I wouldnt even try to grow Armandii in a pot. They are very vigorous. I put one at the side of our summer house and it has grown to the top and is now covering half of the roof within 2 years.
So, my advice would be to find somewhere suitable to plant it in the ground and maybe buy a new clematis more suitable for a pot, which would still need to be much bigger than the one shown.
I’ve seen one Armandii cover a summerhouse roof and it’s now three quarters of the way up a 40 year old cedrus deodara ... not quite as big as this one yet but heading that way ... https://www.flickr.com/photos/29476346@N08/3438866071
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
I’ve seen one Armandii cover a summerhouse roof and it’s now three quarters of the way up a 40 year old cedrus deodara ... not quite as big as this one yet but heading that way ... https://www.flickr.com/photos/29476346@N08/3438866071
That it could! The flower bed is very narrow, only 8 inches wide. It is all open dirt other than about 5 inches if gravel just under the blocks. The cement wall goes down about 4 feet as well. It is actually my garage wall. It was my intent to plant it there if it was growing well enough. I wanted something temporary to test out how well it did in the indirect lighting there.
The trellis does have more to the top part of it out of frame, but not much. I have my deck rails directly next to it though that the clematis can fill out onto. That was the thought at least. I also have a fence on the other side I can trellis it onto as well.
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So, my advice would be to find somewhere suitable to plant it in the ground and maybe buy a new clematis more suitable for a pot, which would still need to be much bigger than the one shown.
https://www.flickr.com/photos/29476346@N08/3438866071
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.