C. President is in pruning group 2, Nellie, so should never be cut down to ground level (in fact, no Clematis should ever be cit down to ground level). In its first few years it should be cut down by no more than a foot or so, ie. just above a pair of leaf-buds to encourage branching, and this should be done in spring. Once the plant has branched and is established, you should retain the framework and only give it minimal pruning. Check out this link for full information:

https://www.rhs.org.uk/advice/profile?PID=346#section-2

Optimal pot size for a clematis is 18" in height and diameter. As C. President isn't one that can be cut down each year, it should have been planted in a pot of this size straight away to avoid any future disturbance. What size pot are you using at the moment?

Oops! That should of course read 'In its first few years it should be cut down to no more than a foot or so above the ground', NOT 'cut down by about a foot...'...!

My typing fingers obviously had a mind of their own yesterday...!