Lyn-- I will tell you what works in my area Texas 8A. Because Clematis like damp not soggy and you need a trellis or support, I would go with a bigger pot. Here I try to use containers at least 16 to 18 inches or bigger so if a watering is missed the plant will be okay. I fill the bottom with pinecones and compost if you have it, which help keep even moisture in the pot. Then a good potting mix, but not a moisture control one. Because clematis have great root systems if it starts looking droopy and has been watered, it may need an occasional soaking from the bottom. Hope this has been helpful.
Edited to say : My clematis seem to be heavy feeders. They are around roses here and get what the roses get. Fertilizer, Epsom Salts, Alfalfa Tea, Compost etc.
May you and your home have an overabundance of blessings!
