It is not a requirement but most clematis growers recommend it. You will have a tidier, more manageable plant that often will produce more flowers than one left unpruned. It helps a lot to know what kind of clematis you have to determine when and how much to prune. If you don't know, paying close attention to bloom time can help. Very early blooming clematis (before May) generally require minimal pruning unless you want to control size. They bloom on old wood (growth produced the previous season or before), so if pruning is desired, they should be pruned as soon after the bloom cycle as possible so as not to affect flowering the following spring. Mid season clematis - those that bloom in May and June (and sometimes into July) tend to bloom on old wood and sometimes as well later on newer growth. These are typically pruned lightly - about a third - in early spring, around the same time you prune roses in your area. Often, many growers will prune again lightly after the first bloom to encourage a later rebloom on those that flower on new growth as well. Late season clematis (July - September) are best approached with a harder pruning. These bloom on new growth and if left unpruned, can produce long, leggy vines with flowering clustered only on the top of the vine. It sounds more complicated than it is :-) The vast majority of vines fit into the second category - the large flowered hybrids - so that should simplify things. And pruning instructions are only guidelines, not requirements. If the vines are newly planted, any clematis regardless of type benefits from hard pruning down to the third set of buds at the end of its first season (late winter/early spring). This encourages the development of a strong root system and the generation of multiple stems from the root crown and often, increased resistance to wilt. A good many growers will continue this practice in the second and sometimes the third season as well. It is hard to hurt an established clematis by pruning regardless of type, so if your vines are large, messy or unwieldy, pruning at any time can provide a necessary rejuvenation. Pruning at the wrong time or harder than typical for that type of vine will only impact flowering time and usually only for a single season.

Best to ask this question on the Clematis Forum (link supplied below). There are different pruning rules depending if it's a Group 1, 2, or 3. Basically, if you cut it back, do it in late winter/early spring, and try to do it so you have two good nodes left on it. Depending on what type it is, you may or may not get flowers next year (some bloom only on old wood, some bloom only on new wood, and some bloom on both old and new wood). But regardless, pruning it back shouldn't kill it -- the worst that will probably happen is that you would lose some height/width and it might not flower for one season. If you're not sure what type it is, the Clematis Forum people could probably identify it for you if you can post some pics of the bloom. Good Luck! Here is a link that might be useful: GardenWeb's Clematis Forum

You can prune back now if you want. There is no "rule" that requires one to wait until spring before pruning. I routinely cut back containerized clems in fall before I shift them to a less visible storage area for winter.

Clematis pruning can tend to be overly and unnecessarily complicated. Having grown a vast number of these vines for many years, IMO it can be simplified into those you prune and those you don't :-) In the 'no prune' category are any evergreen types and those that may bloom either in late winter or very early spring (usually species). And even these can be pruned as desired, but immediately after flowering, as they bloom on previous seasons' growth. Most of the rest - spring and summer blooming LFH's and the late summer/fall bloomers - do get pruned back on an annual basis, primarily to tidy the vine and keep the flowers at eye level. Various sources will tell you that you need to observe when the vine blooms and then prune either lightly (if a type 2/early to mid) or hard (if a type 3/mid to late). But in areas with a long growing season - like GA or here in the PNW - you can take all these vines back pretty hard and not worry excessively about 'light' or 'hard' pruning. All a hard pruning will do for a type 2 (light) is just push the flowering out a few weeks. I think even our friend Babs, who lives in NH and is a very experienced grower of clems, does a hard prune on type 2's. It just makes life so much easier, especially if one grows a lot of different clematis, to simplify matters into either the prune or no-prune camp and ignore the rather nit-picky or "refined" pruning instructions :-) It is almost impossible to kill or even severely damage a clematis by pruning.........and there is also NO rule that says you must prune.