Hi
If I were you I'd plant those plants in wooden or terracotta pots. They protect roots far better than polyethylene. Stand every pot on a piece of styrofoam, this will insulate roots at the bottom from cold.
When winter arrives get 4 pieces of polystyrene or wide and short planks and build sumthing like a box around the pots and stick some insulating stuff between - old papers, dry leaves or sawdust.
You may also wrap the pots in several layers of bubble wrap or even old blankets. The thicker the better. Cover the top of the soil too (with bark mulch, sawdust etc.)but do not bury bases of plants too thick. It may cause rot of the bases. Remember to water your plants when the weather is warm.
When it's frosty and sunny you should protect the tops of plants from the sun. Evergreens get burned easily in winter. Also do wrap your Thujas with a piece of string before first snowfall cuz heavy snow can bend and break their branches. Best way is to use that green mesh which protects conifers from snow damage and "sunburn".
I live in zone 6 and Nandina drops it's leaves every year here but resprout nicely every spring.
Hope my info will be helpful.

Cheers

Martin, I am reading this with great interest, because I for one love nandina domestica, but did not know it would be hardy up to the higher zones.

We have a house in Mass which is in a pocket that is generally z5b, could be z6a, but I try to be safe in my plant selections. I'm not that familiar with plants beyond my native z8b in south Alabama.

Can I get the nandina to grow up there if I plant it in the right spots? Will it have the clusters of red berries that make it so pretty in the winter down here?

How about the dwarf nandina like FirePower? Would they also become diciduous that far north? Or would they simply succumb to the cold?

Thanks for your information.

Well, I have grown my Nandina 'Fire Power' for two years but it never flowered, maybe it will this year but I may never see it since I moved to Germany. If you plant it in a sheltered spot and give it some protection over winter it'll be fine. You should wrap the plant in a piece of garden fleece (in case there's no snow) so it won't shed its leaves. Even if harsh winter kills all the aboveground parts it'll simply regrow from the base.