Posted: 11/06/2013 at 15:07
That pot is way too small and is also thin plastic which means it will heat up quickly in sunshine and freeze easily in winter thus alternately cookin,g or freezing the roots.
If you can't plant it in the ground, I suggest you get yourself a decent ceramic, frost proof pot at least 60cm wide and deep and then transfer your clem, planting it 4" deeper than it is now. Alternately, use one of those terracotta look alike plastic pots but line the inside with bubble wrap as heat and cold insulation. Use the best quality compost you can afford - John Innes no 3 with some added fibrous matter from a peat free Levington's type compost. Mix some slow release fertilser into the compost before planting - blood, fish and bone or pelleted chicken manure - and make sure your clematis is kept watered.
Give it a liquid feed at least once a week during the growing season and a boost of specialist clematis feed as a top dressing every spring.