Once the days are short and cold, you can’t expect kale to keep growing. But you can help it survive almost indefinitely. The trick, which also works for Swiss chard and other hardy greens, is to keep the soil around the roots from freezing.
Start by applying a thick blanket of organic mulch — straw or shredded leaves. That will be enough if you are in zone 7 or the warmer parts of zone 6. If it’s colder, use bales of straw and old windows (or clear plastic) to build a lean-to cold frame.
Place a row of bales close to the long side of the kale row. They should be to the north if the row runs east-west, to the east if it runs north-south. Put another row on top so you have a wall about 3 feet high. On each short side, make a sloping wall by butting single bales firmly against the back wall, then topping them with partial bales.
Now use the windows (or clear plastic sheeting) to cover the front, making sure the cover does not touch the plants. At the top, windows can just lean against the straw. Plastic should be draped and held in place on top of the bales by rocks or a heavy board. At the bottom, where the cover touches the ground, mound on a few inches of soil to hold it and seal out drafts.
At this point, you should have a structure whose sloping, clear roof faces south or west. Use loose straw to fill in any gaps in the walls. That’s it. Throw a heavy blanket over the cover when night temperatures are predicted to fall below 20ºF; and open the frame at the top on warm, sunny days or you’ll cook your kale before you bring it indoors. Don’t try this with root crops; mice and voles will colonize the bales of straw if you feed them beets and carrots!