Tree roots will grow up through even the tiniest holes into a raised bed, so I honestly don’t see that as a solution since clematis don’t do well with root competition. I can’t imagine growing a clematis as large as one of the montanas in a container because supplying moisture for that large a plant would be difficult and because changing out the soil every few years would be a challenge. Is there a spot farther down the fence where you can plant it and then train it to fill the area where you want to see it?

In my garden in areas with tree roots, I do very large pots of annuals or really tough perennials, and I have to either put a saucer under the pot or raise the pot on feet of bricks so that tree roots don’t grow up through the drainage holes and fill the pot with roots.

I don't know, Babs. I spoke with a clematis guy from Clematis mail order site who writes articles, etc. He seemed to think that 2ft by 2 ft and a foot or more could work, so I will give it a try. These are established roots several inches thick so they won't be changing direction and growing vertically. There is no other area that would work due to my neighbor's trees as well as mine, and this is a empty space on the fence between two Sweet autumns. I actually have never changed the soil i in the bottomless sandbox where I planted a schizophragma Moonlight (very similiar in appearance to a climbing hydrangea,) and that was 15 years ago or so. The proper soil composition , providing nutrients and keeping it moist will be critical factors. If it works, it will be lovely.

“These are established roots several inches thick so they won't be changing direction and growing vertically. ”

In my case, the red maple grew new roots and sent them up into a whiskey barrel that was sitting on the ground under the tree. After just a few years the entire container was one tight mass of fine roots, so much so that I had to use pruners under the pot to cut the roots that had grown to 1/4”diameter as they went up into the pot so that I could tip the pot to remove the pot-shaped mass of roots and put in new, useable soil. This was under an established tree with large roots that had been in place ~30? years. Just sharing my unfortunate experience.

I'd have to agree with Babs. It is not the established roots you need to be concerned about but the much smaller and often very dense feeder roots. These are being generated and regenerated constantly and will infiltrate any good soil that becomes available.

My inherited garden has raised beds constructed 20-30 feet away from any trees yet the feeder roots are a constant problem in this bed. As I use it primarily for annual crops and edibles, I dig out masses of dense, congested but tiny feeder roots every spring before planting for the season. This of course would not be necessary for the clematis but you may very well need to spend much closer attention to water and nutrient needs.

I guess time will tell with the roots, it's a small area of 2 by 2 and is 6 feet from the tree. Perhaps because it is a small raised bed it might be less problematic. Althought not a clematis, my schizophrama moonlight has done outstandlingly in a bottomless plastic sandbox immediately next to the pine tree which it has grown upon. I am following Dan Long's suggestions on how to' do this from Brushwood Clematis and keeping my fingers crossed. Yes, it will definitely need to be monitored for water and nutrients.