Bumping this down. Will do the 5 gal.

5G would be the minimum size I'd consider for any long term container growth of clematis. And I'd not expect that leaving it alone in that size container for 10 years or so is doing it any favors :-)

Even though ES is a more compact form, clematis vines of pretty much any size can produce some seriously large root systems. And the bigger the vine, the larger that root system can be. Having a plant rootbound in a container is putting it at an extreme disadvantage - root congestion stunts growth and too high a root to soil ratio increases the frequency of watering and fertilizing required.

If you simply cannot plant in the ground - the preferred choice - then go for the 5G pot at a minimum. And do not wait 10 years before repotting - after 3-4 years, pull, root prune, refresh with new potting soil and replant. And don't forget that half way sunken in the ground or not, this is still a container plant and will need more careful attention to watering and fertilizing than an inground plant would.

Thank You Gardengal. I had no idea they had such large root systems. They seem like such gentle plants. This week into the 5 gal. for a year (from its present 3 gal.) and then to a larger size.

Didn't know they could be root pruned. Yep. All potted plants get watered with very diluted fertilizer. Except succulents!