I have around 8 very pretty, frilly medium sized ornamental cabbages in
the front yard, in front of some cedar bushes, and though we've had
frost (and snow) for a few weeks now, the cabbages are a bit larger and
more colourful than ever. What I'm wondering, since this is the first
time I've had these lovely purple centered green cabbages (bought at a
nursery and planted late summer), is how best to keep them going.

I'm in Eastern Ontario zone 4/5 and we often get wickedly cold, bitter
winters with lots of snow. I'd like to know if I should dig these
plants up, or just leave them overwinter and cover them with lots of
straw. Maybe it's too late to dig them up, though the roots seem rather
close to the surface. Right now the ground is frozen, so I would
imagine no digging would be possible, but it's supposed to warm up a
bit in the coming days. (Rain, and wet snow). If they were dug up at
some point, when should that be (as in, what's the point of no return
when the cabbage would be damaged by the cold?), and would they be
stored in a cold, dark shed, covered, and/or pruned? Or would they be
potted and put in a window like Geraniums? Thanks for any input.