Kale

Fans of “Cheers” might remember when kale literally was a punchline. In the ensuing 34 years, however, kale’s popularity has increased enormously – deservedly so, in my opinion. Kale is quite nutritious: it’s full of fiber, surprisingly high in protein, and a rich source of vitamin C. Kale also tastes great, despite what Woody Boyd might say. Try a baby kale salad with lemon-tahini dressing, or add kale to fruit smoothies to dial back the sweetness. Homemade kale chips are always a tasty idea.

Kale is easy to grow in the home garden. It’s cold-hardy, so with just a little extra effort you can have fresh kale year-round. This article describes kale varieties and tells how to start kale and keep it free of diseases and insects. It also includes techniques for growing kale all year long. In Virginia, September is a great time to start kale as a winter crop.

Kale varieties

Kale belongs to the species Brassica oleracea, the same species as cabbage, cauliflower, and Brussels sprouts. Kale falls under the acephala variety of B. oleracea, i.e. “headless.” (Collards are also B. oleracea var. acephela, and we’ll cover them in a future article.) The name kale comes from the Latin word caulis, meaning stem. Historians believe that kale has been a distinct crop for thousands of years, originating around the Mediterranean Sea.

Kale varieties include:

• Lacinato/Tuscan/dinosaur: Heavily crinkled leaves which are tender and delicious raw. Leaves can be quite skinny, only a couple of inches wide, or as wide as 6”. Colors range from light green to dark green or almost black. In my experience, Lacinato kales are magnets for insect pests, making them useful as “trap crops” (see below).
• Curly, green and red: Leaves are tough unless harvested young but are delicious and soften nicely when sliced and cooked, such as in soups. Their curliness adds texture and visual appeal.

  

• Red Russian: Flat wide leaves with long skinny lobes and red-purple stems and veins. These are very tender, even when mature.
• Siberian: Moderately curly with green leaves that have white stems and veins. Fairly tough leaves, so mostly eaten cooked. As the name suggests, this variety is extremely cold tolerant.

Within these general varieties there are cultivars that represent different combinations of taste, color, texture, cold tolerance, etc. There are also many hybrid varieties. Cornell University maintains a list of kale varieties with helpful ratings from growers.

Starting kale plants

Outdoors To start plants outdoors from seed, sow in late winter or summer. The Virginia Cooperative Extension Home Garden Vegetable Planting Guide lists planting dates for each USDA hardiness zone in Virginia. Note that the guide has not been updated with the latest USDA hardiness zone map. Planting dates for each zone are accurate, but consult the USDA map rather than the map in the guide to find your hardiness zone.

Kale germinates between 45℉ and 85℉. Use a soil thermometer to determine optimal planting date(s). Shade cloth or a sheet of clear plastic are useful for lowering or raising, respectively, the soil temperature.

Prepare the soil by loosening it with a broadfork or spading fork, then rake to break up clumps. If starting a new garden bed, work 4” of compost into the top 12” of soil. Otherwise, we recommend no-till methods for established garden beds. Add fertilizer following the recommendations from a recent soil test, raking it into the top 6” of soil. Our Soil test kits are available from the Virginia Cooperative Extension office in the Albemarle County Office Building, 460 Stagecoach Rd. in Charlottesville.

Sow seeds ¼ – ½” deep, spaced 1” apart, in rows 18 – 30” apart. Keep the soil moist until germination, especially if it tends to crust over. After seedlings are well-established, thin them to 18 – 24” inches apart.

Indoors If you are planting indoors for transplanting later, plan to transplant about 4 weeks before the average date of the first frost. That means you should sow seeds 4 – 6 weeks before the target date for transplanting. Remember to harden off seedlings by putting them outside daily for increasing lengths of time, starting 7 – 10 days before transplanting. For details about starting seeds indoors, consult Piedmont Master Gardener Cleve Campbell’s classic article.

Diseases

In general, kale is not seriously affected by diseases, but a few can cause trouble:[symple_column size=”one-third” position=”first” fade_in=”false”]

• Alternia leaf spot: Round brown spots on leaves and, in other Brassicas, heads. The spots usually have yellow halos.
• Black leg: Sunken brown patches form near the soil line. The pathogen damages vascular tissue, so if the patches encircle the stem, the plant will die.
• Black rot: Yellow lesions develop at the edges of leaves, eventually turning brown. Damage may spread to vascular tissue and, if present, the head.
• Clubroot: Plants are wilted and stunted, even when the soil is moist. Roots become swollen and gnarled.

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[/symple_column]Most diseases can be prevented using good sanitation and cultural practices. Pathogens are spread by seeds, so buy from a reputable supplier. If you are saving seeds, don’t keep any seeds from infected plants. Separate outdoor plants by at least 18” to promote air flow, which will keep plants dry and reduce fungal growth. Be sure to remove infected plants immediately: nothing will cure them! Slow the spread of pathogens by washing soil off your tools, then disinfecting them with a 10% bleach solution. Insects also spread disease, so follow suggestions for insect control (see below). At the end of the growing season, clean up all plant debris. If possible, rotate crops and avoid growing kale where Brassicas have grown for the past two years.

A fungicide spray program might be useful to prevent infection, but that won’t do anything to cure established diseases. Consult the VCE Pest Management Guide for details about fungicides for kale. When using fungicides, always follow the instructions on the label and use personal protective gear.

Insect pests

Insects, on the other hand, can be extremely damaging to kale (see picture). Major pests include:[symple_column size=”one-third” position=”first” fade_in=”false”]

• Aphids: Tiny green, yellow, or gray insects that suck sap out of plants, causing wilted or shriveled foliage. Short life cycle; infestations develop rapidly.
• Cabbageworms: Small leaf-eating green caterpillars that develop into white moths. Sometimes called cabbage whites. Hard to see but can reduce a leaf to a skeleton very quickly.
• Cabbage loopers: More small green voracious leaf-eating caterpillars, identifiable by their hunched appearance (see picture). Develop into brown moths.
• Flea beetles: Tiny and hard to see, but their damage is unmistakable: many small round holes on leaves. Usually won’t kill a plant but the damage reduces vigor.[/symple_column]

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Prevention is the best way to handle insects. Clean up all plant debris at the end of the season to reduce over-wintering any insects. Rotate crops to keep seedlings away from insects that do survive the winter. Scout plants for damage and eggs – check the undersides of leaves (see picture) – and trim affected leaves. A light row cover will keep moths from laying their eggs on your plants without reducing growth much (see below).

Some kale varieties, especially Lacinato, are more attractive to insects. It’s not clear why: perhaps the sugar content or the toughness of the leaf tissue. Regardless, I often find a skeletal Lacinato plant next to a healthy Red Russian. Thus, try growing a few Lacinato plants interspersed among the rest of your kale as a trap crop, grown not for harvest but to attract insects. The insects in your area might have different preferences, so try several kale varieties and note the outcomes.

Insecticides can be helpful for preventing insect damage or for treating severe infestations, but avoid causing harm to pollinators: spray carefully, so insecticides don’t drift onto flowering plants, and spray in the evening when bees aren’t likely to be active. Detailed information about insecticides can be found in the VCE Pest Management Guide. Always follow the instructions on the label and use personal protective gear when spraying insecticides.

Year-round kale harvest

Kale is cold-tolerant and can handle a hard freeze of 26-28℉, even when plants are unprotected. In fact, flavor improves after a frost because the plants respond by making sugars as cryoprotectants. To keep kale alive all winter, protect plants with row covers supported by hoops. Piedmont Master Gardener Ralph Morini’s article on row covers details how to make row covers using inexpensive and easy-to-find materials.

Covers will keep kale alive, but cold temperatures and short days slow its growth over the winter months. For a solid winter harvest, start kale several weeks before the first frost.

Row covers can also keep insects away in summer. There are materials made specifically for this purpose that are lightweight and water-permeable with a fine weave that excludes insects. I can personally attest that row covers have kept my kale going all summer, long after unprotected plants would be shredded by caterpillars. One lesson I learned the hard way: put up covers early, before any sign of insect damage!

Harvesting technique is also key to extending a kale crop. Use the “cut-and-come-again” method: trim from the outside of the plant, leaving the inner core where new leaves appear. The plants will keep growing and can eventually extend several feet high (see picture). Side stalks may branch from the main stem.

When it warms up in late winter, over-wintered kale will bolt, i.e. flower. This tends to make the leaves bitter. At this point, pull up the plants and start a new crop. Or, simply cut off the flower stalks. In my experience, the plants settle down and grow normally. But if temperatures drop below freezing again, the plants will bolt anew.

Final thoughts

If you’re hankering for a kale salad and all you have around are old, tough leaves, give them a massage. Cut out the central stalk plus any large veins, then slice thinly. Add a little lemon juice and salt; the acidity will help break down cellulose fibers, and the salt will provide grit to intensify the massage. Then crush the leaves in your hands for a minute or two, until the leaves are dark and soft. Add some croutons and cheese, toss with your favorite dressing, and tuck in.

 

References and further reading

Featured photo: Conall. CC BY 2.0 (cropped)

The Winter Harvest Handbook, Eliot Coleman

Alternia Leaf Spot and Head Rot of Brassica Crops University of Minnesota Extension

Black Leg University of California Agriculture and Natural Resources

Black Rot of Brassica Crops University of Minnesota Extension

Clubroot University of Minnesota Extension

Cut and Come Again Vegetables Michigan State University Extension

FoodData Central: Kale US Department of Agriculture

Frost dates by Zip Code National Gardening Association

Growing Collards and Kale in Home Gardens University of Minnesota Extension

How to Grow Kale in Your Garden Utah State University

Leafy Green Vegetables Virginia Cooperative Extension

No-till in the Home Garden: Why and How Ralph Morini, Piedmont Master Gardeners

Pest Management Guide – Home Grounds and Animals Virginia Cooperative Extension

Plant Hardiness Zone Map US Department of Agriculture

Row Covers Ralph Morini, Piedmont Master Gardeners

Starting Seeds Indoors Cleve Campbell, Piedmont Master Gardeners

Vegetable Varieties for Gardeners: Kale Cornell University Extension

Virginia Home Gardens Vegetable Planting Guide Virginia Cooperative Extension

Virginia Tech Soil Testing Lab