I am in your zone. Do you have it in the garage or outside? I have about 10 tree roses, and four have been in the ground for years. The others are dragged into the garage after thanksgiving. I prune them back as I would any other. Take off the winter kill which is much less on the garaged ones. The tree rose, or standard, is a grafted rose with graft up high at the top of the trunk. That does make the vulnerable graft more exposed to the elements and freezing.....and have we ever had THAT this year.
I would prune it back at the correct time.
Hope this helps.

Thanks so much. I have the tree rose in the ground and it's been there for quite a few years. It just seems to be slowly deteriorating. IN the summer all the leaves and flowers are pretty high and there is a lot of bare wood below it. I just wonder if I will kill the rose if I cut way down below where leaves come out. I would do it before the rose starts to leaf out. Of course, our weather may kill it before that!!!!

If you cut it below where the leaves have come out year after year , you'll never get the "Polar Joy" blooming part back. It's only up at the top where it was grafted onto the colder hardy 'stem' that supports it.

Does Polar Joy have a hardier 'stem'? It's a rugosa, and I thought the 'tree' part was formed by training. No grafting involved.

I was hoping that somebody would show up who had experience with one. If it is cut back hard, my expectation is that growth would start from somewhere. The question is how likely is it to come from somewhere desirable. If a 'shrub' form of the rose is acceptable, then it isn't really an issue.

From the website RosesbyPing:
"The first truly hardy tree rose. This rose is not grafted and is the same rose from root to head."
So, there are no grafts to worry about. Still, wouldn't cutting off the branching top cause it to push new growth from - just about anywhere? Searching for information specifically about pruning this unique rose tree, I came across this from eHow (see link.)

I'm not sure I agree with her pruning timing in point number 1, pruning at the end of the season, but the other points sound sensible. And here's a thought: Is it possible that you have *never* pruned this rose? You say you've had it "quite a few years" so maybe you need to get busy and give it a pruning over this spring to stimulate new growth. That said, I think cutting the entire top off would certainly be a unnecessary and drastic step. Remove dead and crossing canes, shorten some of the laterals, open up the center - same as you would for any rose, and give it some fertilizer this spring and see what happens. I bet it rewards you with a lot of bloom.

Here is a link that might be useful: How to prune a Polar Joy tree rose

Thanks everyone. I guess I didn't prune it enough the first couple years-I just cut off the dead flowers and pruned down a few inches. I would never cut the entire top off, but I guess I will have to try and prune a few of the brances hard and see what happens. The rose tree is really hardy. I'm in the NW suburbs of Chicago and do nothing to it except throw some fertilizer down. It's been beautiful and blooms all summer.

REGARDLESS of where the graft is, and thank you SO much for the correction, i would prune as a regular rose at the top where you want to encourage new growth. If you have shoots off of the trunk, cut them down all the way to keep the standatrd form. Now i want one.