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Kale Seed Starting Tips

Seedling Kale 2

Kale grows best in the cool weather of spring and fall. Mature kale plants can tolerate temperatures as low as 10 to 20°F (-12 to -7°C).

Time kale seed sowing so that plants come to harvest before very warm and hot weather; hot weather will trigger bolting and seed-stalk formation.

Bolting can be slowed by picking the oldest leaves first, but the flavor of kale will suffer in warm and hot weather. The best-flavored kale has been touched by frost.

Kale matures in 55 to 65 days depending on the variety.

Kale seed starting
Kale seedlings

Kale Sowing and Planting Tips

  • Kale is grown from seeds or transplants.
  • Kale seeds are viable for 4 years.
  • Start kale indoors 10 to 6 weeks before the last spring frost; sow seed in early spring or late summer or a fall or winter crop.
  • Sow seed ¼ to ½ inch (6-13mm) deep.
  • Sow seeds 4 inches (10cm) apart; later thin seedlings to 16 to 18 inches (40-45cm) apart; use the thinnings in salads. Make sure there is good air circulation around maturing plants to avoid disease.
  • Space plants 16 inches (40cm) apart in a staggered pattern or in rows 18 inches apart.
  • Kale grows best in full sun but can tolerate light shade.
  • Kale prefers a soil pH between 6.0 and 7.0.
  • Seed germinates in 5 to 7 days at or near 70°F (21°C); sometimes seed can take up to 2 weeks to germinate if the soil is cold.
  • If sowing seed in summer for fall harvest, place the seed in a folded damp paper towel placed in a plastic bag, and kept in the refrigerator for 5 days before sowing.
  • Keep the soil evenly moist until seeds germinate then keep the soil moist until seedlings are well established; once plants are established mulch with straw to keep the roots cool and moist.
  • Add aged compost to planting beds in advance of sowing; compost will feed the soil and aid moisture retention.
  • Kale grows best when daytime temperatures are in the 60s°
  • Kale can tolerate light frosts in the spring and freezes in the fall.
  • Kale planted in the fall can winter over under straw and will begin growing again early in spring.
  • Avoid planting kale where cabbage, broccoli, Brussels sprouts, or cabbage family crops have recently grown.
  • Make successive sowings every few weeks for an extended harvest.
  • Fertilize with an organic fertilizer such as fish emulsion at half strength.
  • Aphids and flea beetles can attack kale.

Interplanting: Interplant kale with beets, leaf lettuce, onions, spinach, and Swiss chard.

Container Growing Kale: Grow kale in a container a minimum of 8 inches (20 cm) wide and deep.

Kale Planting Calendar

For spring harvest:

  • 6-4 weeks before the last frost in spring: start seed indoors for transplant out later.
  • 4-2 weeks before the last frost in spring: transplant seedlings into the garden.
  • 2 weeks before the last frost in spring direct sow in the garden; the minimum soil temperature should be 45°

For fall harvest:

  • 14-12 weeks before the first frost in fall: start seed indoors.
  • 10-8 weeks before the first frost in fall: transplant seedlings into the garden.
  • 10-6 weeks before the first frost in fall: transplant seedlings into a plastic tunnel or cold frame.
Kale seedlings
Time kale seed sowing so that plants come to harvest before very warm and hot weather; hot weather will trigger bolting and seed-stalk formation.
  • ‘Red Russian’ purple stems, flat, smooth, grey-green leaves.
  • ‘Toscano’ also called dinosaur kale, dark green, savoy leaves.
  • ‘Winterbor’ ruffly, blue-green leaves.

Botanical Name: Brassica oleracea

Kale is a member of the Brassicacea (Cruciferae) or cabbage family.

More tips: How to Grow Kale.

Written by Stephen Albert

Stephen Albert is a horticulturist, master gardener, and certified nurseryman who has taught at the University of California for more than 25 years. He holds graduate degrees from the University of California and the University of Iowa. His books include Vegetable Garden Grower’s Guide, Vegetable Garden Almanac & Planner, Tomato Grower’s Answer Book, and Kitchen Garden Grower’s Guide. His Vegetable Garden Grower’s Masterclass is available online. Harvesttotable.com has more than 10 million visitors each year.

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    • You can test the germination rate of the two colors of seeds. Place 10 or 20 between moist paper towels and slip the towel and seeds into a large plastic baggie. After a few days count the number of seeds that have germinated and divide by the total number of seeds to get the germinations percentage.

    • Plants produce seeds after they flower; the flowers drop seed. Each plant will be different. Most vegetables are annuals–meaning they will sprout from seed, grow, flower, and set seed in one season. Check the seed packet of the vegetable, it should tell you the number of days from seed to maturity. Soon after the plant reaches maturity, it will flower. Allow the flowers to drop or dry up and you can then harvest the seeds. If a kale plant reaches maturity in 60 days, it will likely begin to flower in the next 10 to 20 days, the flowers will dry up after a week or two and you can harvest the seed. In this example, seeds would be harvested in about 90 to 100 days.

    • Kale can be harvested cut-and-come-again which means you can take two or three leaves for a meal and then back another time for additional leaves. If you have several plants, harvest each cut-and-come-again in order; that will give the first plant harvested time to regrow new young leaves for the next harvest.

    • You can sow kale seeds 1 to 2 inches apart. Once the seedlings come up, thin the plants from 6 to 10 inches apart. Choose the strongest seedlings to grow on. Use scissors to simply cut the weaker seedlings off at soil levle.

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