Not so easy to discover the clematis with a particularly long flowering period within the richly stocked Clematis family. There are 4 species that can adorn themselves with this attribute. You can find out what these are here:

Clematis viticella
The Italian clematis should not be missing in any garden. Their unsurpassed flowering splendor extends from June to September. In addition, the species is completely hardy and thrives in almost any location.
Clematis texensis
If you are looking for the children of the sun among the clematis, you will find them in this species. Where it is sunny, warm and sheltered, this clematis unfolds its colorful splendor from June until well into October
Clematis orientalis and tangutica
The members of this clematis line impress with yellow flowers from June to October/November and decorative seed heads in autumn, which also serve as winter decorations in the garden.
tips and tricks
All summer-blooming clematis with a long flowering period belong to pruning group 3 and are pruned back in November/December or early spring.