Putting it in a 5 gallon pot would be wonderful and potting mix to start. I would use Osmocote Plus in a month or so according to directions. I have never heard of soaking a clematis in water. Put it in semi-shade to start.

I'm sure the other amendments would be great but I keep things simple and have not run into problems.

I agree with Shelia almost completely :-) My only qualifications to her remarks would be that I'd add the Osmocote now while potting up. I do the same for all the clematis I grow in containers. And I wouldn't add any other amendments.....just a good quality potting soil. Adding those sorts of amendments to potting soils tends to alter drainage and aeration, and not for the better.

And I would plant it deep. Remove any leaves that may be present to prevent rots but a deep planting should simulate more stemming from the root crown.

Clematis don't like to be moved or planted. If you could, I would put it straight into it's new home. Whenever I move a clematis, it seems to do almost nothing on the top. It spends the first year in its new location growing roots.

"Clematis don't like to be moved or planted"

I don't understand this statement. I have moved clems many, many times without any significant interruption in their growth or performance. Most plants will show some reaction to transplanting but with proper care, recover quickly

And they don't like to be planted??? What does that even mean?

Clematis like to be left alone. The fewer new homes the better. Seems pretty self explanatory to me.

^^^ This seems to be an assumption on your part that is not borne out by reality. Even clematis specialty websites discuss when and how to transplant.....and even divide.....an established clematis vine.

And as previously stated, I have done so many, many times in my 50 odd years of gardening, as well as being a clematis collector.

There's a lot of scientific evidence that plants do better if planted directly in the ground instead of in pots and later planted in the ground. But, you have your opinion and I have mine.

It's not a young plant. It's 5 years old. I would guess over 90% of clematis purchases go in the ground. If this didn't work, don't you think we would have heard a lot about it?

5 minutes on google yielded...

https://www.gardenersworld.com/how-to/grow-plants/how-to-grow-clematis/

"As a rule, most clematis do best in garden soil rather than in containers."

https://www.longfield-gardens.com/article/6-tips-for-growing-clematis/

"Clematis are long-lived, and they don’t like to be moved, so choose the planting site carefully."

Can you name a single clematis vendor who recommends potting up their plant instead of placing in the ground? The people who actually have money on the line if giving poor advice.

"Can you name a single clematis vendor who recommends potting up their plant instead of placing in the ground?"

No. But that is not very surprising. Why would a vendor suggest a methodology that would indicate that the plants they ship are not mature enough to planted directly in the ground? And TBH, most specialty vendors DO ship out at least 2 year old plants in quart sized or larger containers.

However, most clematis authorities - not vendors - do recommend growing on young, small vines for at least a season or two before direct planting. From the ICS: "Packaged clematis, including those from most mail-order sources, are commonly less than two years old and not very strongly rooted. Usually it is better to grow one of these on for a few months in a 1- or 2- gallon pot than to put it straight into the ground. Young clematis do not object to having their root systems confined for a while, whereas if they are abandoned in a big hole at that tender age they may have trouble getting established.

Even if your clematis is a better-rooted specimen in the standard gallon pot, you might consider burying it pot and all in the planting hole, lifting it in the Fall when it has done all it can for the year, removing the pot, and setting the plant back in. This lets the root system develop undisturbed, and also gives you a chance to move the clematis to another site, if you've had second thoughts. (Bear in mind that potted clematis may come with the stake fastened to the pot. No matter when you elect to take out the plant, remove the staple first!)"

And many, many vendors and clematis authorities promote growing clematis in containers rather than in the ground for all manner of reasons and some - like Raymond Evison - breed clematis specifically intended for that purpose. Linda Beutler, the current ICS president just did a webinar on growing clems in containers and you would need to look hard to find a clematis authority more knowledgeable or experienced than her!

You seem very adamant that your way is the only "correct" way to grow clems but almost 50 years of growing them personally and professionally (including time at a clematis specialty nursery) would indicate that is simply not true. And this is easily borne out from a host of other very reputable sources. But I have no interest in arguing this point with you. Feel free to do what ever you want but please don't come into these discussions making these absolute pronouncements that they can only be successfully grown your way. It simply isn't true and there are 100's of growers that can attest to that.

When I first started growing clematis about 20years ago, I bought them from a couple of vendors at the San Francisco Flower and Garden Show. Their recommendation was to keep in a pot for a year so their roots could develop and then plant in the ground. These were growers from Northern California.

I find beefing a clematis up by growing in a pot for a year or so really helps with survival here. I have been growing them for 40 years and a boost in a pot can really be helpful.

Can we get some photos of the plant(s)?

What city are you in?

On the issue of growing Clematis in a pot, everything depends on the species. If the species has a root system that can fit comfortably in a container, there is no limitation on its growth if it stays in a container. On the other hand, a large species like Clematis Montana is going to grow 30 feet high and wide and will have a root system the size of a large car. That plant will have limits on its growth in any container.

The above is completely separate from the point that during their first year or two, most growers keep them in containers. I am talking about long-term plantings.

While I am new to the genus, my limited experience so far has been that I can transplant them in Spring with no harm, but when I have tried to transplant between containers - with a change of the soil substrate - in late Summer or early Fall, I often get die off and a few have died completely. I started with succulents, and it has taken me a while to respect that you cannot abuse many genera the way that you can abuse succulents and cactus.

In a container, you cannot let Clematis go dry for more than a few days or they die off quickly. I lost a few over Winter that I let dry up.

Here is the Container Clematis Youtube video that @gardengal48 (PNW Z8/9) referenced:

https://www.youtube.com/watch?v=HxAlSMYvj-o

They marked their content as "for kids" and as a result you cannot save the video on Youtube. Really aggravating mistake, and when you try to explain this to them they argue with you that they marked it correctly. I hate to point out to them that in their list of Zoom conference participants most of the faces look over 60 years old, none of them look under 30, and zero of them look like kids.