Posted: 09/05/2014 at 13:42
I find the group 3s work best for me in my sunny south facing border, or group 2s that can be treated as group 3s which means cutting them back hard in spring, giving them a feed and then watching them grow and flower over summer - training them as they develop.
I have Betty Corning, Princess Diana, Sunset, Arabella (non self clinging), Chrystal Fountain and Hendryetta (non clinging) all very happy and their flowers don't fade in the sun. I also have Dr Ruppel, Omoshiro, Star of India and Westerplatte in beds to the north of the house but which get full sun from March to October and they'd be good on your obelisk too.
I also have Red Robin which is a group 1, flowering now with lovely whorled seed heads to follow. It just gets pruned to keep it in bounds on its clematis panel.
One clematis per obelisk is ample as over the years their vigour will increase and they'll easily cover it. Do not mix tow clems from 2 runing groups as you'll never untangle or distinguish which stems to prune and which to leave.