The kale and collards (which is nothing more than a variation of the same plant species) are the close relatives of the wild cabbage which grew by sea cliffs in the Mediterranean long before the beginning of farming. They are extremely cold tolerant and can often survive through the winter to produce tender leaves in the spring. Very young kale leaves can be added raw for salads, but the larger leaves develop a strong flavor and texture that benefits from cooking. The kale that has been frozen is much sweeter because the plants protect their leaf cells from freezing by reducing a little stored starch to the soluble sugars.
Kale is probably the most nutritious of all foods. It is low in calories and enormously abundant in vitamin C, beta-carotene, folic acid, lutein, iron, calcium, and fiber. It is full of sulforafane and indoles, which are very effective in protecting your body against cancers. Research suggests that a diet rich in kale and other members of the cabbage family may reduce the risk of hormone-related cancers, including chicks, ovary, and prostate cancers in half.