An oft-recommended rule of thumb for container gardening suggests to look for plants hardy to 1 - 2 zones lower than where you are to help maximize your success of overwintering. The loostrife is generally listed as hardy to Zone 3.

Yes a container can freeze harder and quicker than the ground, however they can thaw quicker than the ground as well. Meaning an issue for container plants is reducing a potential freeze/thaw cycle that can hurt the plant's roots. You would want the pot to gradually thaw in spring.

There are various ways people have suggested to protect containers in winter including wrapping (many use bubblewrap or may mound with leaves, etc), sinking in a larger container or even one of those ice chests, placing it in a spot with a warmer microclimate (eg., against a wall), etc.

The only thing you can do to find out is try some of these things and see what happens. You might be quite surprised! You can do some research to check the hardiness of the plants that you want to grow and go from there.

I don't think you'll have a problem with the goosestrife. That stuff is practically impossible to kill!! It makes a wonderful cut flower and the bees love it.

I did (i think!) kill my blueberries i had overwintered outside. They were in smallish containers and the temp locally did hit -26F (thats without wind). Its OK because being the dummy i am, planted them in soil with a PH way too high. Live and learn.

Yes, it's important to use pots that have some weatherproofing capacity. I use a combination of foam and fiberglass pots for best results. I do have ceramic and terra cotta containers, but I'm always afraid they'll crack. I move as many containers as possible up against the brick wall of my apartment building. I mulch with 2 or 3" of pine bark, and for the plants I can't move I wrap with bubble wrap or foam wrap. Once this winter after we'd had a thaw I had to run out with flannel sheets and cover my climbing rose during a blast blizzard. I left them covered for four days. Same with my peony.

Using all these measures I've managed to keep a number of perennials alive. Good luck with yours.

Judith

Personally I have plans of building a place to help keep them warm - a framework of PVC pipe and some thick plastic, cut to the sape of the "walls" and "celing" and then use duct tape to connect them over the framework. I'm still working out how to do a door though. Perhaps a largeish zipper ducttaped on. I figure if I do that, and get some old blankets from goodwill to pile around my plants, they should be ok.

Marie, you're in a much warmer zone than I'm in. I don't think you need to go to all that trouble.