Do you have to get clematis in the ground for winter freezes? We do get frosty mornings here, but usually not any "all day" freezes. I have several in pots and have not over wintered them before.
Clematis in pots
Here they freeze when their left in pots...Your in a higher zone so it might work for you..I'm in zone 5 and we get lots of snow and ice..
According to the British Clematis Society, http://www.britishclematis.org.uk/containers.html
Type:
Containers made from wood, stone or earthenware (terracotta) provide excellent root protection against summer heat and winter cold. While containers made from plastic fail to insulate the roots of clematis when subjected to extremes of temperature, they can give years of trouble-free service provided they are sited very carefully.
Special attention should be paid to ensure that the holes for drainage are adequate. If necessary, the existing holes should be enlarged so that the clematis roots do not become waterlogged.
Size:
Your container should be at least 45cm (18") deep and 30-45cm (12-18") wide. The larger and deeper the container, the better.
Thanks for the advice!