Since the 1850s, hundreds of varieties of clematis have been propagated. It may be difficult to know exactly which variety you have, but you can narrow it down based on the time of year that it blooms as well as its flower size. There are three main types of clematis, designated as groups A,B and C. Some botanists further divide group B into 2 groups, B1 and B2. Knowing which of the three types you have also tells what time of year is appropriate to prune your clematis.
The group A type of clematis are the early flowering varieties. These vines flower in early spring sometime during the month of April and May. Examples of these types include Clematis alpina, Clematis macropetala and Clematis montana. Prune this type of Clematis immediately after blooming or no later than the end of July. The following year's blooms will appear on the growth generated from this pruning. At this time you can also do any shape or size pruning if you wish to reduce your clematis' size.
Group B1 varieties of clematis are large-flowering hybrid types. These types bloom twice.They first bloom in mid-June and then again on the current year's growth later in the summer, although the blooms in the second flush are smaller in size than the first flush. Common group B varieties include "Nelly Moser," "Miss Bateman" and "Duchess of Edinburgh." These should be pruned in mid to late winter by removing all dead and weak stems. The remaining growth should be pruned back to the uppermost pair of green buds.
B2 types of clematis start blooming at the same time as B1 but do not have a pause in their bloom cycle; instead, they bloom continuously through the summer into the fall. Like B1 type, the first blooms are on the previous year's growth, and blooms that appear later occur on the present year's growth. It can be difficult to differentiate between types B2 and C since they both bloom from summer into fall. The difference lay on which season's growth they start blooming.
Group C clematis are late-flowering types. These varieties bloom on the upper 2 to 3 feet of the current year's growth. They are the longest blooming of the clematis as they start to bloom in early summer and continue on into the fall. Common type C clematis include the "Jackmanii" types and clematis "Viticelli." These can be cut back to a height of 2 to 3 feet from the ground every year in February or March.