Answer: The only white flowered clematis that would be cut to the ground in late fall or late winter/early spring would be the sweet autumn clematis, it has fragrant, starry white flowers and blooms in the fall.
The other clematis would be pruned in the spring, but how hard to prune it depends on what variety it is. If in doubt, trim off any winter killed stems and trim the rest to about three feet. Then watch the blooming pattern and timing to determine when the best time to prune would be.
Here is a description of general clematis care and pruning. Your plant is most likely in Group B or C because it has large flowers. You may need to cut and paste the complete url into your browser to make it work correctly.
Enjoy your clematis!
Q&A Library Searching Tips