Answer: A newly planted clematis usually will not need pruning for a few years. They tend to become established slowly and may take three years or so to reach their normal growth rate and blooming potential. Eight to twelve feet would not be unusual for most types of clematis, although some such as the fall blooming clematis can reach 25 feet or more.
Pruning clematis depends on when it blooms. Some varieties bloom on new wood; these can be pruned back very short each spring or not pruned at all. Some bloom only on wood from the previous year, and these would not be pruned until after they have bloomed for the season. Some will bloom early in the season on older wood and then later in the season bloom again on the current year's growth. These can be pruned selectively to control size and encourage maximum blooming over a longer period.
Unfortunately, there are many white flowered clematis so you will need to observe your plant in order to decide how to prune it.
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