Clematis the Queen of all flowering vines, can be cut back to control the size but the best time of year to do this depends on the variety. In general the evergreen or early to bloom clematis such as the Clematis armandi should be cut back after the finish flowering. Summer bloomers pruned in early spring and if you know the name of the clematis type you have then I can be more specific. If in doubt cut back by one third or less and you won't kill the clematis. In my garden I also cut off long, wild clematis growth during the summer right after the vines blooms. Another option is to never prune your clematis. Some I just let scramble up a tree or over an arbor and the vines grow long and wild and bloom well. I fertilize my clematis every spring with Osmocote slow release plant food. Check out my YouTube videos to see more clematis pruning tips and views of my garden.  Keep growing, Marianne Binett