Typical advice for container grown plants is to go to 2 zones lower. But with some protection, you could grow zone 4, even z5 plants.

Most important is to provide a very well draining potting mix.
Is there a reliable snow cover in your area?
Is it very windy?
Are you able/willing to do some protection: mulch, protected location, covers of some kind?
What plants were you growing that didn't survive?

I found few links -
http://garden.lovetoknow.com/wiki/Gardening_Zone_3
http://northernshade.ca/plant-list/
http://www.midwestgardentips.com/best_performing_perennials_n-z.html

Rina

This post was edited by rina_ on Tue, Oct 21, 14 at 15:49

Rina...thank you for repIying.
I don't garden a big lot and, in my ignorance, I bought all zone 5 plants for the containers. Some of them did survive over last winter but many died. I left them in the garage for the winter.
I will check out the websites you suggested.

Thanks so much for the info.

Were they all perennials? Just in case some of the ones that died were annuals...Rina

Hi Rina.
Yes, they were all perennials.
I will choose more carefully next time.
Checked out your suggested sites and got some ideas.
Thanks.