I live in Colorado, zone 5. I cut my clematis to the ground last fall. It came back healthy, but a tangled mess of 4 foot vines that I cannot separate, Will I damage them if I cut them back to 6-9 inches?
Some clematis bloom early on last years growth so cutting them back now may forfeit this year’s bloom. Perhaps a better approach may be to thin out the vines a little and train the strongest growth up your trellis or support. Try to establish upright growth and see what happens this season. If it blooms you will know that you can cut it back hard every year if you need to. Good luck.