How do I winterize kale? - Plant Care Guide

To winterize kale effectively and extend its harvest season, the primary strategy involves selecting cold-hardy varieties, providing physical protection from harsh cold and wind, and ensuring consistent moisture. Unlike many tender vegetables that die with the first frost, kale is remarkably cold-tolerant, and proper winterization can allow it to continue producing fresh leaves even through snow and freezing temperatures, often sweetening its flavor.

What is kale and what makes it cold-hardy?

Kale (Brassica oleracea var. sabellica) is a highly nutritious, leafy green vegetable belonging to the cabbage family (Brassicaceae). It's renowned for its robust flavor, health benefits, and remarkable ability to withstand cold temperatures, often surviving well beyond the first frosts that kill off most other garden vegetables.

What Kale Is:

• Appearance: Kale typically grows as an upright plant with large, thick, often curly or ruffled leaves, though some varieties have flatter or lacier foliage. Common colors include various shades of green, but ornamental kales also boast purple, red, and even white coloration.
• Growth Habit: It's a non-heading brassica, meaning it doesn't form a tight head like cabbage. Leaves are harvested individually or by cutting the entire plant from the top.
• Nutritional Powerhouse: Rich in vitamins K, A, and C, as well as antioxidants and fiber, making it a popular "superfood."

What Makes Kale Cold-Hardy:

Kale's exceptional cold-hardiness is due to several physiological adaptations that allow it to survive and even thrive in temperatures that would destroy most other vegetables.

1. Cryoprotectants (Natural Anti-freeze):

   • As temperatures drop, kale plants increase their production of natural sugars and other compounds (like betaines and proline) that act as cryoprotectants (similar to antifreeze). These compounds lower the freezing point of water within the plant's cells, preventing damaging ice crystals from forming and rupturing cell walls.
   • This is why kale often tastes sweeter after a frost – the conversion of starches to sugars for cold protection enhances its flavor.

2. Cell Membrane Stability:

   • Kale's cell membranes are more flexible and stable in cold temperatures compared to less hardy plants. This allows them to withstand the stresses of freezing and thawing cycles without breaking down.

3. Dormancy/Slowed Metabolism:

   • While not truly dormant like a tree, kale slows its metabolic processes dramatically in cold weather. It shifts energy from rapid growth to maintenance and cold protection, allowing it to conserve energy.

4. Tough Leaf Structure:

   • Compared to tender lettuces, kale leaves are naturally more fibrous and robust, giving them a degree of physical resilience against cold, wind, and even light snow loads.

5. Root System:

   • Its relatively strong root system helps anchor the plant and draw what little moisture is available from cold soil.

These adaptations allow kale to tolerate temperatures down into the teens Fahrenheit (-10°C to -7°C) and often even lower (single digits Fahrenheit or colder) with proper protection, making it an excellent candidate for winter harvesting in many temperate climates.

What are the benefits of overwintering kale in the garden?

Overwintering kale in the garden offers several significant benefits for both the gardener and the plant, extending the harvest season, enhancing flavor, and providing a fresh source of greens during typically lean months.

1. Extended Harvest Season:

   • Fresh Greens in Winter: The most immediate and satisfying benefit is having access to fresh, homegrown kale leaves long after most other garden vegetables have succumbed to frost. In many regions, you can harvest kale even when there's snow on the ground, sometimes from late fall all the way into early spring.
   • Early Spring Boost: Overwintered kale plants often rebound vigorously in early spring, providing a very early harvest of tender, new leaves before newly planted spring crops are ready.

2. Improved Flavor and Texture:

   • Sweetness After Frost: As mentioned, cold temperatures trigger the kale plant to convert starches into sugars as a natural antifreeze. This process significantly sweetens the flavor of kale, making it less bitter and more palatable, especially for varieties that can be slightly tough. Many gardeners find overwintered kale to be the most delicious.
   • Tenderization: Light frosts can also help to slightly tenderize the leaves, making them more enjoyable to eat.

3. Cost Savings and Food Security:

   • Having fresh, homegrown vegetables readily available in winter reduces your reliance on grocery stores for greens during the colder months, potentially saving money and enhancing your household's food security.

4. Reduced Food Miles:

   • Harvesting from your own backyard in winter significantly reduces the "food miles" associated with produce that often has to be shipped long distances from warmer climates, supporting a more sustainable lifestyle.

5. Garden Activity in Winter:

   • For passionate gardeners, being able to go out and harvest fresh produce in winter provides a rewarding connection to the garden during a time when most other plants are dormant.

6. Nutrient Cycling (If Allowed to Die Back):

   • Even if the kale eventually succumbs to extreme cold, allowing it to remain in the bed and then tilling it under in spring (or letting it decompose if using no-till methods) returns valuable organic matter and nutrients to the soil, improving its health.

Overwintering kale transforms it into a dependable, delicious, and deeply satisfying winter harvest, truly extending the productivity and enjoyment of your garden into the colder months.

What kale varieties are best for overwintering and cold hardiness?

Choosing the right kale varieties is paramount for successful overwintering, as some types are significantly more cold-hardy than others. Selecting cultivars specifically bred or known for their resistance to freezing temperatures will yield the best results.

Here are the kale varieties best for overwintering and cold hardiness:

1. 'Winterbor' Kale:

   • Characteristics: This is often considered the gold standard for cold hardiness. It's a tall, upright variety with very finely curled, dark blue-green leaves. It can tolerate temperatures well into the single digits Fahrenheit (around -10°C to -15°C) or even lower with protection.
   • Flavor: Maintains good flavor and texture in cold.
   • Best For: Reliable overwintering in Zones 5-7 with protection, and even Zone 4 with heavy cover.

2. 'Lacinato' Kale (also known as Dinosaur Kale, Tuscan Kale, or Black Kale):

   • Characteristics: Distinctive dark blue-green, heavily textured, bumpy (savoyed) leaves. Often more tender than curly kales when mature.
   • Cold Hardiness: Very good cold tolerance, typically surviving into the low teens Fahrenheit (around -10°C) without much protection, and colder with cover. It also benefits significantly from frost sweetening.
   • Flavor: Excellent flavor, often preferred by chefs.
   • Best For: Overwintering in Zones 6-8, and 5 with protection.

3. 'Red Russian' Kale:

   • Characteristics: Flat, deeply lobed leaves with reddish-purple stems and veins. It offers a unique texture and color.
   • Cold Hardiness: Good cold tolerance, often surviving into the low teens Fahrenheit. The reddish coloration can intensify with cold.
   • Flavor: Tender and sweet, even without a hard frost.
   • Best For: Zones 6-8, and 5 with protection.

4. 'Redbor' Kale:

   • Characteristics: A striking ornamental and edible kale with intensely ruffled, deep purple to red leaves. Very upright growth.
   • Cold Hardiness: Excellent cold tolerance, similar to 'Winterbor', with colors becoming more vibrant in cold.
   • Best For: Adding visual appeal and edibility to the winter garden in Zones 5-8.

5. 'Siberian' Kale:

   • Characteristics: Large, broad, slightly frilly leaves, typically blue-green. Known for its extreme cold hardiness.
   • Cold Hardiness: One of the most cold-tolerant varieties, reportedly surviving down to 0°F (-18°C) or even colder with snow cover.
   • Best For: Gardens in very cold climates (Zones 4-6) looking for reliable overwintering.

Tips for Success with Cold-Hardy Varieties:

• Proper Planting Time: Plant in late summer/early fall to allow plants to establish and grow to a good size before the onset of truly cold weather.
• Good Drainage: Even the hardiest kale will succumb to root rot in soggy, frozen soil. Ensure excellent drainage.
• Mulch: A thick layer of organic mulch around the base provides extra insulation.
• Physical Protection: Even cold-hardy varieties benefit significantly from row covers or cold frames in colder zones.

By choosing one of these proven cold-hardy kale varieties, you set yourself up for a successful and delicious winter harvest.

What is the best timing for planting kale for overwintering?

The best timing for planting kale for overwintering is late summer to early fall, several weeks before your first expected hard frost. This allows the kale plants to become well-established and grow to a good size before the onset of truly cold weather, which is crucial for their survival through winter.

The exact timing will vary significantly by your USDA Hardiness Zone and local climate:

1. For Zones 7-8 (Milder Winters):

   • Timing: Plant seeds or transplants in late August to early September.
   • Reason: This gives the plants ample time to grow robust roots and a decent amount of foliage before winter. They'll likely provide a good harvest in late fall and then continue producing through winter, especially with minimal protection.
   • Harvest Window: Late fall, winter, and early spring.

2. For Zones 5-6 (Colder Winters, but possible with protection):

   • Timing: Plant seeds or transplants in mid-July to mid-August. You need an earlier start to ensure they are mature enough to withstand colder temperatures.
   • Reason: The goal is to get large, established plants that have built up their cold-hardiness before temperatures drop significantly.
   • Harvest Window: Late fall, potentially intermittent winter harvests on milder days, and an early spring flush. Protection is highly recommended.

3. For Zone 4 (Very Cold Winters, most challenging):

   • Timing: Plant seeds or transplants in late June to mid-July. This requires careful planning to avoid summer heat stress.
   • Reason: The longest establishment period is needed before hard freezes begin.
   • Harvest Window: Late fall, with very limited winter harvests even with heavy protection. Main benefit is a very early spring harvest.

Key Principles for Timing:

• Maturity Before Cold: The aim is for the kale plants to reach near-mature size (e.g., 6-8 sets of true leaves, 8-12 inches tall) before consistent freezing temperatures arrive. This ensures they have sufficient energy reserves and tough leaves to withstand the cold.
• Avoid Summer Heat Stress: For earlier planting dates in hotter climates, provide some afternoon shade or use heat-tolerant varieties to prevent bolting before winter arrives.
• Seed vs. Transplant: Starting from transplants (purchased or started indoors 4-6 weeks prior) can help you hit the optimal outdoor planting window, especially if your summers are hot or growing season short. You can buy kale seeds or kale plant starts.

By precise timing of your planting, you give your cold-hardy kale varieties the best possible chance to survive winter and provide a continuous, delicious harvest.

What physical protection methods are best for overwintering kale?

Even the most cold-hardy kale varieties benefit significantly from physical protection methods to ensure successful overwintering and extend their harvest through the coldest months. These methods create a warmer, more stable microclimate around the plants, shielding them from harsh winds, intense cold, and heavy snow.

Here are the best physical protection methods for overwintering kale:

1. Mulching (Essential Base Layer):

   • Purpose: Insulates the soil and plant crowns from freezing temperatures and protects shallow roots.
   • Method: Apply a thick layer (4-6 inches / 10-15 cm) of organic mulch (e.g., straw, shredded leaves, pine needles) around the base of the kale plants in late fall, before the ground freezes solid.
   • Benefit: Keeps soil temperatures more stable, preventing freeze-thaw cycles that can heave plants out of the ground.

2. Row Covers (Fabric Covers):

   • Purpose: Provides a lightweight, breathable barrier that traps heat and offers protection from light frosts, wind, and even snow.
   • Types: Use a heavyweight (1.5-2 oz/sq yd) floating row cover directly over plants or supported by hoops.
   • Method: Lay the fabric directly over the plants (it's light enough not to crush them) or drape it over garden hoops to create a mini-tunnel. Secure the edges with rocks, soil, or pins to prevent wind from blowing it away.
   • Benefit: Can raise temperatures inside by 5-10°F (3-6°C), extending harvest significantly in Zones 5-7. Allows light and water to penetrate. Easily removed for harvesting.

3. Cold Frames:

   • Purpose: A more substantial, permanent or semi-permanent structure that creates a warm microclimate.
   • Construction: A bottomless box (wood, brick, concrete blocks) with a transparent lid (old window, polycarbonate sheet) that traps solar heat.
   • Method: Build the cold frame over the kale plants. The lid can be propped open on sunny days for ventilation and closed at night or during cold spells.
   • Benefit: Provides excellent protection, raising internal temperatures by 10-20°F (6-11°C), suitable for Zones 4-6. Great for continuous winter harvesting. Many cold frame kits are available.

4. Cloches / Hotkaps:

   • Purpose: Individual covers for single plants.
   • Types: Can be glass bell jars, plastic domes, or DIY from milk jugs.
   • Pros: Good for small patches or individual plants.
   • Cons: Limited in size, requires daily venting on sunny days to prevent overheating.
   • Best For: Providing initial protection to newly planted fall kale.

5. Hoop Houses / Low Tunnels:

   • Purpose: Larger versions of row covers with hoops, suitable for covering entire rows or beds.
   • Method: Bend PVC pipes or metal hoops over the bed and drape heavy-duty row cover fabric or clear greenhouse plastic over them, securing tightly.
   • Benefit: More internal space than cold frames, good for multiple plants, offers significant cold protection. Many hoop house kits are available.

6. Snow Cover:

   • Natural Insulation: In areas with consistent snow, a layer of snow acts as excellent natural insulation, protecting plants from extreme cold.
   • Method: Don't remove snow from kale if it's there.

By layering these physical protection methods according to your climate and the severity of winter, you can dramatically increase the chances of successfully overwintering kale, enjoying fresh greens throughout the coldest months.

How do I harvest overwintered kale to ensure continued production?

Harvesting overwintered kale correctly is crucial to ensure the plant continues to produce fresh leaves throughout the winter and into early spring, rather than succumbing to stress or premature bolting. The "cut-and-come-again" method is key.

1. Harvest from the Bottom Up (Cut-and-Come-Again):

   • Method: Always harvest the outer, larger, lower leaves first. Use clean, sharp gardening snips or a knife to cut the leaves off cleanly, close to the main stem, but leave the central growing point intact. Also leave at least 4-5 inner, smaller leaves on the plant.
   • Why: This method allows the plant to continue producing new leaves from its center. Removing only the older leaves ensures the plant can still photosynthesize and sustain itself.

2. Harvest Selectively, Don't Strip the Plant:

   • Quantity: Only take what you need for a meal or two. Avoid stripping too many leaves from a single plant, especially in cold weather, as this can severely stress it and reduce its ability to recover.
   • Frequency: Harvest as needed, perhaps once or twice a week, or less frequently during very cold spells when growth is slow.

3. Harvest on Milder Days (If Possible):

   • Timing: If temperatures are very low, or the plant is frozen solid, avoid harvesting. Harvesting frozen leaves can cause cellular damage.
   • Thawing: Wait for a milder day when the leaves have thawed and are pliable. This minimizes damage to the plant and ensures better texture for eating.

4. Be Gentle with Protection:

   • If using row covers or cloches, carefully lift or remove them for harvesting to avoid damaging the plants. Replace them securely immediately after harvesting.

5. Monitor Plant Health:

   • Regularly inspect your overwintered kale plants. If a plant looks stressed, yellowing, or appears to be struggling, reduce harvesting from it or stop entirely until it recovers.
   • Remove any yellowed, dead, or mushy leaves (from cold damage or disease) promptly, but be careful not to remove too much healthy foliage.

6. Spring Harvest and Bolting:

   • As spring arrives and temperatures consistently warm up, your overwintered kale will put on a final burst of tender new growth. This is an excellent time for a final, abundant harvest.
   • Bolting Signal: Eventually, as temperatures rise consistently above 70-75°F (21-24°C), the kale plant will "bolt" – send up a tall central flower stalk. This is a sign that its life cycle is coming to an end. Once it bolts, leaves will become more bitter.
   • Final Harvest/Removal: You can still harvest leaves until they become too bitter, or you can allow the plant to flower (which provides food for pollinators) and then remove it to make way for spring/summer crops.

By following these harvesting practices, you can enjoy a continuous supply of sweet, tender kale throughout the winter, extending your garden's bounty into the leanest months.