Posted: 01/06/2015 at 22:55
Clematis roots are thick and fleshy and like deep, cool root runs. It is always recommended that they be planted at least 4" deeper than they were in their pot as this encourages them to form extra shoots and increase flower power. Your pot will need to be at least 60cms wide and deep and preferably deeper if your clematis is to stay there more than a couple of years.
Clematis are gross feeders so you will need a very good compost to which you will have to add extra slow release food for clematis or roses every spring as well as regular watering and occasional liquid feeds throughout the growing season.
Clematis cirrhosa is hardy down to -5C and armandii will cope down to -10C but neither will like having frozen roots so, depending on how cold your winters are, you may get away with just protecting the pot with external bubble wrap well ahead of any cold spell.