Sandra...Clematis "Jackmanii" blooms on the new vines that it will grow this season..So, I would go ahead and hard prune him down to about 6-12 inches from the soil above a healthy leaf bud..if not you'll have blooms HIGH UP and not from the ground up..don't forget to fertilize and water well..clematis like to have their roots stay moist..your new ones from last year will appreciate the same treatment...Jeanne

It is never too late to cut back clematis if you missed it earlier this season. In fact, my Julia Correvon and Purpurea Plena Elegans got badly infested with aphids during a time when I did not have much time to devote to gardening. The infestation was so bad that I just last week chopped all the vines down to within a couple inches of the ground. They will be back up shortly and will bloom as usual, albeit just a little later than normal.

Oh, Thank you Jeanne and OrwellianzQ! I appreicate the help. Now I have the confidence to do what I know must be done! :)
This is a wonderfully helpful web site. Thanks again!
Sandra