Most single hydrangeas tend to reach and exceed the width of 3 feet some I suspect that the root system will comingle and compete for water and nutrients. Perhaps a half wine barrel with sufficient water drainage holes, instead?

Thanks Luis_pr! I was thinking that it might be tough to return/exchange this gigantic trough since I bought it through the mail! So I was hoping to put a plant in it one way or the other but wanted to do it in such a way that it would look pleasant while I'm waiting for it to grow. I wonder if some smaller annuals might fill out the sides of the pot at first? Would that be a viable strategy?

That is what some people often do at first or when they get panicles that lose foliage near the base in most summers.

My understanding of the term "frost proof" when used on containers is that the container itself won't be destroyed/damaged if the temperatures reach frost level. So you can leave it outside year round. It doesn't mean that it will protect the plants inside from frost.

Where do you live? USDA grow zone or nearest major city. And what type of hydrangea?

Hi Popmama - I believe I'm in zone 6b (Boston)

I assume the type of hydrangea you want to put in your pots is a big-leaf macrophylla type hydrangea like Endless Summer or Bloomstruck. You are a full zone above me in an area that is more well known for people growing big-leaf hydrangeas. You could grow two in that pot for the summer with some annuals as floral_uk suggests. Then plant them out in your yard in the Fall.

In my area, no way they would survive in a pot over the winter unless you put that pot in a garage or shed and water it periodically through the winter.

Thanks Popmama - yes, big leaf hydrangeas. I ultimately found out that I can still return the very large pot (the 3 foot wide one) as UPS will pick it up for me. Given this, I am ordering a more suitable container (still frost proof and somewhat insulated but more of a single tree/shrub kinda pot). This is preferable to me as frankly, when the other pot arrived, it looked like a cooler or a bathtub to me, lol. It was rectangular and kinda huge. I'm still getting an XL sized pot but it's a round one that I think will look lovely with a shrub in it. Thanks for your suggestion, though. I wish we had a better yard for them. I could still give the plant away to someone who has a better yard, though, once it outgrows its pot.