Kale (Brassica oleracea) is a leafy vegetable in the Brassica or cole crop family. It is usually grouped into the cooking greens category with collards, mustard, and Swiss chard, but it is actually more of a non-heading cabbage. The leaves grow from a central stem that elongates as it grows. Kale is a powerhouse of nutrients. The plants can be quite ornamental, with leaves that can be curly or tagged, purple or shades of green. It is considered a cool-season vegetable and can handle some frost, when mature.
|Botanical Name||Brassica oleracea|
|Plant Type||Biennial grown as an annual leaf vegetable|
|Mature Size||2 feet|
|Sun Exposure||Full sun to part shade|
|Soil pH||5.5 to 6.5|
|Bloom Time||Early spring|
|Hardiness Zones||7, 8, 9|
|Native Areas||Southern and Western Europe|
How to Grow Kale
Kale is easy to grow from seeds (much easier than cabbage), either directly seeded into the soil or started indoors and then transplanted. It can be grown as a cut and come again vegetable, so a few plants may be all you need.
Kale is a member of the cabbage family, which is notorious for rot diseases and attracting insect pests. It can be susceptible to black rot and club root as well as aphids, cabbage loopers, cabbageworm, cutworms, flea beetles, and slugs. The best defense is to monitor the plants often for signs of eggs or feeding. Be sure you know which pest is in evidence and treat accordingly.
Since kale is grown for its leaves, not flowers, it can handle full sun to partial shade. Sun exposure is just one factor in growing healthy kale plants. They can handle more sun if they are given plenty of water to cool the soil. Partial shade will be required if the weather is warm and dry.
Kale plants like to grow in a rich soil, high in organic matter with a slightly acidic pH. You’re growing it for the leaves, so the high nitrogen content provided by organic matter is crucial.
Keep your kale plants well-watered. Along with cool temperatures, moist soil helps keep kale leaves sweet and crisp, rather than tough and bitter. Mulching under the plants will keep the soil cool and moist, the way kale likes it.
Temperature and Humidity
The optimal soil temperature for planting kale is 60 to 65 degrees Fahrenheit. All varieties prefer cool temperatures and will be sweetened by a touch of frost. Hot weather turns kale bitter. Kale is a biennial that is usually grown as an annual. It will last through winter in most zones, with adequate protection, but will collapse if exposed to heavy frosts or snow. It can be grown throughout the winter in USDA Zones 7 through 9 if winters are mild and there is adequate water.
When planting, use 0.5 cups of 5-10-10 fertilizer for each 10 feet of row. Mix it into the top 3 to 4 inches of soil. Keep your kale growing with side dressing throughout the growing season. Use compost or feed with some type of high nitrogen fertilizer (the first number on the fertilizer label) like fish emulsion.
Kale can be direct seeded in the garden or started indoors and set out as transplants. Start plants indoors about six weeks before your last expected frost date. Kale seeds germinate quickly in warm soil and should be up within five to eight days.
Cover seeds with about 1/2 inch of soil and don’t allow the seeds to dry out before germinating. Plants will grow more slowly outdoors than indoors under lights.
Transplant seedlings after danger of frost. Set plants out with about 16 inches of spacing between plants. This gives them room to spread out and still allows for air circulation.
You can direct seed in cold climates as soon as the soil can be worked and the soil temperature is at least 45 degrees Fahrenheit. Kale matures quickly, in about two months or less, so if you prefer you can start your plants later or even plant multiple crops by succession planting.
In warm climates, kale can be direct seeded in late summer/early fall, as well as in the spring. A winter crop of kale in warmer climates can be much sweeter than a summer crop.
Varieties of Kale
It's hard to find bad-tasting kale. The curly-leaved varieties tend to hang on longer in cold weather, but can be slower to get established than the flat-leaved types. Look for these varieties:
- "Hanover Salad" is a fast grower and an early producer. It is nice for fresh use.
- "Lacinato" is the puckered heirloom kale from Tuscany. It is sometimes listed as Tuscan or dinosaur kale. The thick leaves are hardy enough to harvest after a snowfall.
- "Redbor" has magenta leaves with curly edges, giving this kale a mild, crisp flavor and texture.
- "Red Russian" has smooth, tender leaves with purple veins and edges. It is especially slug resistant.
- "Vates" is a dwarf, curly, bluish-green kale that is both heat and cold tolerant. It is derived from "Dwarf Blue Curled Scotch."
You can harvest a few young leaves, but expect to wait approximately two months for mature plants from seeds. Check the days to maturity on the seed packet or plant label for more precise timing.
You can harvest very young leaves to use fresh in salads or allow plants to mature and use as a cooked green. Harvest older leaves by removing the larger, outer leaves and allowing the center of the plant to continue producing. Kale will be good throughout the summer months, but especially good after a light frost.
If you need to store picked kale, place it in the refrigerator and keep it moist but not sealed. It can retain its crispness for a week or two.