Collard leaves grow in a loose rosette. Harvest the bottom leaves first.
Collards will fill a pot for winter. These are not nearly full size yet. To harvest from a pot, you can pick the plants just about clean, which will leave the pot looking bare for a while, or you can pick just a few leaves at a time to add to soups and salads. 
As you harvest collard leaves from the bottom up, the lower stem will be bare, making the plant look tree-like. New leaves grow from the center through cool weather. When plants that have been in the garden all winter send up a bloom stalk early in spring, it's time to take them out. 
Cold-hardy collards are always better after frost sweetens their leaves; this makes them perfect for fall gardens.
A bunch of cut collard leaves will make a pot full of greens that you can also freeze for later.If you don’t live in the South, you might not see collards very often; they are a leafy, cool-weather vegetable very popular for cooked greens. However, collards grow well throughout the country. A relative of cabbage, broccoli, Brussels sprouts, kohlrabi, and kale, this upright, dark green, waxy plant is a little like a cabbage that doesn’t make a head. It is one of the most cold-hardy of all vegetables, able to withstand temperatures in the upper teens. In Zone 8 and southward, collards often provide a harvest through the entire winter. You can plant them in spring and fall, although fall-planted collards are favored because the leaves are sweeter when kissed by frost.
Soil, Planting, and Care
Set out spring transplants 3 to 4 weeks before the last frost; in late summer, plant 6 to 8 weeks before the first frost for fall and winter harvests.
Like all vegetables, collards like full sun, but they will tolerate partial shade as long as they get the equivalent of 4 to 5 hours of sun to bring out their full flavor.
Plant in fertile soil because collards should grow fast to produce tender leaves. They need fertile, well-drained soil with a pH of 6.5 to 6.8 to discourage clubroot disease. To be sure about your soil pH, test the soil with a do-it-yourself kit, or by using your regional Cooperative Extension office.
Work nitrogen-rich amendments such as blood meal, cottonseed meal, or composted manure into the ground before planting. Feed your plants with Bonnie's Herb & Vegetable Food as they develop and each time you harvest to keep lots of leaves coming on. Since the plants produce so much foliage that gets harvested often, regular feeding goes hand-in-hand with regular harvesting.
Collards are easy to transplant. Set plants deeply so that about half the stem is buried. Space transplants at the distance given on the Bonnie label. If you don’t have the label, a good general spacing is 36 inches apart. After planting, water and fertilize.
Collards like a nice, even supply of water. Water regularly, applying 1 to 1.5 inches of water per week if it doesn’t rain. You can measure the amount of water with a rain gauge left in the garden. Mulch with compost, finely ground leaves, weed-free hay, or finely ground bark to keep the soil cool and moist and to keep down weeds. Mulching will also help keep the leaves clean.
Harvest and Storage
Harvest leaves when they are up to 10 inches long, dark green, and still young. Old leaves may be tough or stringy. Pick the lower leaves first, working your way up the plant. You can even harvest leaves when frozen in the garden, but be careful because the frozen plant is brittle.
Of course, wash the leaves thoroughly because soil often clings to the undersides. Collard leaves will keep for several days in the refrigerator.
Troubleshooting
Keep a soil pH of 6.5 to 6.8 to discourage clubroot disease. Although worst on cabbage, Brussels sprouts, and turnips, clubroot affects all members of the cabbage family. The best way to avoid problems is to keep the garden clean. Insects that like collards include cabbage loopers, slugs, imported cabbageworms, cabbage root maggots, aphids, and flea beetles. Disease problems include black leg, black rot, clubroot, and yellows. To prevent diseases from building up in the soil, don’t plant collards or other cole crops in the same spot each year. Rotate with a non-cole crop for 2 years before returning to the same spot.