Clematis can be a confusing group of plants to prune, since they are not all pruned the same way. There are three methods that can be applied to major groups depending on the time of year the plant flowers. The earliest flowering clematis bloom on old wood, while later flowering types must produce new growth in order for flower buds to form. Prune carefully, since vines are usually well entangled.

Group A: Early-flowering Clematis

Plants in this group bloom in early spring, generally in April and May, from buds produced the previous season. Prune these plants immediately after flowering, but no later than the end of July. This allows time for new growth to produce flower buds for the next season. Remove shoots that have bloomed. You can prune out more vines to reduce the size or to form a good framework of branches, but avoid cutting into woody trunks. Plants in this group include: C. alpina, C. macropetala, C. armandii, C. montana and C. chrysocoma.

Group B: Large-flowered Hybrids

Large-flowered hybrids bloom in mid-June on short stems from the previous season's growth and often again in late summer on new growth, though these blooms are usually smaller. Prune in February or March by removing dead and weak stems, then cut back the remaining stems to the topmost pair of large, plump green buds. This cut could be a 6 inches to 18 inches from the stem tips. Plants in this group have the tendency to become leafless at the base as they mature. You can underplant with low, spreading perennials to help conceal the stems. You may be able to force a flush of new growth from the base by cutting the vine back to 18 inches immediately after the flush of bloom in June. Plants in this group include: 'Nelly Moser,' 'Miss Bateman,' 'Lasurstern,' 'Duchess of Edinburgh,' 'Mrs. Cholmondeley' and others.

Group C: Late-flowering Clematis

Plants in this group flower on the last 24-36 inches of the current season's growth. Some types begin blooming in mid-June and continue into the fall. This is the easiest group to prune since no old wood needs to be maintained. In February or March cut each stem to a height of about 24-36 inches. This will include removal of some good stems and buds. Eventually the length of the bare stem at the base will increase as the vine matures. Plants in this group include: C. viticella, C. flammula, C. tangutica, C. x jackmanii, C. maximowicziana, 'Perle d'Azur,' 'Royal Velours,' 'Duchess of Albany' and others.

http://clematis.org/

Check this out, there is an American Clematis Society, and an International Clematis Society...Maybe they can help you better.

http://dspace.dial.pipex.com/clematis/

Thanks for the links, Oui. Both sites have great information.

I'm sure "The President" will make a dramatic groundcover for your beds with its large blue-purple flowers. Please post photos of it in bloom!

I had a neighbor up in Virginia that could GROW ANYTHING. She had one clematis vine in her yard that was white. That clematis bloomed like crazy and had the largest blooms. I was not interested in the flower at that time. But I do remember my other neighbor questioning her in detail as to what she did to make the vine so beautiful. They were trying to root part of it so the other neighbor could have one. The neighbor with the perfect clematis said she never pruned it...And all she did was loosen the soil 3ft all the way around the plant. The blossoms on this vine were HUGE I mean the size of a dinner plate. The base vine was at least 6-8" thick. I think I will call that old neighbor today and question her about it.
I will let you know what she tells me.

I do know she asked me to collect some Alabama wild morning glory seeds for her. I did, and brought them to her next time I came up from Alabama to Virginia. Morning glory flowers usually get 1 - 2". A few months later..I was walking through her glorious flower garden and there was this vine that looked like a morning glory but the bloom was as big as a dinner plate. I asked her what kind of flower is that..To which she replied those are the morning glories you brought me..She was always kind of evasive about what she did to to get the blooms SO HUGE..hummmmm I wonder if she will ever tell me her secret...

I do know she made her own mulch.

Yep I am calling her today...