Can they be over wintered? Can they be grown from seed? Or if we buy them big, how do we propagate it?
Mine over-wintered and are blooming now. You can buy seeds so I'm certain you can save the seeds after they bloom. These will die when temps get too high though. I like them. I didn't expect the yellow blooms.
Grown as annuals, not perennials. They'll take some very cold temps but they are not going to overwinter in your zone - they are mainly grown for a Fall to early winter display.
Kales do not tolerate summer heat, and plants set out in spring will bolt - start from seed in mid-summer or purchase transplants for a good fall show. Seeds must be sown under cool conditions for seedlings to thrive..... Sow your seeds about 6-10 weeks before the first frost - if you are still having warm temps 6 - 10 weeks before first frost, you may have to use your refrigerator for germination.
It should be mentioned that there are ornamental kale varieties that are both good to eat and which make excellent summer foliage displays in the perennial garden.
This site discusses a couple of good varieties, including my favorite, "Redbor" which has deep red-purple frilled foliage, grows to 2 1/2 feet or so and stands hot weather well.
It will overwinter like other kales and flower in spring, but you'll need new plants each year for the foliage display (seed is readily available).