Often grown for fresh winter greens, collards (Brassica oleracea var. Acephala) can endure numerous frosts and keep producing under cold conditions which prevent other members of the Brassica familymembers. In actuality, the cool temperatures which precede frost improve the sweetness in the leaves. Collard green plants have been biennials which overwinter at U.S. Department of Agriculture plant hardiness zones 8 through 10, with a few varieties hardy to USDA zone 6.
One Tough Vegetable
With normal weather patterns, collard plants can survive temperatures down to approximately 20 degrees Fahrenheit, so long as they’re growing in well-drained soil. The exception is when a hard freeze follows a period of unusually warm weather, which does not permit the plant to acclimate gradually to lower temperatures. As the weather cools before frost, the plant converts starches in the leaves to sugars and changes the structure of their proteins. This chemical reaction gives credence to the adage which collard greens are always sweeter after the first frosts.
Repeated Winter Harvests
Collard greens are typically planted in Mediterranean climates in late summer for winter harvests and set out about six to eight weeks prior to the first autumn frost. The leaves are ready to harvest about 75 to 85 days after transplanting seedlings. Harvest young leaves less than 10 inches long to get the very best flavor, cutting them in the bottom of the plant. As the greens are picked from the bottom, fresh leaves will continue to emerge on the shirts of the plant. Collard greens can even be picked frozen.
Greatest Growing Conditions
Collard greens develop best in locations with at least four to five hours of sunlight, in well-drained soil amended with 1 inch or more of composted manure worked into the ground. A soil pH of between 6.5 and 6.8 helps prevent disease. Set the plants 18 to 24 inches apart with 3 feet between rows. Collard greens need at least 1 inch of rainfall or supplemental watering each week. Although not as disturbed by diseases and insects as many different Brassicas, collards shouldn’t be planted where other brassicas have grown recently to prevent club black and root rot diseases. Cabbage worms and flea beetles can occasionally attack plants. Hand pick cabbage worms and use floating row covers to keep flea beetles from damaging the leaves.
Spring Bolting Period
As a biennial plant, collard greens which overwinter start to produce seeds the second season early in the spring. When the plant flowers, seedpods form on the plant. You may continue to crop through this “bolting” interval, cutting lower leaves during the early spring.