Living in an urban area, i have to be creative about gardening spaces. This is one of 3 garden areas i work for myself. In the above photo from left to right are; red russian kale, collards and garlic covered with straw mulch. We've had several frosts so it was time to cover the garlic. You want to wait till after a frost so that pests won't over winter in the mulch. All organic gardeners struggle with pests enough during the season, you don't want to help them with a nice cozy over wintering place. You can also see row cover hoops over the red russian kale. This type of kale can take some frosts but it's not as cold hardy as winterbor. Once the weather starts getting really cold I'll cover the row so i can keep harvesting longer.

This photo shows another view of the same garden space. In the lower left is parsley which is still fine. You can see i already have a row covered. Under the row cover are chard and carrots. Carrots can take the cold. You can even over winter your carrots in the ground with no protection and they'll be fine to harvest next early spring. You'll want to harvest the carrots as soon as the ground unfreezes though, or they'll use the energy from the root to flower and go to seed. The reason i have the carrots covered is to keep the ground from freezing so i can harvest them longer. Row covers can allow you to harvest your crops much longer than you would without them. If you have some small cold frames, and put those under row covers you could harvest most of the winter. This is why it's good to plan having fall crops a good size by the time it starts getting cold. They won't grow much during the winter, but the extra protection allows you to harvest much longer and possibly all winter long.