Can you tell us what zone your in? That will probably make a difference. :)

Zone 6a

Well, I wish I could be more help, but I'm in 9b, so our conditions are pretty far apart. Hopefully someone will be along who closer to you than me!

I don't recommend standards (rose trees) in this area, and I'm on the border of 6 & 7. If you must, then you'll need to put it in a pot and bring it into an unheated garage during the winter.

But, really, don't do it.

You would have to keep it in a shed or garage for the winter but otherwise it will do fine. I have a rose tree I keep in my shed over the winter and it's lasted several years. You do have to be sure to water it some at least once a month all winter though. Dehydration will kill a rose quicker than cold.

Knockouts are supposed to be hardy to Zone 5, but even in z6 you'll probably get a lot of cane dieback in some winters, and so the exposed rose part of the tree could be vulnerable to dying completely.

Here's page with care tips for rose standards depending on zone. If I grew rose standards I would probably wrap them up a bit for insulation and stick them in my unheated garage, or else use my cold room in the basement.

You could also just bring them indoors if you have a spot with lots of light for it. Roses do need to go dormant for the winter.

http://www.midwestgardentips.com/winter_protect_standards.html

My $0.02... Knock Outs are hardy to the tips here in borderline 5b/6a. I know several people who actually have their Knock Out tree roses planted in the yard and provide no protection whatsoever and they survive outside for years (not sure how many years though). I have 6 tree roses including one double knock out tree rose and I haul them all into a garage for the winter. As much as the graft is hardy (root stock is hardy to zone 5 and so is the scion), all it takes is a couple of nasty freeze thaw cycles to possibly damage it or kill it altogether and for that reason I don't take any chances.