I cannot stress too much that proper pruning of these young plants is absolutely critical for their future growth and development. All too often I have seen a perfectly good little clematis which has been planted out and allowed to develop one very long, spindly growth. And the owners have complained that either it's a weak plant or it doesn't flower well when the only real problem is how it's been treated. A plant like this will take years to develop properly if it ever does. Roots and top growth need to develop proportionately.
Top growth needs to be restrained until root growth is sufficient to support it.
This will also encourage both branching and the development of multiple stems from the buds under ground. This is particularly important for group B clematis which are notoriously reluctant to fatten up at the base. Two or more years of this treatment may be necessary for the plant to develop a satisfactory framework, but the rewards in terms of future flowering and general appearance are well worth it. During this time flowering is not sacrificed, rather delayed until later in the season. Late season pruning of clematis is not recommended in areas which experience cold winters.