A mature montana clematis can be as large as 25 feet high. Covered with white or pink blooms, it produces an extraordinary show early in the clematis season. Provided with good growing conditions, it will scale large evergreens or the garage roof in exuberant growth (montana, after all, has its roots in the French "monter," or "climb"). In smaller spaces, montanas need pruning, which can present some challenges because of its size. Knowing how it grows will enable you to prune it.
Plan pruning with the knowledge that a montana clematis is a Class 1 clematis. Class 1 clemati bear flowers on "old" wood, that is, last year's woody stems. (Class 2 clemati bloom on both old and new wood, and Class 3 set blooms only on new growth; pruning strategies for these types differ from Class 1 measures.) Some nurserymen divide classes into A, B and C; Class A corresponds to Class 1.
Cut back both woody stems and new growth just after the clematis has bloomed. Because montanas bloom in late spring/early summer, pruning right after blooms have finished gives the plant time to send out new growth stems that will bear flowers the following spring. Ordinarily, the light pruning described in Step 3 is all that is needed to shape your vine, and there is no requirement that pruning be done every year if you like the shape and size of your clematis.
Cut stems back a foot or two, unless your clematis has gotten completely out of control. If you must remove long-established woody stems, be prepared for slightly fewer flowers the following spring; even a long summer season may not permit thick stems of considerable age (anywhere from 20 to 50 years) to recuperate completely. If old stems appear dead in early spring, wait for the plant to leaf out before cutting them back; thick old stems may take a little extra time to return to life than younger, thinner ones.
Get help if you must cut back thick old stems, which can run 10 to 20 feet in length. Although your clematis appears light and airy, old stems can be of considerable weight. Pulling an old stem out of a tree while standing on your ladder is dangerous. Either seek out a helper to pull down long cuttings or at least make certain you are standing on solid ground when you pull them down.
During or after pruning, examine and strengthen, if necessary, the supports you have provided for your growing vine. With blooms out of the way and full leaf yet to come, this is the best time to repair trellises and add sturdier supports for further growth and direction.