Last revised February 11, 2010
Kale is a cold-hardy crop producing "greens" high in nutritive value. It is not well adapted to hot weather. Best quality is produced where summers are cool or when it is grown into the fall or winter. Varieties of kale "greens" are of two types. Scotch types have gray-green and very curled and crumpled leaves while Siberian types are blue-green and less curled. Both dwarf and tall types are available with the dwarf types being preferred. Collards are similar in nutritive value but much more tolerant of warm weather.
Kale. Photo credit: Alex Stone, Oregon State University
Blue Max, Georgia Southern, Heavi-crop, Top Bunch, Vates. For trial: Morris Heading, Carolina, and hybrids: Hi Crop, Green Glaze (some resistance to imported cabbage worm, diamondback moth, and cabbage looper reported).
See the Vegetable Variety Selection Resources page to find varieties that have been shown to perform well in the Pacific NW.
Collard and kale seed numbers about 144,000/lb. Plant seed 0.5 inch deep. Use hot-water treated seed and fungicide treat seed to protect against several serious seed-borne diseases. Hot water seed treatments are very specific (122 F exactly, for 25 to 30 minutes; the wet seed then quickly cooled and dried). The seed treatments are best done by the seed company, and can usually be provided upon request.
For collards, seed the spring crop in April, fall crop in July. If direct seeding, space rows 2 to 3 feet apart with 4 seeds per foot, 2-4 lb seed/acre. When plants are 2-3 inches tall, thin them to a spacing of 12 to 24 inches apart. If transplanting, transplant when the plants are 6-8 inches tall and plant 12,000 to 14,000 plants/acre.
For kale, seed a spring crop as early as possible and seed a fall crop from July to early August. Space 1-3 feet between rows and 3-10 inches between plants. Use 3-5 lb seed/acre of "Scotch", and 1.5 lb/acre of "Siberian" types (see above for descriptions).
Most kale in the Willamette Valley is now transplanted using greenhouse-grown plugs.
Well-drained loams relatively high in organic matter are suitable for collards and kale. Cover crops may be turned under to maintain organic matter. The desirable pH is between 5.5-6.5. If the pH is too high, manganese is frequently unavailable which results in a chlorotic condition of the leaves. If the pH is too low, an application of lime is recommended.
The following recommendations are general. It is advisable to have a soil test done for each field to be planted. For more information on fertilization and lime requirements for collards, kale, and other cole crops, see Broccoli.
Nitrogen: 60-100 (N) lb/acre (1/2 at planting and 1/2 at thinning)
Phosphorus: 80-120 (P205) lb/acre
Potassium: 60-120 (K20) lb/acre
Sulfur: 30-50 (S) lb/acre
Boron: 0-4 (B) lb/acre
Magnesium: As indicated by soil test (may need 60-120 lbs MgO per acre)
Copper, boron, and zinc - as indicated by soil test.
Maintain uniform soil moisture for tender growth and maximum use of soil nutrients. As much as 12-14 inches of water may be needed. Soil type does not affect the amount of total water needed, but does dictate frequency of water application. Lighter soils need more frequent water applications, but less water applied per application.
An approximate yield for collards is about 150 cwt/acre. Collards are considered to be mature when a large rosette of leaves form in the crown of the plant. The plants may be harvested by machine or cut by hand. Grade the harvested plants by removing decayed or damaged leaves. Generally four wrapper leaves are left on to protect the center rosette.
Kale leaf yield can be as much as 400 cwt/acre. Kale can be harvested three ways: whole plant, bunched leaves, or "stripped" leaves. "Stripped" kale is pre-packaged for fresh market. In all methods, yellow or damaged leaves must be removed before packing. Whole plant harvest in not currently practiced in Oregon. Kale in Oregon is usually harvested on demand 2-4 times over a 4-month period but leaf harvest may be done weekly when demand is high. A skilled laborer can harvest and bunch 4 boxes/hour. Yields are about 3200 dozen 1-lb bunches/acre.
Leafy greens such as collards, kale, rape, Swiss chard, and beet greens are handled like spinach. Because of their perishability, they should be held as close to 32 F as possible. At this temperature, they can be held for 10 to 14 days. Relative humidity of at least 95 % is desirable to prevent wilting. Air circulation should be adequate to remove heat of respiration, but rapid air circulation will speed transpiration and wilting. Satisfactory precooling is accomplished by vacuum cooling or hydrocooling. These leafy greens are commonly shipped with package and top ice to maintain freshness. Research has shown that kale packed in polyethylene-lined crates and protected by crushed ice keeps in excellent condition for 3 weeks at 32 F but only 1 week at 40 F and three days at 50 F. Vitamin content and quality are retained better when wilting is prevented.
Collards are packaged in bushel baskets, crates and cartons, 24 pack, 23-24 pounds; 1.4-bu, 23-24 pounds; or crates and cartons, 12-24 bunches.
Kale is commonly packaged 1 or 2 dozen bunches/box. Bunches are 4-8 leaves or about 1 lb each.