Welcome Rosti. I am in Washington as well, but on the wet side. I have grown autumn clematis with no invasiveness problem. It did get very thick and robust (which sounds ideal for your situation), but I ended up digging it out because I had planted it to grow up within a mature lilac and after a couple snowy winters, it just weighed the lilac branches down too much. If you planted it on a strong trellis, that might be a good choice. Virginia creeper also comes to mind, or an ivy other than English (which IS invasive in Washington). Here's a link to your county to double check invasiveness before you plant:
http://wnps.org/invasive_speci...
Good luck, and let us know what you decide on.
