What type of clematis have you attempted to grow in the past? The type III clematis, especially the viticellas, are better suited for the south as the type IIs can often brown out in the heat of the summer in my experience. Also, for your pot, choose one the largest possible sized ones you can get. Clematis' root systems can get reasonably large over time and a smaller pot is going to require watering multiple times per day and even still, the clematis is not going to thrive if its root system is limited in size. Is there some reason you can't plant the clematis in the ground at the base of one of the posts on your pergola? I think you would have much better luck and a better experience than trying to grow them in a pot.

nckvilldudes, thank you for your response. The reason I can't plant it in the ground is the deck is concrete and 4 feet off the ground. In building the pergola my husband poured concrete around the post for extra stability. In the winter we get very high winds up here. So I guess if they need very large pots it will not be feasible for me to plant one.
I did not care what type I had just thought it would be enjoyable to look at and smell.