I live in South Dakota and planted a young, 18 inch tall golden clematis (clematis tangutica) in the fall of 2007. It is now about 13 feet tall and 8 feet wide. It is a beautiful plant and I love it. It has gotten very bushy on the top and I would like to prune it a bit. Should this be done in the fall, winter or spring? And do I need to be careful how much I take off? Thanks for any help you can provide.