“We have several clematis vines in our garden and do not know their names. Do you advise cutting them this fall or leaving them alone?”
— Laura Anderson, Deerfield
Different species and varieties of clematis have more specific pruning requirements for best results. If left unpruned for long periods of time, clematis vines can become overgrown with dead stems and dense growth that produce few flowers.
More vigorous varieties, like sweet autumn clematis, can overrun support structures and adjacent plants with flowers higher up and less visible. Clematis that have been planted in the last three years will likely need little to no pruning, other than pruning out any dead stems or nipping back a fast-growing stem, as they are getting established.
Group one clematis bloom in spring on growth from the previous year. If any of your clematis vines have a big flush of flowers in spring and no later flowers, then they are likely in this group. Aggressively pruning these vines in fall or early spring before they flower will reduce or eliminate flowering, so it is best to prune as needed, right after they have finished flowering in spring.
Prune to thin any dense tangles of stems to open up the remaining stems to light and more air circulation — how aggressive you prune will depend on the vine’s vigor and type of support it is growing on. This will stimulate new growth that will produce flowers the following year.
Group two clematis bloom in late spring or early summer with sporadic flowers on new shoots and old stems later in the season. The vines that mostly bloom on old stems will flower heaviest in late spring, and those that flower mostly on new shoots will bloom heaviest in summer.
For these clematis, lightly prune to thin out the stems before growth begins in spring and then again, if needed, when the first flush of flowers begins to fade. The stems with flowers can be cut back. If any of your vines flower more heavily later in the season, then you can be more aggressive when pruning before growth begins. If any of the clematis are overgrown, cut the vine back by about half to rejuvenate it while sacrificing the earliest blooms that occur on the old stems.
Group three clematis flower in late summer or fall on new growth produced that season. These vines should be pruned just before growth begins in the spring and can be pruned aggressively if needed. Since they flower on new growth, you will still get good flowering from healthy vines. It’s also OK to let the more aggressive vines ramble in your garden without cutting them back if desired. Some varieties will flower earlier and for a longer period of time if pruning is minimized.
Most clematis I see growing in home gardens are not in need of much pruning, except for sweet autumn clematis, which can become a large brush heap if not cut back on occasion in spring. You may be just fine without doing much pruning on your clematis vines, other than a more drastic pruning at the expense of some blooms in the short term every few years. Reduced flowering of a clematis vine can also be due to increasing levels of shade in a garden as adjacent trees grow.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at [email protected]. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.