I've actually cut my clametis back to the ground after in 5 years because it was getting overgrown and leggy. It is in full sun all day. I usually wait unti it sends out leaves to know what is still green. I cut back any dead branches. Make sure you water it well if you have periods of no rain and fertilize with a weak solution of a high phosphorous fertilizer weekly all summer. This will promote more flower this season and next.