When to Plant Your Clematis    http://www.homeofclematis.net/html/pl_02.htm

Whenever the ground is workable clematis can be planted. Clematis react to the seasons much like bulbs. In the summer and fall there is vigorous root growth storing energy for top growth and flowers the following spring. Come spring, clematis will produce two or three feet of top growth before you notice any root activity. For this reason, in mild regions fall planting is ideal. If one chooses to plant in spring or summer, it is useful to periodically pinch out the growing tips.

If you choose to plant in the fall, you will want to prune the following spring, no matter which pruning group your clematis belongs to. Although this will inhibit flowering for the first year, it will help promote root development and produce a bushier plant.

If planting is done in the heat of the summer, it is essential that sufficient water is provided to keep roots moist and cool.