Clematis are usually talked about as belonging to one of three pruning groups. Group 1 (sometimes called group A) typically bloom in early spring on old (last season's) growth, and they require little or no pruning. If you decide you need to prune, the best time is immediately after flowering. (You are probably thinking, "Why would I prune if I don't have to?" More on this later.) Pruning soon after flowering will give the plant the maximum amount of time to regrow stems that will form the buds for next season. The evergreen Clematis armandii belongs to group 1.
Group 2 (group B) clematis bloom in late spring or early-summer, bearing large and sometimes double flowers. They are the trickiest group for pruning because they bloom, to some extent, on both old and new growth. As a rule, blooms form on new growth that sprouts from last year's stems, as well as new stem growth from the current year, so it is desirable to preserve as much of the old growth as possible. Many in this group have a heavy bloom in late spring followed by a second, usually lighter, bloom in summer. A light trimming immediately following the first bloom flush may increase the number of flowers in the second flush. Generally, last year's stems are the ones that produce the largest flowers and double blooms, so your second flush of blooms, which appear on the current year's stems, might look quite different from the first flush. 'Nelly Moser' is an example of Group 2 clematis.
|Clematis 'Nelly Moser'|
Group 3 (or group C) are mid-to-late summer blooming and they bloom entirely on new season growth. One of the more familiar clematis in this group is 'Jackmanii', a very vigorous growing and heavily-flowering variety. Because clematis in this group bloom on new growth, they perform better with a heavy cutback at the end of the winter, perhaps leaving only 6-12 inches of healthy stem. Lack of pruning for this group can result in leggy-looking plants that have the flowers clustered only at the top of the plant.
In addition to maximizing bloom, there are other reasons to consider pruning. If your plants are young, as some of mine are, you might choose to prune to encourage more branching and a better overall shape as they mature. Or, if you have neglected pruning in the past, you might prune because you want to reduce the size of the plant, or because you need to free it from a nearby plant to which it has attached itself, or because there is no other way to untie the knot that the stem has become (yes, we've all been there!). So where (that is, how far back on the stem) you prune your clematis will depend on why you are pruning it. Locate a pair of healthy buds and make the pruning cut directly above them.
There is a lot of information available on the internet about how to prune clematis and some of it is contradictory, but the bottom line is that you should not be afraid to prune. Clematis grow so quickly that by next year, they will have forgiven any mistakes you might make this year. But if you are still afraid to prune, take this year to observe how your plants grow, both when they bloom and where the blooms appear (old vs. new stems), and make notes for pruning time next year.
As for me, I'll find a warm day in late February or early March and prune all our clematis. We don't have any Group 1 plants (unfortunate--I really like that evergreen 'Armandii') but I know I have a least one Group 3, which I plan to cut nearly to the ground. The others are Group 2's (I think) so I will do a light pruning on these, except for the two that I planted last year. These two need a heavier pruning to encourage branching.
One final word of caution: watch your plants carefully once the weather warms up so that you can train the the fast-growing stems where you want them to grow. Otherwise, overnight (or so it seems), they'll tie themselves into that knot we talked about earlier. There is nothing more delicate than a clematis vine twined around itself, and it is almost impossible not to damage the stem in your attempt to unwind it. So keep a close check on it until you see that it is growing where you want it to grow.