Collard leaves are rich in calcium (226 mg per cup, cooked), vitamins B1, B2, B9, and C (which may be leached by cooking, however), as well as beta-carotene (pro-vitamin A). They are high in soluble fiber and contain multiple nutrients with potent anti-cancer properties: diindolylmethane, sulforaphane and selenium.
Researchers at the University of California at Berkeley have recently discovered that 3,3'-Diindolylmethane in Brassica vegetables such as collard greens is a potent modulator of the innate immune response system with potent anti-viral, anti-bacterial and anti-cancer activity.
Most collards are biennials which are grown as annuals. While they are taller than kale, and the leaves are of a different color and shape, botanically both kale and collards are known as Brassica oleracea var. acephala . Where they are adapted, they will commonly reach a height of 3-4 ft.
Collards are heat-tolerant but also winter-hardy in regions that have first frosts around October 30 or later (roughly Virginia and southward). They do best in a slightly acidic soil with a pH of 5.5 to 6.5.
The seeds can be sown 6 weeks before the last frost date. Rapid growth is said to favor more tender greens. Sown in early summer, collards can be harvested at a time when other Brassicas and green leafy vegetables are few and far between. In the South, collards are also sown in the fall. Plant on GrowBiointensive recommended 12" centers. Once mature, the harvest period for biennial collards can last up to 24 weeks. Plants may need staking.
If bolting is a problem, try planting smaller seedlings. Cabbage loopers, blister beetles, and cabbage webworms can all be handpicked. Older or stressed plants can especially suffer from aphids. An aromatic companion such as garlic or mint may help as will a daily blast of water on the leaves.
Collards should be harvested about once a week. The rate at which you harvest can have an important effect on yields, with the correct rate stimulating growth and an excessive rate stunting it. Optimally, cut the leaves where they join the main stem when they are full-sized but not yet woody. Keep the main stem and the central growing tip on the plant uncut so that each plant will continue to produce. The flavor is sweeter and better in cold weather, particularly after a frost.