I would put the pot in a protected location, like the garage if door is closed all day. Hydrangeas are hardy outside, in the ground, because dirt ususally stays the same temp for winter. Snow protection is very helpful with that. In a pot, the dirt is REALLY cold, experiences more fluctuation in temps as pot sides get cold or warm from daylight, cold wind. Pot may crack with freeze/thaw cycle and dirt expansion. So with protection, the potted shrub may make it thru winter. Do you have a hole in dirt you could put hydrangea into outside, then fill around it with dirt, mulch, until ground is workable in Spring? Might be an easier solution, temps would be more stable than garage. Maybe sidewalk redo folks could make you a hole in frozen dirt for bush to stay until spring? Too bad about the timing.

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.

Pick a strong vertical stem/leader and cut everything else. Periodically clean all side shoots from that leader up to 3-4' height and let rest grow. Not all 'shrub' PG are suitable to be grown into 'tree' PG. You have to pick one that already have a defined leader. Since tree PG command higher prices than shrubs most 'strong leader' shrubs have been already selected out from the stock by growers and in commerce you'll see shrubs that are good to be grown as a shrubs, not a single trunk trees.

Most likely neglected 'Tardiva' in tree form. Basic advise would be to remove now anything growing below 'graft point' (which is not a truly 'graft point', but just a 'cut point' in reality) and...post a picture of it in a leafless state (Nov-March) to see how it could be shaped/refined. However, I have a suspicion that it could be easily converted back to a shrub form, if desired. In this case don't remove this large stem growing from the base and....post a picture of it in a leafless state (Nov-March) to see how it could be shaped/refined. :-))