I wouldn't prune all of the canes. If you would like a multistem shrub, cut all of the smaller canes back to the ground, leaving 3-4 of the more sturdy ones. They can also be trained as a standard or tree form, by leaving a single upright cane. In that case, select the largest one to remain. They all may appear dead now, but wait until foliage begins to appear and I believe you will find that they are very much alive. If the stems have branches, some of those may be dead, which is a normal occurance with paniculatas, and can be removed after the plant has produced foliage. Rb

I think I might have misinterpreted your question. When you said "hydrangea tree, I thought you were referring to the PeeGee variety, which is often trained as a tree. (You can buy it from nurseries already started as a tree form.) You can read up on PeeGee and see a few photos here. If you were referring to potting any type hydrangea, be it PeeGee 'tree' form or any 'bush' form, then the answer to your initial question in general is "yes". PeeGee is tough and will withstand our winters and bloom just fine. But other varieties, it's touch and go depending on our weather. So that's why I pot my more unusual or delicate varieties, so they have protection in the garage. I find Target has nice decorative plastic pots that aren't terribly expensive. I have Nikko in the ground as well as All Summer Beauty, Domotoi and Endless Summer. It's hit or miss whether I get a good bloom show, our winter can be hard on them. This winter was Endless Summer & Domotoi's first winter in my flowerbed. ES is loaded with blooms, but Domotoi isn't showing any yet. It's a later bloomer, but still, I think it might have to be potted, just too darn cold here for it. I just went out & took some pics for you. Here's 3 of my potted ones. On the deck is a "Pia" (a dwarf variety, hardy in our zone but I wanted it potted for the deck). In the gray pot on the right is "Masja" (a beauty and full of buds this year!) and in the largest pot in the back is "Ayesha". That's the one with the drastic curly-q outer limbs. Some of my "in the ground" hyds also get curly-q on the outer stems, but not as drastic. Ayesha is already in a whopping big pot, but still would do better in a bit larger one. I forget how many gallons the Ayesha's pot is, but it's the largest size you can find at most retailers. Here's my PeeGee tree form, by the pond. Looks a little sparse right now, it's just beginning to leaf out. As you can see, it takes up minimal ground space, it's the upper branches that need space. One of the mistakes they made with it (and why it was probably reduced to $5) is they didn't prune to get the "V" high enough off the ground. I'm trying to train it to a better shape, which takes a long time. So if you decide to buy a PeeGee tree form, look for one that has has a longer trunk for balance when it matures (I'd suggest 4' from ground level, minimum). I use good potting soil (I like Fertilome's "Ultimate" potting mix, it's supposed to be a good mix for container planting). I use water crystals in the soil at planting time. Then I add a good 2" layer of mulch to the tops of the pots. I find I have to drench them regularly in the heat. I feed them with Schultz Bloom Plus for Acid Loving plants because I prefer bluer blooms. I also keep them under a tree where they get dappled sunlight. I bring them into the garage around late October (I don't let a hard frost get them). In winter I water maybe once every 10-14 days, no fertilizer since they're sleeping. And I bring them out of the garage in late April/early May. I wait till the trees have leafed out so the canopy it creates is added protection should we have a late spring frost.

"I don't hear much about growing "unique" on this forum and I haven't seen one at a nursery (that I am aware of)." That's because 'Unique' is an older form of paniculata (introduced in 1968) with very large flowers produced on a very large growing shrub and has been overwhelmed in the marketplace by the newer, showier and more compact recent intros. There is also a lot of confusion as to whether or not most plants sold as 'Unique' really are that plant :-)) A true Unique produces massive flowerheads (16" long by 10" wide) with a preponderance of very large (2") sterile florets - fertile florets are sparse and completely hidden. Unique also develops minimal pink tones with age. It is about double the size of Pinky Winky, both in the size of the shrub and the size of the individual flower panicles. Unique is a very robust and upright variety that lends itself well to training into tree form. Early in my horticultural career, Unique, together with PeeGee and Tardiva were about the only paniculatas available for planting, both as shrub forms as well as the standard or tree forms. It is only in recent years (last 30 or so, with most emphasis on the last decade) that the market has been inundated with other paniculata cultivars.

@emmarene9 Pitts are not quite as common here, at least where I am. I chose it simply because it seemed like a plant that would comply with all the demands, not be tempermental and look good. @Embothrium You stand out as the sole person to regularly rail at the idea of a plant being grown in the form of a bouquet. And I am apparently a person who can't fathom how or why anyone would, or could, find that form repulsive! Every time I read one of your attempts at describing the form, I see that you stray markedly far from the concept of a bouquet. There's nothing about the form that requires shearing other than your prejudging. Calling it deer or goat browsed sounds kind of "chewed up" looking, which would be a result of poor maintenance, which is not hardly a prescribed requirement. Bare "stems" ... eventually they get big and become trunks. Bark interest would be entirely a product of what a particular plant offers, Many have spectacular bark and exposing it to view is the only way it can be appreciated ... no differently than with any standard tree. I think your mind is locked in tight to some erroneous and inaccurate concepts -- probably because you're taking illustrations literally -- which, of course, is not how they are intended or should be taken. I've tried to inform you of that before. More than once actually. So I'm pretty sure you're not going to make the leap. I do get that you are repulsed by this form, or the idea of it. And that's OK. What puzzles me is why you insist on bringing it up over and over and over again. I would think you'd just look the other way and move on since we've been around the block about it so many times already. Are you a member of Antifa? I'm asking because that's how they do, too. They can't just disagree and go their own way. They feel like they have to shout you down and do their level best to make sure that you cannot express yourself without a lot of antagonism. At any rate, I have already long ago duly noted your objection and promise not to forget it.