Heirloom Kohlrabi Planting Guide: From Seed to Harvest

Heirloom kohlrabi (Brassica oleracea) is a hardy biennial plant often cultivated as an annual, admired for its unique, swollen stem and mild, slightly sweet flavor. This cool-season vegetable, part of the brassica family alongside cabbage, broccoli, and kale, is especially cherished for its adaptability, resilience, and the vibrant color and taste of its heirloom varieties.

Growing heirloom kohlrabi offers gardeners a chance to cultivate an ancient, open-pollinated crop that has been passed down through generations, providing a healthier, diverse harvest. Unlike hybrids, heirloom varieties retain their natural flavors and distinct characteristics, offering a richer taste and a visually striking addition to any garden.

This heirloom kohlrabi planting guide walks you through every step, from starting seeds indoors to harvesting mature, flavorful stems, ensuring a bountiful, nutritious crop that adds variety and beauty to your gardening experience.

Quick Start Heirloom Kohlrabi Planting Guide – Complete Planting Guide Below

Heirloom / Open Pollinated Kohlrabi is a hardy biennial grown as an annual. Kohlrabi seeds can be started indoors 4-6 weeks before the last frost, then set out transplants when there are 6-8 true leaves and nighttime temps remain above 30°F. Direct sow Kohlrabi seeds 3-4 weeks before the last frost in spring, or in late summer for a winter harvest. Plant Kohlrabi in full sun, ¼ – ½” deep, 1″ apart in rows 12-18″ apart.

Thin Kohlrabi plants to or set out transplants 3-8″ apart. Germination of Kohlrabi seeds should occur in 5-17 days in soil 55-75°F. For extended harvest of Kohlrabi, stagger plantings throughout a month in spring or fall plantings. For tender sweet Kohlrabi, harvest when stem reaches 2″ in diameter. For general use and storage crop harvest at 3-5″ in diam. Fall Kohlrabi crops will remain hardy to 10°F.

The Complete Planting Guide for Heirloom Kohlrabi: From Seed to Harvest

Understanding Heirloom Kohlrabi

Heirloom kohlrabi varieties differ from modern hybrids due to their open-pollinated nature. Unlike hybrids, which often prioritize uniformity over flavor, heirlooms maintain a diverse genetic profile that contributes to their unique flavors, appearance, and hardiness. In the garden, heirloom kohlrabi offers two main types: Purple Vienna, with a rich violet bulb, and White Vienna, a pale, green bulb with a milder taste. These traditional varieties are robust and adaptable, making them excellent for organic or sustainable gardening.

How to Grow Heirloom Kohlrabi: Season and Climate

When considering how to grow heirloom kohlrabi, timing is crucial. Kohlrabi thrives in cool weather, making it an ideal crop for both spring and fall planting. Here’s how to get the timing right:

• Spring Crop: Start heirloom kohlrabi seeds indoors 4-6 weeks before the last frost. Once seedlings reach 6-8 true leaves and nighttime temperatures remain above 30°F, they are ready for transplant.
• Fall Crop: Direct sow seeds in late summer, about 8-10 weeks before the first expected frost, for a late fall or winter harvest. Kohlrabi is frost-tolerant and can survive temperatures as low as 10°F, making it a reliable crop in colder weather.

Preparing the Soil

Kohlrabi grows best in rich, well-drained soil with a slightly acidic to neutral pH range between 6.0 and 7.5. Follow these steps to prepare your soil for planting heirloom kohlrabi:

1. Soil Preparation: Loosen soil to a depth of 6-8 inches, adding compost or well-rotted manure to enhance fertility.
2. Nutrient Boost: For a nutrient boost, incorporate bone meal or rock phosphate, which supports strong root development and is particularly beneficial for brassica plants.
3. Moisture Control: Heirloom kohlrabi requires even moisture to thrive. Mulching around plants helps regulate soil temperature and retain moisture, which is essential for a healthy crop.

Planting Heirloom Kohlrabi: Sowing and Spacing

When planting heirloom kohlrabi, consider both indoor seed starting and direct sowing methods:

• Indoor Seed Starting: Plant seeds ¼–½ inch deep in seed trays filled with high-quality potting mix. Once germination occurs (typically in 5-17 days at temperatures between 55-75°F), thin seedlings to ensure they have adequate space.
• Direct Sowing: Sow seeds ¼–½ inch deep directly into the soil, spacing them about 1 inch apart. Rows should be spaced 12-18 inches apart to allow for good airflow and sunlight. When seedlings reach 3-4 inches in height, thin them to 3-8 inches apart.

For extended harvests, stagger plantings throughout a 2-3 week period during the spring or fall. This approach ensures a continuous yield of tender, flavorful kohlrabi stems.

Transplanting Seedlings

Once seedlings are ready for the garden, transplant them when nighttime temperatures consistently stay above 30°F. Be mindful of spacing, as kohlrabi needs room to develop its signature swollen stem. Space plants 3-8 inches apart in rows spaced 12-18 inches apart. After transplanting, water thoroughly to reduce transplant shock and encourage strong growth.

Care and Maintenance for Heirloom Kohlrabi

A few key practices will keep your kohlrabi healthy and productive:

• Watering: Provide consistent moisture, aiming for 1-1.5 inches of water weekly. Mulching helps retain moisture and prevent soil from drying out.
• Fertilization: Add a balanced organic fertilizer every 3-4 weeks, particularly if leaves start to yellow, which may indicate a nutrient deficiency.
• Weed Control: Kohlrabi struggles against weeds, so it’s essential to keep the bed weed-free. Regular hand weeding or using a thick layer of mulch will prevent weeds from competing with kohlrabi for nutrients and water.

Harvesting Heirloom Kohlrabi

Harvesting kohlrabi at the right time is essential to enjoy its best flavor and texture. Kohlrabi is at its peak when the swollen stem, often mistaken for a root, is firm and tender. This versatile vegetable can be harvested in different stages, depending on its intended use—whether for fresh eating, cooking, or storage.

When to Harvest Kohlrabi

• Size and Maturity: For the best taste, heirloom kohlrabi should be harvested when the bulbous stem reaches 2–3 inches in diameter. At this size, the flesh is tender and sweet. If allowed to grow larger, the bulb may become woody or fibrous, particularly in warmer weather.
• Variety Considerations: Different heirloom varieties have varying ideal sizes. For example, smaller varieties like Purple Vienna are best harvested at around 2 inches in diameter, while larger varieties like Superschmelz can be left to grow up to 8 inches without becoming tough or woody.
• Fall Crops: Kohlrabi grown for fall harvest can often remain in the ground longer, as cool temperatures improve flavor and help prevent the stems from becoming woody. Kohlrabi can survive light frosts and even temperatures as low as 10°F, making fall an ideal season for harvesting a sweeter, more flavorful crop.

How to Harvest Kohlrabi

• Harvesting Bulbs: To harvest, use a sharp knife to cut the kohlrabi bulb just above the soil line. Avoid pulling the plant from the ground, as this can damage the bulb or disturb nearby plants.
• Harvesting Leaves: The leaves of kohlrabi are also edible and can be harvested at any time. Cut leaves individually from the plant or harvest a few at once. Leaves can be used like any leafy green, adding a nutrient-dense option to your diet.

Tips for a Continuous Harvest

• Succession Planting: For a continuous supply of kohlrabi, stagger your plantings every 2-3 weeks during the spring or fall growing season. This method provides a steady harvest of fresh bulbs over an extended period, rather than a single large harvest.
• Overwintering: In mild climates, kohlrabi can be left in the ground over winter. Mulch around plants to insulate them from fluctuating temperatures. Overwintered kohlrabi will often resume growth in early spring, providing an early harvest.

Signs of Overripe Kohlrabi

It’s important to monitor kohlrabi plants regularly to ensure you catch them at the right time. Here are signs that a kohlrabi bulb may be overripe:

• Tough or Woody Texture: Larger, overgrown kohlrabi bulbs often become tough and fibrous, particularly in warmer temperatures. These are best used in cooked dishes where they can soften.
• Cracking: Overripe kohlrabi may crack or split as it grows. While cracked kohlrabi is still edible, it won’t store as well and may lose some of its natural sweetness.

After Harvesting

Once harvested, remove the leaves if you plan to store the bulb for an extended period, as the leaves pull moisture from the bulb and reduce its shelf life. Kohlrabi can be stored in the refrigerator or preserved for longer-term storage. Refer to the “Storing and Preserving Kohlrabi” section for detailed storage options.

With these harvesting tips, you can ensure your heirloom kohlrabi reaches the table at its freshest and most flavorful stage, whether enjoyed raw in salads or cooked in a variety of dishes.

Managing Pests and Diseases in Heirloom Kohlrabi

Growing heirloom kohlrabi comes with the joy of cultivating traditional varieties, but like all brassicas, it can face challenges from certain pests and diseases. Maintaining plant health and using preventative measures are the best strategies for protecting your heirloom crop. Below is a guide to the most common pests and diseases affecting kohlrabi, along with organic solutions to keep them under control.

Common Pests and Organic Control Methods

1. Cabbage Worms
   • Identification: Cabbage worms are green caterpillars that feed on the leaves, creating holes and leaving frass (droppings) behind.
   • Control: Handpick worms regularly, and use row covers to prevent butterflies from laying eggs on the plants. Introduce beneficial insects, like parasitic wasps, which prey on cabbage worms. Organic insecticides like Bacillus thuringiensis (Bt) can also help control infestations without harming beneficial insects.
2. Aphids
   • Identification: Aphids are tiny, soft-bodied insects that cluster on the undersides of leaves, sucking sap and causing leaves to curl and yellow.
   • Control: Spray aphids off with a strong stream of water or apply insecticidal soap for severe infestations. Encourage natural predators like ladybugs and lacewings, which feed on aphids. Planting companion plants such as marigolds or nasturtiums nearby can also help repel aphids.
3. Flea Beetles
   • Identification: Flea beetles are small, black or brown beetles that create tiny holes in leaves, often causing significant damage to seedlings.
   • Control: Use floating row covers to protect young plants, especially during early growth stages. Apply diatomaceous earth around plants as a natural deterrent. For larger infestations, neem oil can help reduce flea beetle populations organically.
4. Cutworms
   • Identification: Cutworms are fat, grayish larvae that chew through plant stems at the soil line, often cutting down young seedlings.
   • Control: Place collars around seedlings to protect stems, or sprinkle coffee grounds and crushed eggshells around the base of plants as a deterrent. Encourage birds in your garden, as they help control cutworm populations.

Common Diseases and Prevention Methods

1. Clubroot
   • Identification: Clubroot causes swollen, misshapen roots and leads to stunted plant growth, yellowing leaves, and wilting.
   • Prevention: This soil-borne disease thrives in acidic conditions, so maintain a soil pH of 6.5-7.5. Apply lime to raise soil pH if needed, and rotate crops to avoid planting kohlrabi or other brassicas in the same spot each year. Avoid moving soil from infected areas to new planting zones.
2. Black Rot
   • Identification: Black rot causes yellow V-shaped lesions on leaf edges, which eventually turn black and spread, weakening the plant.
   • Prevention: Black rot spreads through infected soil and plant debris. Use certified disease-free seeds, avoid overhead watering, and remove any affected plants promptly to prevent the disease from spreading. Crop rotation and keeping a clean garden are essential in managing black rot.
3. Downy Mildew
   • Identification: Downy mildew appears as yellowish spots on the upper side of leaves, with grayish mold underneath. It thrives in cool, damp weather.
   • Prevention: Space plants to improve airflow and avoid excess moisture on leaves by watering at the soil level. Applying an organic copper fungicide can help protect plants, especially in humid conditions. Rotate crops to minimize fungal build-up in the soil.

General Preventive Measures for Healthier Plants

• Crop Rotation: Rotate kohlrabi with non-brassica crops each season to prevent soil-borne diseases and pest infestations.
• Proper Spacing: Space plants according to recommended guidelines (3–8 inches apart in rows 12-18 inches apart) to reduce humidity around plants and improve air circulation.
• Mulching: A layer of organic mulch, such as straw or shredded leaves, helps prevent soil-borne pathogens from splashing onto leaves and keeps moisture levels steady.
• Clean-Up: Remove any dead or diseased plant material promptly and clean up garden debris after each season to reduce overwintering pests and diseases.

By staying vigilant and implementing these organic pest and disease management strategies, you can enjoy a robust and healthy heirloom kohlrabi crop, free from major pest and disease issues. Regular monitoring and preventive care ensure a bountiful harvest, preserving the unique qualities of your heirloom varieties.

Storing and Preserving Heirloom Kohlrabi

Proper storage and preservation techniques for heirloom kohlrabi allow you to enjoy this versatile vegetable long after the growing season. Kohlrabi can be stored fresh, frozen, or even pickled, each method helping retain its crisp texture and mild flavor. Here’s how to store and preserve kohlrabi for both short-term and long-term use.

1. Short-Term Storage

Fresh kohlrabi can last in the refrigerator for up to two weeks with proper preparation:

• Remove the Leaves: Cut off the leaves as they pull moisture from the bulb, which shortens its storage life. However, don’t discard the leaves—they can be used like other leafy greens in salads, soups, and sautés.
• Refrigerate in Crisper Drawer: Place the bulbs in the crisper drawer of your refrigerator. Store them in a breathable bag, such as a plastic bag with holes or a mesh bag, to maintain humidity and prevent wilting.
• Storage Life: When stored correctly, kohlrabi bulbs can stay fresh in the fridge for about 1-2 weeks. Regularly check for any signs of softness, as this indicates they’re beginning to lose quality.

2. Long-Term Storage Methods

For longer storage, freezing or pickling kohlrabi are excellent options that preserve its flavor and nutritional value.

Freezing Kohlrabi

Freezing is one of the best ways to preserve kohlrabi for use throughout the year. Follow these steps for best results:

• Preparation: Wash and peel the kohlrabi bulbs to remove the tough outer skin. Cut them into slices, cubes, or julienne strips based on how you plan to use them later.
• Blanching: Blanch the prepared kohlrabi by boiling it for 2-3 minutes, then immediately transfer it to an ice bath for an additional 2-3 minutes to stop the cooking process. Blanching helps preserve the texture, color, and nutrients during freezing.
• Drying and Packing: Drain the kohlrabi and pat it dry. Place the pieces in a single layer on a baking sheet and pre-freeze for 1-2 hours to prevent them from clumping. Then, transfer the frozen pieces to airtight freezer bags or containers.
• Storage Life: Frozen kohlrabi can last up to 10-12 months in the freezer. Label your bags with the date to keep track of freshness.

Pickling Kohlrabi

Pickling is a fantastic way to preserve kohlrabi and add a tangy twist to its natural flavor. Pickled kohlrabi makes a flavorful addition to salads, sandwiches, and charcuterie boards. Here’s a simple method:

• Preparation: Wash, peel, and cut the kohlrabi into matchsticks or thin slices.
• Prepare Pickling Brine: In a saucepan, combine 1 cup of vinegar (such as white, apple cider, or rice vinegar), 1 cup of water, 1 tablespoon of salt, and 1 tablespoon of sugar. Bring the mixture to a boil, stirring until the sugar and salt dissolve.
• Optional Add-Ins: Add spices and herbs to the brine for extra flavor, such as garlic, dill, mustard seeds, or red pepper flakes.
• Packing: Place the kohlrabi in sterilized glass jars, leaving about ½ inch of headspace. Pour the hot brine over the kohlrabi, ensuring it’s fully submerged.
• Seal and Refrigerate: Seal the jars with lids and allow them to cool. Store in the refrigerator for 24 hours before eating to allow flavors to develop. Pickled kohlrabi can last up to 1 month in the refrigerator.

Root Cellar Storage

If you have a root cellar or a cool, dark basement, kohlrabi can also be stored there for a few months:

• Preparation: Leave about an inch of stem on each bulb, but remove the leaves.
• Storage Conditions: Place the kohlrabi bulbs in a container filled with damp sand, sawdust, or straw to help retain moisture. Ensure the storage temperature is between 32-40°F with high humidity (about 90-95%).
• Storage Life: Stored this way, kohlrabi can last 2-3 months, depending on the conditions. Check periodically for any signs of mold or decay.

3. Using Preserved Kohlrabi

Preserved kohlrabi is versatile and can be used in various dishes:

• Frozen Kohlrabi: Thaw and roast, add to stews, or use in stir-fries for a fresh, crisp flavor.
• Pickled Kohlrabi: Serve as a side dish, chop into salads, or add as a garnish to sandwiches for a tangy bite.
• Cellar-Stored Kohlrabi: Use as you would fresh kohlrabi—peel, chop, and add to raw salads or cook as desired.

With these storage and preservation methods, you can enjoy your heirloom kohlrabi harvest well beyond the growing season, bringing the flavors of your garden to the table all year long.

Saving Seeds from Heirloom Kohlrabi

Saving seeds from heirloom kohlrabi is an excellent way to preserve unique varieties and ensure a continuous supply of open-pollinated plants with consistent flavors and growth habits. Since kohlrabi is a biennial, it will flower and produce seeds in its second year. Here’s a step-by-step guide to successfully saving heirloom kohlrabi seeds:

1. Selecting Parent Plants

• Choose the healthiest and most vigorous plants to save seeds from. This helps maintain desirable traits, such as flavor, hardiness, and resistance to pests or diseases.
• Ensure the plants are true heirlooms, as hybrids won’t produce reliable offspring.

2. Overwintering the Plants

• Because kohlrabi is a biennial (Brassica oleracea), it needs to overwinter before it will bolt, flower, and produce seeds in its second year.
• In climates with mild winters, simply leave a few selected plants in the ground over winter. For colder climates, plants can be dug up in late fall, stored in a root cellar or cool location, and replanted outdoors in early spring.

3. Bolting and Flowering

• Once the temperature warms up in spring, kohlrabi plants will start to bolt, sending up tall flower stalks.
• These stalks produce small, yellow flowers, which eventually develop into seed pods. Allow the pods to mature and dry fully on the plant.

4. Harvesting Seed Pods

• When the seed pods have dried and turned brown, they are ready to be harvested. Be mindful to collect the pods before they begin splitting open, as they can drop seeds and self-sow if left on the plant for too long.
• Carefully cut the stalks with seed pods and place them in a paper bag to catch any seeds that might fall during the drying process.

5. Cleaning and Storing Seeds

• Once the pods are completely dry, gently crush them to release the seeds. Separate the seeds from the chaff (the dried plant material) by hand or using a fine sieve.
• Place the seeds in a cool, dry area to ensure they are fully dry before storage.
• Store the seeds in a labeled envelope or jar, noting the variety and the date of harvest. Keep seeds in a cool, dark, and dry place, such as a pantry or dedicated seed storage container, to maintain viability.

6. Seed Viability

• Kohlrabi seeds generally remain viable for 3-5 years if stored properly. Conducting a germination test before planting each season can help ensure a strong crop.

By saving heirloom kohlrabi seeds, you’re not only preserving unique genetic lines but also fostering a sustainable and resilient garden. Plus, each season, your saved seeds adapt slightly more to your specific growing conditions, improving their performance over time.

Conclusion

Planting heirloom kohlrabi is an enriching experience that brings a unique, nutritious crop to the garden. With careful planning, regular maintenance, and proper harvesting, heirloom kohlrabi rewards gardeners with a consistent and versatile yield. Embrace this hardy, open-pollinated brassica in your garden for a sustainable, flavorful addition to any season’s harvest. Whether you’re enjoying it fresh, roasted, or stored for winter, heirloom kohlrabi will undoubtedly become a favorite in your garden rotation.

FAQ: Heirloom Kohlrabi Planting Guide: From Seed to Harvest