Collard leaves are ready to pick as soon as they reach usable size. They will be most tasty when picked young–less than 10 inches long and dark green. Older leaves will be tough and stringy.
Collard greens are ready for harvest 75 to 85 days from transplants, 85 to 95 days from seed.
Harvest leaves from low on the stem first and work your way up the stalk. Pick leaves from the outside of the plant and work inward. Be careful not to damage the stem where new leaves emerge. Leave at least four leaves at the top of the plant (the growing crown); that will allow the plant to grow new leaves for future harvest. Regular harvest and even watering will keep the plant producing new tender leaves.
Collard leaves are most flavorful in cool weather. Leaves will be sweeter if harvested after frost; cool temperatures cause carbohydrates in the leaves to turn to sugar.
In mild-winter regions, collards will produce new leaves nearly all winter. In hard freeze regions, protect collards from temperatures in the low 20°sF—use row covers, plastic tunnels, or cold frames. When temperatures in the teens are predicted, cover collard plants to keep the leaves from freezing. Still, frozen leaves can be cooked.
Overwintered collards will bolt and produce flowers in the spring; then plants should be removed and replaced.
Collards planted in the spring and grown into the summer will be bitter if hit by a summer heat wave. But new leaves generated in the fall when temperatures become cool again will be tasty. The roots of summer grown collards should be generously mulched and watered for best flavor.
After harvesting collard greens, wash the leaves thoroughly to remove any soil that may be clinging to the bottom of leaves.
Store collard leaves for several days to a week in the refrigerator. Place leaves in a perforated plastic bag wrapped in a damp paper towel in the vegetable crisper of the refrigerator to keep leaves moist and to avoid drying.
Collard greens will store for two to three weeks at 32° to 34°F and 90 to 95 percent humidity (moist) with some air circulation.
You can harvest the collard plant whole and keep the leaves fresh indoors for a few weeks by setting the roots in moist soil or sand.
If you cook collard greens whole, stems will become tender.
More tips at How to Grow Collards.