Answer: There are different bloom periods for different types of clematis. Cultural conditions and the weather can also contribute to a longer bloom season, as can pruning.
If your clematis blooms on old wood, it will bloom early in the season. Some varieties will rebloom later in the season on the current year's growth as well. By careful pruning of these plants, ie prune back about a third of the plant in early spring to force blooms in mid season, you may be able to extend the apparent season.
Clematis plants do well in a cool and evenly moist soil that is well drained. Adequate soil moisture and high fertility will help the plants bloom to their fullest potential, especially those that bloom on new growth only. The longer you can keep them in active growth the longer they will bloom.
Finally, some gardeners will plant two different types together to try to maximize the impact.
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