Where are you located Stephanie? In my part of the country it is too cold to have clematis in pots outdoors in winter, so I would want a bright spot near freezing for them to do well. If you are in a warmer part of the world like BorS in the PNW, that wouldn’t be an issue.

10” deep seems too shallow to me for a clematis since you should plant them deeper than they were in the pot by several inches. I don’t necessarily think that using smaller pots for smaller plants is a good idea for clematis since they tend to have good sized root systems. I grow my nonvining clematis like Sapphire Indigo up the center of a tall ringed support like a rose obelisk or leaning against a shrub because I think they look better than sprawling, so I would be more likely to get a large diameter, deeper pot than your earth box.

Piilu gets to between 5’ & 6’ in my garden and I grow it on a 5’ obelisk so it grows up and then either flops or travels to a nearby obelisk. It would also be fine on a flat trellis. Regardless, climbing clematis need to be able to grab something not much thicker than a pencil, so something with fat bars won’t work well.

Thanks so much for your helpful replies so far! I am in Washington DC. The deck gets a lot of full sun and no shade. The patio underneath does have part-shade areas. I will go ahead and nix the idea of using the earthboxes (I was hoping to put them to use somehow, but I will just put them away in shed for now). Do I really need 20 inch diameter pots for both varieties though? I'm just not sure my patio can handle four more large pots. And will either one do ok in full sun or do either of them need part shade?

Alternatively, can I plant the clematis (one Sapphire Indigo clematis and one Piilu Bi-color clematis) together in the same pot? If I got a large enough pot (how big)?

I am hoping gardengal48 will weigh in on how big is big enough and whether two in a pot will work because she has more experience with pot culture. But pots in the DC area should be fine. You will want to keep a close eye on moisture to keep them happy, and you might want to check out mixes for long term container growth on the container forum. Regular potting soil breaks down over time and loses pore space for movement of air and water, so often materials that will help retain pore spaces is added to the mix used in long term pots.

My clematis do best in full sun, but I am farther north, so I don’t know if in your area they benefit from part shade or not. Is there room just off the patio for a paver you can set the pots of clematis on? Or use two of the clematis on the patio and two on the deck?

To extend interest in pots with large perennials, I often plant something with long term flowers and/or contrasting foliage that has shallow roots which won’t compete with the larger, deep rooted plant such as annual candytuft, Veronica Georgia Blue, golden Irish moss, or golden creeping Jenny. This adds interest during the times between bloom.

IME Earthboxes are great for growing greens, peas, beans, onions, and other relatively shallow rooted veggies or annual flowers like marigolds, petunias, and other flowering annuals often grown in pots. Not so great for perennials or woody plants IMO.

I know you posted to the container gardening forum, but here is the direct link. You can look at posts that talk abou container soil mixes like 511.

https://www.houzz.com/discussions/container-gardening-dsbr0-bd~t_29858

I’ve grown clematis in pots in Philadelphia for years. I had a Henyri for a decade. I use regular potting soil; 511 is too loose for my area. I have a very elaborate container garden and grow everything from roses to bamboo and hydrangeas. I never ask “can I,” I just try it. It usually works. You have to use big pots, though, and fertilize regularly.

Hummingbird has an informative page re how to grow clems in containers, you might find it helpful. They are in the far north, so you may not need containers as big as their own minimum when it comes to wintering. That said, it also depends on cultivars plus big ones are better for root growth anyway.

FWIW I am in 7a and use ones at least 20" for clems, and protect them in winter.

All of my clems are grown in containers. I use big ones, as the vines develop big root systems....7G is about my minimum size and some are even larger, depending on the size of the vine. My largest is probably about 20G. I use the 5-1-1 mix as I do for all my containerized plants (a big collection of trees, shrubs and vines and couple of mixed perennial pots). I water often and fertilize every spring with Osmocote and often hit them during the growing season with a water soluble fert (Dyna Gro) as well.

Winter protection is not necessary in my climate......which is a good thing, as I have no idea what I'd do with 40+ containers in winter!!

I have grown hf young and vile de Lyon in a 12" container but I move it indoors for winter