Although growing times vary between varieties, most kale is ready for harvest between 45 and 75 days after transplanting.
You will need at least six square inches of growing space per plant regardless of your container type. Choose an area with full sunshine if you’re planting during the fall, and an area with partial shade if you're planting in the spring. Avoid low-lying areas and spaces where water tends to collect and/or flood.
Make sure that you plant your kale in fertile, loamy soil, amended with nitrogen, phosphorus, or potassium if it is low in these nutrients. Sandy or clay-like soil will hurt the kale's flavor and production ability. Kale prefers soil with a pH between 5.5 and 6.8.
If you're starting your seeds or starts indoors, plant them between five and seven weeks before the last frost. If you're going to start your kale outside, plant the seeds two to four weeks before the last frost or at least 10 weeks before the first frost in the fall.
Kale seeds are small and should only be planted ¼ to ½ inch deep. Space the seeds about three inches apart, and thin to 6-9 inches as your plants mature.
When your seedlings are about 4 inches high, you should start hardening off for planting in the garden. At this point, your kale seedlings should have at least four developed leaves. It takes four to six weeks for your seedlings to reach this stage.
Spread a thin layer of fertilizer evenly over the growing area. Follow the directions for your specific type of fertilizer for precise amounts. For compost and mulch, spread a layer a few inches deep. For seaweed powder or rock dust, apply a thin, even sprinkle. Kale responds well to fish emulsion and compost tea.
Depending on the amount of sun your plants receive, you may have to water them as often as every day. Aim for an inch of water a week.
Fertilize your kale plants during the growing season every six to eight weeks. Fertilizer helps the kale grow strong and robust and keeps it producing healthy, sweet leaves.
Harvest the kale about 70-95 days after sowing and 55-75 days after transferring to your garden. The plant should be at least eight inches tall before you harvest the leaves. Be aware that growing time varies according to each variety, so be sure to look up the appropriate time before harvesting. Pick off the outside leaves first if just harvesting individual leaves. If harvesting the entire plant, cut the stem down to about two inches above the soil with one clean cut. This will allow the plant to continue to produce leaves. Don't leave the leaves on the plant for too long after they are ready for harvest. Doing so will produce bitter, tough leaves.