I'd try an annual vine - they grow very quickly, flower profusely and generally have more 'fullness' than a clematis will achieve in a similar length of time. And they are far better suited to a smaller container.

Thanks gardengal48. I've been thinking about Morning Glory...our neighbor has it and it does grow like a weed.

Morning Glory is a great choice for summer beauty/coverage on a balcony/terrace. You won't have all the babies that would come up in your yard next year. But do you want it to stay green in the winter? Or do you stay inside in 6a & not care? I've checked on my favorite vines & they are not hardy in z6. When you find out here what will grow fast enough for you, you might look at Brushwood Nursery in PA that sells only vines & climbers: http://www.gardenvines.com/shop/

I agree with the morning glory suggestion. They grow very fast and you won't have the reseeding problem on a balcony. However they will wilt if not provided with enough water. I'd get the deepest pot possible, something tall and plastic but with a narrow opening to minimize evaporation.

I grow morning glories in pots (16") and they do wonderfully. They do like lots of water, but will quickly cover a trellis or fence. In our zone 6 they start blooming around August and continue through October.