Clematis- Elsa spath

All clematis are better in the ground than in a pot - they need a nice cool root run, preferably in shade, with the tops in the sun, so if you can provide a tall, large pot that's shaded, with the heads of the plant in the sun, then put it in a container, but I'd recommend in the ground every time. Remember they're not twining plants, they need a thin support to hook their leaves around, so a thick trellis no good, get some clematis mesh or similar, fix the support in place (it'll need to be as high and wide as the plant grows), then plant your clematis. Half a day's sun will be okay, but you might be better with Hagley Hybrid or Nelly Moser - these prefer a lightly shaded position, less sun than most. They, however, flower earlier than Elsa Spath, so I'd chance it with a Group 2. (Someone's done a blog recently on clematis pruning and the Grouping system, check the blogs)