In general brassicas are good cool weather vegtables, most tolerate light frost, some will stand low 20's for a brief period. The brassica group includes broccoli, cabbage, cauliflower,kale, mustards, pak choi,radishes, rutabaga,turnips and others. English peas, snow peas, snap peas are cold hardy until they enter their bloom cycle. Good for planting in early spring.
And no, root vegetables are not in general more cold hardy than other vegetables, Beets, parnips and carrots can compete with brassicas like turnips and rutabagas. Potatoes cannot.

Agree with above. IME, carrots, spinach and kale are the champs.Turnips, broccoli and brussels sprouts are also very tough. Other brassicas, such as the Asian ones (pak choi et al) are slightly less tolerant of hard freezes. But all of these can easily take multiple light frosts.

Spinach is perhaps the hardiest. I plant it in mid Sept (next week) and overwinter it for spring (zone 6 here, not 5). No cover, no nothing.

Good info everyone.....I will be planting some more spinach soon!

I planted a good amount on the prescribed fall planting date for my area - but it was very hot for the week-2 after - only 2 came up.

I didn't realize that spinach was considered hardy amongst the hardy.....must have more!

Well temperature-wise, what's the difference between a hard frost and a light frost? Also this is just a guess, but excluding kale and spinach, are leafy green things (lettuce, cabbage, chinese cabbage) considered less hardy? Also I planted peas, but they're not forming flowers. The night time temperatures where I'm at range from the 40's to 60's. What temperature do peas need to form pods? I live in Northern Michigan, in the upper part of the lower peninsula.

What is overwintering?

Corn salad and miner lettuce are hardier still than the more common veggies. Russian kale is hardier than regular kale, which is itself a top performer in winter. It will overwinter in Michigan without cover (overwinter=survive in the winter, presumably to be eaten in spring). Lacinato kale is also better than kale, and also somewhat unknown.
And arugula is also a top winter performer. Let us not forget fava greens either. And parsnips are on a par with carrots.

Garlic is the hardiest of them all, but of course we are restricting ourselves to things that can be eaten in winter. Let me mention the wonderful flavor of garlic shoots, 5-10 inches high, just out of the ground in February. Just let some garlic plants go to seed, and pick the heads. Plant at the same time as regular garlic...

We are building a cold frame with a sliding glass door panel that I got for free. It's double pane heavy glass, and I'm intrigued to find out how much warmer it will keep things. We're hoping we can harvest things for a long while here in New Hampshire. It will be fun to see how the experiment works.

The special autumn edition of Fine Gardening magazine had an article about greens and lettuces that are good for colder weather. As far as lettuces, to sum up: if it has "winter" or "artic" in the name, it's been bred for cold resistance. I bought some "Winter Desnity" romaine and some "winter Marvel" butterhead, among others. The French use cold frames much more than we do, and evidently there are many French bred varities of lettuces that do well in cold weather - d'Hiver means of winter.

I also have Elliot Coleman's Four Season Harvest book. In addition to corn salad (mache), miner's lettuce (claytonia or winter purslane), arugula and spinach, he suggests endive (especially frisee type), Chioggia radicchio, buskshorn plantain (minutina), mizuna, sorrel, escarole, chicory, and even swiss chard. He also mentions several other of what I think of as the more unusal salad greens.