Posted: 12/02/2016 at 12:40
Clematis are very hungry, thirsty plants which need deep, cool root runs so a planter on a south facing wall is not going to be ideal. I would suggest instead a pyracantha which is evergreen and can be trained to cover a trellis and will provide year round interest. It will have blossom in spring and berries in autumn and green foliage all year and not require much pruning except to take out stems that insist on growing away from the wall.
A repeat flowering climbing or rambling rose is another option but will need regular feeding and dead heading to keep looking good.
Whatever you choose, buy or make the biggest planter you can afford and fit in the space and fill it with John Innes no 3 compost after putting crocks over the drainage holes to stop it getting blocked and water-logged. Top dress with extra fertiliser every spring and water regularly especially in hot weather and strong, drying winds.