Bonnie, I found out accidentally that clematis vines will root well by layering. I have a couple that had crept along the ground and by the time I saw that growth they had rooted- I went to pick it up and attach it to the trellis and it wouldn't budge! The woody stems seem to root best, so if you can lay a piece of the vine horizontal and pin it to the soil it should root by fall. I'm sure some rooting hormone would help speed the process. If I'm intentionally layering I usually lightly scrape the part of the stem that will make contact with soil with a knife, preferably at a leaf node, dust the wound with rooting powder, then pin it to the ground. A little soil over top of the wound is helpful too- it keeps it from drying out. I think mine had some mulch tossed over them that achieved the same effect.
