Posted: 07/04/2016 at 07:56
Paul - for clematis, it's the depth that's more important really to allow roots to get down deep. It's why clematis don't do so well in shallow areas. When potting on any plant, always go just a size 'up' if you know what I mean - ie, don't take a plant in a 3" pot and put it in a 10" one. It allows roots to develop evenly and fill the pot nicely and support top growth. Keeping them sheltered from weather while small is also important, whatever the plant, until they're big enough for the big bad world
Just check what aspect your particular clematis do best in as well, when you plant them out. Some are happy in any aspect, some need a bit of shade or the flowers bleach, some need a sheltered position. Giving them the optimum conditions will give them the best chance to do well.