Rule of thumb is to select a plant that is winter hardy two zones colder than your zone. So, you would look for something winter hardy to zone 4. (Are you sure you're zone 6? I thought Central NJ had been redesignated as 7.) Here's a link to the USDA interactive zone map; it will tell you what zone you're in based on your zip code http://planthardiness.ars.usda.gov/PHZMWeb/InteractiveMap.aspx

Another thing to consider is the alternate freezing/thawing that can take place throughout the winter. Choose a container that will collect less of the sun's heat on a sunny and mild winter day, or put a container within another to shade it.

You're right - I'm in 7a. My plan is to place them in a pot inside larger plastic square pots. I have two braided hibiscus trees on the front porch in embarrassingly bad shape and need something hardier. Thanks for the info! I'll visit Gasko's soon and see what's there.

The pot-within-a-pot sounds like a very good idea. You might even be able to fit a little bit of insulating material in there for winter. Check around and you'll probably find something nice to top off the potting mix as a mulch.

You might want to check out Rare Find Nursery in Jackson. They have a very nice selection of desirable trees, shrubs and perennials. Their annual "Dog Days Sale" is coming up Aug. 17-19, I believe. Check their website, though, before going, for days/hours they're open.