When to Plant Kale Seeds

Kale (Brassica oleracea) doubles your seeding delight with sowing opportunities in spring and late summer. In U.S. Department of Agriculture plant hardiness zones 7 through 10, this cool-season biennial produces all winter. Seed-sowing times depend on your region, and calendars aren't your best guides. Soil and air temperatures and other factors provide the cues you need to plant kale in spring and do it again -- in reverse -- for fall.

Soil temperatures control kale's germination and signal your sowing time. As a cool-season cole crop, kale is one of the first seeds you can sow spring. Kale germinates when well-drained soil warms to 40 to 45 degrees Fahrenheit -- no matter where you garden. That's long before most seeds leave their packets. Soil warming varies from year to year according to weather trends. The simplest way to seed kale on time is to use a soil thermometer. Available from garden centers or online retailers, these stemmed thermometers look like meat thermometers for soil. Insert the stem at least 2 inches into soil to check the temperatures where your kale seeds will grow.

With kale seeds sown 1/4 to 1/2 inch deep in a sunny, moist garden, the soil protects the seed from frost. As the soil warms and kale seedlings break the surface, air temperatures come into play. Thinning your seedlings to 2 to 6 inches apart, in 24-inch rows, lowers protection even more. Kale relishes cool weather, but extremes damage the tender seedlings. During unpredictable springs, you may want to delay seeding until conditions stabilize. Delayed seeding just shifts the harvest back. Some cold-tolerant, short-season kale varieties mature in 40 days, but others take 58 days to mature. Keep fabric row covers or other cold protection handy in case spring freezes are forecast.

Historic first and last frost dates help guide decisions on sowing kale for spring and fall harvests. Sow spring seeds four to six weeks before the year's last expected frost. For fall harvests, sow eight to 10 weeks before you anticipate frost. Dates vary considerably. Southern Iowa gardens expect spring frosts to stop by the end of April, but western Montana looks to the end of June. Southern Arkansas gardeners expect frosty kale by mid-November, but southeast Oregon looks to early September. Soil temperatures must cool below 85 F for fall kale germination. Sow late enough to avoid warm soil, but so crops mature in cool weather.

Once you've figured out the dates to start sowing, don't limit kale to a single spring or fall crop. Successive planting keeps tender kale coming continually at its peak. Sow kale seeds at two-week intervals for successive spring crops. In late summer, do the same. For baby kale harvests, plant right up until the ground freezes. Kale withstands a light freeze, and its flavor sweetens with the touch of frost. Extend your season even more by speeding spring's warm up and slowing fall's cool down. Cloches, cold frames and row covers all protect kale and extend your harvests on both sides of the gardening calendar.