I would get a bag of cheap potting soil and get enough fertilizer in there so that the root ball can quickly fill the pot. If the dirt you use is too heavy then the pot will weigh a lot and the roots will have a harder time expanding. Then if you plant it in the ground later use 50/50 whats in the pot + native soil. If you get any saplings just rip them out and the deeper seeds will decompose.

Neither your garden soil or your compost in a pot are going to get your hydrangea off to a good healthy start - they won't provide the right aeration or drainage in a container to encourage good root development. Better is to make or buy a product formulated for use in pots - potting soil. Basically none on the market contain any soil at all, but bulk ingredient usually peat, ground bark, coir, something that will provide aeration without compacting as garden soil in a pot tends to do.

Unlike hydrangeas in the ground, I do fertilize those in pots, but sparingly - using no more than about half the amount listed on product label directions.

I guess I should have clarified that I am wanting to pot it up for the meantime (3 months or so) until I can prep the bed it is going to go in. I will just cave in and go buy a bag of potting soil which is what I was trying to avoid :)

Springwood, when you say so that the root ball can quickly fill the pot, how quick are we talking about? Since it is a bare root I don't want to have to transplant it later so I was thinking of using a pot so that it could develope the root system and then I could plant it in it's permanent place once it is ready.

Another option would be to make time to prep the new bed but I am already behind on my other garden chores :(

Good luck, Melaroma! I would be interested in hearing about this goes for you as I have 2 hydrangeas that I'm trying this with over the summer. Just got too ambitious and wasn't able to get their in-ground spots properly prepared. Planning to move them from their pots this fall.

Err, sorry I didn't see this earlier.

A nice loose potting mix will let roots travel far and fast. Contrast with plants potted with other material like topsoil or tree & shrub mix and you'll see the difference.