Gotta disagree on the Clematis:  I cut mine way back by pulling on the vines until it was about where I wanted it; it had gotten out of hand.  It came back with a vengeance and actually grew past where it was before I trimmed it.  I did not cut it all down by the woody stem tho.  Also, it didn't bloom as before because all the growth was new stuff. So you can trim it back a bit, I have to every year or it would take over the whole yard!