I'm in Chicago too, and I've been thinking about the same sorts of things for various woody plants as well!

I don't know about bush cherries, but you can find good information on overwintering hydrangeas in containers on the hydrangea forum. Definitely check with them - I just sponged up all this info from them doing searches and such. The "Container Gardening" and "Balcony Gardening" people are also really good at knowing how to overwinter container plants.

The general consensus over there seems to be that you can overwinter the more delicate macrophyllas and serratas by wrapping/insulating/mulching them lightly and putting them in an attached garage, and making sure it stays neither too warm or too cold; by putting them in an unheated detached garage and very heavily wrapping/insulating/mulching them, keeping them off the floor of the garage as well; or by burying the pot and insulating heavily as above. People use hay bales, bags of leaves, cellulose house insulation, and even bubble wrap. You can even manage leaving a pot outside against the wall of a house, if you put mega-insulation around it. One thing that folks mentioned was that if you stuff leaves and mulch into the crown of the plant (the usual practice,) you risk rotting the plant out. So some people create airspace by making a small "cage" out of chicken wire, then putting mulch around that.

Other people put space heaters in their garages on bad days, set up funky impromptu "greenhouses", or even make little tents out of tarps and then put an incandescent bulb inside the tent with the plants.

Lots of people push zones with these plants, but generally speaking, your chances for success will be best if you select a plant that's a zone or two hardier than where you live. H. macrophyllas and H. serattas are not nearly as hardy as H. arborescens (these include Limelight and Annabelle) or H. paniculata (the Pee Gees), which are both hardy as far as 3a. (In fact, last year I got two gorgeous Endless Summers and a lacecap, and even though I put them in the *ground* I had an irritating amount of dieback. ...I don't know that I'd fool around with putting them in a pot.)

What you *don't* want to do is put them in a basement, or in any situation (including your garage if we have another freaky mild winter!) where it gets too hot for them. Basically you have to keep an eye on it - If things get too warm and they break dormancy down there in the dark, you are going to have a very sad and confused hydrangea that does not handle the situation too well.

...In general, whenever you're putting something in a pot, as far as overwintering is concerned, the bigger the container the better.

let us know how it goes!