Group 1 the early flowering species which include C. Montana C. alpina, and C. macropetala as a group require little or no pruning. These Clematis will benefit from a light trim after flowering. Some C.montana are vigorous and may need pruning to contain them in the growing space, in which case prune back to size you want, after flowering which will be late May. Avoid
any pruning after June, as the Clematis is then producing wood (and flowers) for next year. Clematis in Group 1 can be left unpruned. Clematis montana is a popular variety as it flowers reliably and requires no pruning, so is one of the easiest Clematis to grow. (see Types of Clematis.)
Group 2 early to mid-season flowering (below centre image) Video How to prune Group 2 Clematis, which require moderate pruning to a framework. These Clematis flower early summer during late May and June, and need to be pruned in late Feb/early March. Group 2 Clematis are pruned back lightly to a framework of branches and buds; avoid pruning too hard, as this may reduce flowering.
Prune off the top growth to a pair of axil buds to make a framework cutting back, not harshly, to a good pair of axil buds. You may remove quite a bit of top growth, some may have buds and shoots, don't worry that's normal just lightly cut back to a good framework and remove any spindly or poor shoots.
In the images above the stems of the Clematis in Feb/March look dead, all brown and stringy. In the second image the Clematis in winter looks dead it definitely is not. Locate the axil buds shown in the second image and in third image, prune back to axil buds and cut off top growth. By pruning time February or early March Clematis are already growing and the top of the plant in particular will have buds and shoots on it. Do not worry that when pruning that you are cutting off this new growth, plenty more will follow.
Group 3 are the late flowering cultivars, and small flowering cultivars Video How to prune Group 3 Clematis . With the late flowering group of clematis pruning is easier simply cut down to about 30 cms in Feb/early March.
Group 3 Clematis tend to be vigorous and need to be pruned hard every year. A reservation about this variety of Clematis is that they are vigorous and can quickly outgrow its space. Most of Group 3 need a lot of room and will overwhelm an adjacent good-sized shrub, such as a large species Rhododendron within a couple of seasons.