Kale is a hardy, cool-season leafy green vegetable in the cabbage family. It does not do well in summer heat but grows best in cool spring and fall weather. It will definitely survive light frosts and usually frigid cold snaps. Cool weather actually makes the kale sweeter and more flavorful, whereas hot weather invariably leads to bitter yellowed kale fit for the compost heap.
'Premier' is a fairly new variety of kale that matures early and is cold hardy. Siberian Kale, Red Russian and Red Ursa are a few know for their cold hardiness. 'Vates', vates blue dwarf, Winterbor are hardy varieties that do not yellow in cold weather.
If you plant kale in the spring you can be harvesting in May or June. If you plant kale in late in the summer, you can harvest it until the ground freezes up in winter. Direct sowing of kale [Seeds planted directly into the ground] for a spring harvest should commence about two weeks before the last frost date in your area. The seeds will germinate in soil temperatures as low as 40 degrees Fahrenheit. Seeds are tediously slow to germinate when sown outdoors in warm weather. For starting seeds indoors, planting should commence four to six weeks ahead of planting time.
Transplants, either puchased from a nursery or started yourself should be hardened off, or acclimated to the outdoor environment. Seedlings grown from seed and nurtured in a controlled indoor environment are in a sense 'fragile and tender'.
Hardening off is a simple process of acclimating these young plants to the mood swings of Mother Nature, in order to ensure their success in the grown up plant world. See: Hardening Off Seedlings
A low nitrogen fertilizer such as 5-10-10 is best, use in moderation unless your soil is abysmal to start. 1 cup per 20 ft of row should suffice. Mix it into the top 1/2 foot of soil. For seed starting indoors use a standard potting mix for vegetables.