Hi Kim, couple of ideas. First, Clematis like their roots shaded, so bear that in mind. They also need regular watering. I believe Sweet Autumn is considered invasive in some areas as it readily escapes into the wild (though it is pretty). If you are adventurous, a stunning combo is New Dawn rose with any deep purple clematis! I think Wayside Gardening and White Flower Farms both offer really nice plant selection and quality. Best of luck!

Sorry you had to wait so long for a response :-) If you had posted this in the Clematis forum, you would have received multiple fast responses!!

Most clematis have big root systems to support all that top growth so you will need big containers....smaller containers will hamper growth and you will not get the height you are looking for.

The 'shaded roots' thing is a clematis myth. What they need is a consistently moist root run and while a shaded root area can help to maintain soil moisture (less evaporation), the shading won't achieve anything if the soil is dry.

Clematis terniflora, sweet autumn clematis, is a house eating moose!! This is a huge, rapidly growing vine that consumes a lot of real estate and not at all suited for a container. It also has a very late season bloom (hence the common name) so will not be in bloom for more than a few weeks in late summer or early fall. And as with other potential invasives, it is not your neighbors you need to be concerned about, but the vine spreading or seeding into natural areas.

One of the easiest, most popular and long booming purple clematis is 'Jackmanii'. Starting in late June or so, it will bloom until frost. It should manage the 10' columns even in a container but how much it might spread horizontally is up for grabs.

While most mail order nurseries will offer a handful of clematis vines, you will get a much larger selection from specialty clematis nurseries, like Brushwood, Joy Creek or Silver Star Vinery.