I understand they are fairly hardy.
Oooooh, you should be
very careful, I think
they are not. They are Mediterranean species, they don't grow naturally here in the Loire valley where I think the winters are milder than in northern Germany : I've never ever seen a pomegranate in a garden here.
In the "Côte d'Azur" ("French Riviera"), great, along the southern part of the Atlantic coast, fine, but where temps go below 4-5 ° below zero several days in a row, it's very, very risky...
When they survive a mild winter they'll never be as strong, as healthy as those that grow down south. But if you can keep olive trees in a shallow pot outside where you live, OK.
At least, I think putting them in a cold frame of some kind against a wall might be a good move.
You wouldn't kill a present from your wife for lack of care, would you?...