Lauren St. Peter of Caribou recently asked for advice on pruning her clematis vine. I realized that I have neglected clematis in previous pruning columns, the reason being that the hundreds of species, hybrids and varieties of clematis fall into one of three groups, each pruned differently. The subject has been waiting for its own column.

All clematis can be divided into three pruning groups based on whether they flower from summer through fall on new wood (Group 1), in early spring on old wood (Group 2), or both (Group 3). If you do not know to which group your clematis belongs, let it grow for a year without pruning and watch it closely to observe where the flowers form.

Clematis vines in Group 1 are the easiest to prune. Just before new growth begins in spring, cut all stems back to strong buds within 12 inches of the ground. This will stimulate growth that will bear flowers toward the end of new shoots later in the year.

Lauren provided enough information to identify her vine as likely a Group 1 (possibly a Group 3) clematis, writing that she has been trimming it back in the fall as she does other perennials and it is doing nicely. She is concerned, however, that pruning has kept it contained, kept it from growing. She is correct; cutting the vine back to within a foot or so of the ground every year will keep it relatively short. If she wants the vine to grow taller, flowering in the canopy of a tree or at the top of a tall trellis, she should not prune it as close to the ground.

If Lauren’s clematis belongs to Group 3, her practice of fall pruning means she is missing the spring flowers that form either on 1-year-old old wood or from short shoots growing off 1-year-old wood. This is the trickiest group to handle. Cutting back hard before growth begins, you miss the early flowers, while cutting back after the spring flush of flowers will sacrifice later flowers that form on long shoots of the current season’s growth. One approach for this group would be to not prune at all, but this could lead to a tangled mess.

Group 3 clematis can be contained by cutting back the entire plant severely every few years just before growth begins, with little or no pruning in the intervening years. With this approach, you miss only the early flowers of one season.

Clematis vines in Group 2 flower early in the season in the leaf axils of the previous year’s stems. Vines of this group should be pruned as soon as the flowers fade, but avoid reducing the entire plant to a framework of thick old stems that may not resprout after severe pruning. Instead, cut back only some of the stems each year, always maintaining at least one young replacement stem ready in case the older stems die. Make each cut above a vigorous bud and avoid cutting within the very old wood. This means, of course, that the vine will get a little larger every year.

To prune a clematis vine properly, you do not need to know what species, hybrid or variety you have, only when it flowers. If you do know what type of clematis you have, you can discover to which pruning group it belongs in Lee Reich’s outstanding book on pruning, “The Pruning Book” (The Taunton Press, 1997). It is a required text in all of my pruning courses.