According to this article from the Harvard School of Public Health, one cup of collard greens contains 357 milligrams of calcium, but a cup of milk has 306. Collard greens also are one of the leafy greens with calcium that is more absorbable. (Some greens like spinach contain oxalic acid which interferes with absorption of calcium).
In addition to calcium, Collard greens contain Vitamin K which plays a role with calcium in keeping bones healthy and strong. Three proteins in bone depend upon Vitamin K to function. Collard greens are a member of the cabbage family. The leafy vegetable has been a regular part of American cooking in the South, aka ‘soul’ food for decades. (They were also popular with the ancient Greeks and Romans). In the South, they are often cooked with black eyed peas. A Collard Greens Festival has been held for several years in East Palo Alto celebrating Southern African-American culture.
In Portugal, soup is made with the greens, potatoes, onions, garlic, salt, savoy cabbage, and pork sausage, although tofu could be substituted for the pork.
The vegetable is usually planted in the early spring. In the South it can also be planted in midsummer and harvested in fall or early winter.