..if you don't hard prune every year..it will just bloom at the top where the new vines are....this is done every late winter/early Spring...Jeanne

I agree with the others! Whack whack in late winter or early spring to a foot or so. Eventually the lower vines will get woody with no leaves if you don't prune it as a type III clematis should be pruned.