Posted: 18/03/2014 at 17:44
More or less any clematis can be trained to grow horizontallly rather than vertically so you really do have to find one that won't exceed the wall length by too much.
I suggest you have a look at alba luxurians, caerulea luxurians, alpina, Beauty of Worcester, Betty Corning (scented), Brunette, Cicciolina, Gravetye Beauty, Hagley hybrid, Jackmanii, Little Nell, Madame Julia Correvon, Perle d'Azur, Polish Spirit, Red Ballon, Sliver Moon, Sunset .
Plenty of variety in form and colour and flowering period. If you do plant one at eache end of the wall make sure they are form teh same pruning group or you'll get in a terrible muddle once they've settled in and start to cover the wall.
I find alpina and Brunette are not hardy enough for my garden but I have all the others doing well.