Kohlrabi, scientifically known as Brassica oleracea var. gongylodes, is a unique member of the Brassicaceae family, which includes cabbage, broccoli, and kale. Also known as German turnip, kohlrabi is prized for its swollen stem, which resembles a turnip and is eaten as a vegetable. It has a mild, slightly sweet flavor and a crisp texture, making it a versatile ingredient in various culinary applications.
Kohlrabi Overview
Botanical Background and Global Popularity
Kohlrabi is believed to have originated in Europe and has been cultivated for centuries as a staple vegetable. It is particularly popular in Germany, Eastern Europe, and India, where it is used in a variety of traditional dishes. In recent years, kohlrabi has gained popularity worldwide due to its nutritional benefits and unique appearance. The plant is cool-season and grows best in temperate climates, where it is valued for its rapid growth and ability to produce a good yield even in cooler weather.
Edible Parts and Culinary Uses
The primary edible part of the kohlrabi plant is its swollen stem, often referred to as a bulb, though it is not a true bulb. The flesh is crisp and juicy, with a flavor that is often compared to a cross between a mild cabbage and a turnip. Kohlrabi can be eaten raw, sliced thinly in salads or slaws, or cooked in a variety of dishes. It can be roasted, steamed, or sautéed and is commonly used in soups and stews. The leaves of kohlrabi are also edible and can be used similarly to kale or collard greens.
Suitable Planting Season
Optimal Time for Sowing Kohlrabi
Kohlrabi is a cool-season crop that thrives in moderate temperatures. The best time to plant kohlrabi is in early spring or late summer, depending on your climate. In temperate regions, kohlrabi seeds can be sown directly into the garden 2 to 4 weeks before the last expected frost in spring. For a fall harvest, sow seeds in late summer, about 6 to 8 weeks before the first expected frost. Kohlrabi is frost-tolerant and can be grown throughout the growing season in cooler climates.
Seasonal Considerations for Different Climates
In cooler climates, kohlrabi can be grown almost year-round, with the main harvest periods in spring and fall. In warmer climates, it is best to plant kohlrabi in the fall or winter to avoid the hot summer months, which can cause the bulbs to become woody and tough. Succession planting, or sowing seeds every few weeks, can help ensure a continuous supply of fresh kohlrabi throughout the growing season.
Ideal Soil Type
Soil Composition and Texture
Kohlrabi grows best in well-draining, fertile soil with a light, crumbly texture. The ideal soil should be rich in organic matter, such as compost or well-rotted manure, which provides the nutrients necessary for healthy plant development. Loamy soil is ideal for kohlrabi, as it retains moisture without becoming waterlogged. Heavy clay soils can be problematic, as they tend to hold too much water, leading to poor bulb development and increased risk of disease. If your soil is heavy or compacted, consider amending it with organic matter to improve its structure and fertility.
Soil pH and Fertility
Kohlrabi prefers a slightly acidic to neutral soil pH, ideally between 6.0 and 7.5. Maintaining the correct pH is important for nutrient uptake, particularly for calcium and potassium, which are crucial for bulb development. Regular soil testing can help you monitor and adjust the pH as needed. Kohlrabi is a moderate feeder, so it is beneficial to incorporate a balanced organic fertilizer or compost into the soil before planting. Side-dressing with compost or applying a liquid fertilizer during the growing season can also help maintain soil fertility and support vigorous growth.
Sowing Tips
Preparing the Soil for Sowing
Before sowing kohlrabi seeds, it is essential to prepare the soil thoroughly. Start by removing any weeds, rocks, and other debris from the planting area. Loosen the soil to a depth of about 30 cm (12 inches) and incorporate a generous amount of compost or well-rotted manure to improve fertility and drainage. If you are planting in raised beds, make sure the soil is well-drained and has a fine, crumbly texture that allows the small seeds to germinate easily. For best results, lightly moisten the soil before sowing to create an ideal environment for seed germination.
Sowing Technique and Seed Spacing
Kohlrabi seeds can be sown directly into the garden bed or started indoors for later transplanting. To sow directly, plant the seeds about 0.6 cm (¼ inch) deep in rows spaced 30 to 45 cm (12 to 18 inches) apart. The seeds should be spaced 10 to 15 cm (4 to 6 inches) apart within the row to allow for sufficient plant development. If you prefer to start seeds indoors, sow them in trays or pots filled with a high-quality seed-starting mix, and transplant the seedlings when they are about 10 to 15 cm (4 to 6 inches) tall and have at least two sets of true leaves.
Protection and Germination
Kohlrabi seeds typically germinate within 5 to 10 days, depending on soil temperature and moisture levels. To protect the seeds and encourage even germination, consider covering the seedbed with a thin layer of mulch or a row cover, especially in cooler climates. This helps maintain consistent soil moisture and temperature, reducing the risk of poor germination or seedling damage. Once the seedlings have emerged, thin them to a spacing of 10 to 15 cm (4 to 6 inches) apart to allow for proper growth and air circulation.
Is Year-Round Harvesting Recommended?
Feasibility of Continuous Harvest
Kohlrabi can be harvested throughout the growing season with careful planning and succession planting. In cooler regions, where temperatures remain moderate year-round, kohlrabi can be grown and harvested continuously. However, in temperate climates, kohlrabi is typically grown as a biennial, with the main harvest periods occurring in spring and fall. To extend the harvest period, consider growing a mix of early, mid, and late-season varieties, or sowing seeds in succession every few weeks.
Succession Planting for Extended Harvest
Succession planting is an effective strategy for extending the kohlrabi harvest over several months. By sowing seeds every few weeks, you can ensure a steady supply of fresh kohlrabi without overwhelming yourself with too much produce at once. This method works particularly well in regions with variable climates, as it allows you to adjust planting times based on weather conditions. In cooler climates, consider using a cold frame or row cover to protect the plants from frost and extend the harvest into late autumn or early winter.
Growth Speed Reference
Germination and Early Growth
Kohlrabi is a relatively fast-growing plant, particularly when grown in optimal conditions. Seeds typically germinate within 5 to 10 days, and the seedlings are hardy enough to tolerate light frosts. During the first few weeks after germination, kohlrabi plants focus on developing a strong root system and producing their first set of true leaves. The plants grow steadily in cool, moist conditions, with the bulbs beginning to swell after about 4 to 6 weeks.
Time to Maturity and Harvest
Depending on the variety and growing conditions, kohlrabi is usually ready to harvest within 55 to 70 days from sowing. The bulbs can be harvested at various stages, from small, tender bulbs about the size of a golf ball to larger bulbs about the size of a tennis ball. For the best taste and texture, it is recommended to harvest kohlrabi bulbs when they are about 7.5 to 10 cm (3 to 4 inches) in diameter. Regular harvesting encourages the plant to produce more bulbs and extends the harvest period.
Considerations for Different Varieties
Different kohlrabi varieties may have slightly varying growth rates and harvest times. For example, early-maturing varieties like ‘Early Purple Vienna’ or ‘Winner’ are often ready for harvest in as little as 55 to 60 days, while slower-maturing varieties like ‘Superschmelz’ may take up to 70 days to reach full maturity. When selecting a variety, consider your local growing conditions and the desired harvest time. Varieties with larger bulbs may be better suited for fall harvesting, while smaller, more tender varieties are ideal for spring and summer.
Temporary Shade for Hot Weather
Heat Tolerance and Risks
Kohlrabi thrives in cool conditions but can suffer from heat stress if exposed to intense sunlight for prolonged periods, especially during the peak of summer. In regions where summer temperatures regularly exceed 26°C (80°F), kohlrabi may benefit from temporary shading, particularly during the hottest part of the day. Providing some form of shade can help prevent the bulbs from becoming woody and tough, ensuring a more palatable harvest.
Methods for Providing Shade
If you are growing kohlrabi in a hot climate or during the peak of summer, consider using shade cloth to reduce the intensity of sunlight. A shade cloth with 30% light reduction is usually sufficient to protect the plants from excessive heat while still allowing enough light for photosynthesis. Alternatively, planting kohlrabi in an area that receives morning sun and afternoon shade can help mitigate the effects of high temperatures. For container-grown kohlrabi, simply moving the pots to a shaded area during the hottest part of the day can also be effective.
Watering Tips
Importance of Consistent Moisture
Kohlrabi requires consistent moisture to produce healthy, tender bulbs. Inconsistent watering can lead to poor bulb development, bitterness, and increased susceptibility to pests and diseases. It is important to keep the soil evenly moist but not waterlogged, particularly during the early growth stages and as the bulbs begin to swell. Overwatering should be avoided, as it can lead to root rot and other fungal diseases.
Watering Technique
For best results, water kohlrabi deeply and regularly, focusing on the root zone. Drip irrigation systems or soaker hoses are ideal, as they deliver water directly to the soil without wetting the foliage, reducing the risk of disease. If watering by hand, apply water slowly at the base of the plants to allow it to penetrate deeply into the soil. Mulching around the plants with organic material such as straw, compost, or shredded leaves can help retain soil moisture and reduce the need for frequent watering.
Mulching to Retain Moisture
Applying a mulch layer of 5 to 7.5 cm (2 to 3 inches) around the base of the plants can help retain soil moisture, suppress weeds, and keep the soil cool. Mulch also helps to reduce water evaporation from the soil surface, which is particularly beneficial during hot, dry periods. Organic mulches such as straw, grass clippings, or shredded leaves are ideal, as they also contribute to soil fertility as they break down.
Recommended Watering Timing
Best Times to Water Kohlrabi
The best time to water kohlrabi is early in the morning, allowing the soil to absorb the water before the heat of the day. Morning watering ensures that the plants have access to moisture throughout the day, which is especially important during hot weather. Watering in the morning also helps reduce the risk of fungal diseases, as the foliage has time to dry out before nightfall. In particularly hot or dry conditions, a second watering in the late afternoon may be beneficial, but avoid watering too late to prevent the plants from staying wet overnight.
Adjusting Watering Based on Weather
Watering frequency should be adjusted based on weather conditions. During cooler, wetter periods, less frequent watering is needed, while in hot, dry weather, more frequent watering may be necessary to maintain consistent soil moisture. Monitoring the soil moisture level regularly and adjusting your watering schedule accordingly will help ensure that your kohlrabi plants receive the right amount of water at the right times.
Watering Frequency Reference
General Guidelines
Kohlrabi typically requires watering once or twice a week, depending on the weather and soil conditions. In sandy soils, which drain quickly, more frequent watering may be necessary, while in heavier soils, less frequent watering may be required. The key is to keep the soil consistently moist but not waterlogged, particularly during the growing season when the bulbs are being produced.
Indicators of Proper Watering
To determine whether your kohlrabi plants are receiving enough water, check the soil moisture level at a depth of about 15 cm (6 inches). If the soil feels dry at this depth, it is time to water. Additionally, wilting leaves, particularly in the early morning or evening, can be a sign that the plants need more water. However, if the leaves remain wilted during the heat of the day but recover by evening, it may be a normal response to high temperatures rather than a sign of water stress.
Pest and Disease Control
Common Pests Affecting Kohlrabi
Kohlrabi is relatively hardy but can be affected by a few common pests, including aphids, cabbage worms, and flea beetles. Aphids are small, sap-sucking insects that can weaken plants and spread diseases. Cabbage worms are caterpillars that chew holes in the leaves, causing significant damage. Flea beetles are small, jumping insects that leave tiny holes in the leaves, giving them a “shotgun” appearance.
Integrated Pest Management Strategies
To manage these pests, gardeners can use a combination of cultural, mechanical, and biological control methods. Regular monitoring and early intervention are crucial. Insecticidal soaps or neem oil can be effective against aphids and flea beetles, while row covers can protect young plants from cabbage worms. Encouraging beneficial insects, such as ladybirds and lacewings, can also help keep pest populations in check. Additionally, maintaining healthy, well-watered plants can make them less susceptible to pest infestations.
Common Diseases and Prevention
Kohlrabi is also susceptible to several fungal diseases, including downy mildew, clubroot, and black rot. Downy mildew causes yellowing and browning of the leaves, while clubroot is a soil-borne disease that causes swelling and distortion of the roots, leading to stunted growth. Black rot appears as dark, V-shaped lesions on the leaf margins.
To prevent these diseases, ensure good air circulation by properly spacing plants and avoiding overhead watering. Crop rotation and removing plant debris at the end of the season can also reduce the likelihood of disease recurrence. In cases where fungal diseases are detected early, fungicidal sprays can help manage the outbreak.
Seed Preservation and Storage Methods
Harvesting and Preparing Seeds
Kohlrabi seeds can be saved from mature plants that have bolted and produced seed stalks. To save seeds, allow the plants to flower and produce seed pods. Once the pods have matured and dried on the plant, they can be harvested by cutting the seed stalks and placing them in a paper bag. After the seeds have fully dried, they can be cleaned and stored for future planting.
Optimal Storage Conditions
Store kohlrabi seeds in an airtight container in a cool, dry, and dark place. A temperature of around 5°C to 10°C (41°F to 50°F) is ideal for seed storage. Properly stored kohlrabi seeds can remain viable for up to 5 years. Labeling the container with the date of harvest and the variety name can help you keep track of your seed inventory.
Preserving Edible Parts and Storage Duration
Post-Harvest Handling
After harvesting, kohlrabi bulbs should be handled gently to avoid bruising, which can lead to rapid deterioration. The bulbs can be stored in the refrigerator for up to two weeks. To store them, trim off the leaves and place the bulbs in a plastic bag with ventilation holes. The leaves can also be stored separately in the refrigerator for up to a week, though they are best used fresh.
Short-Term and Long-Term Storage
For longer storage, kohlrabi bulbs can be blanched and frozen or stored in a root cellar. To freeze kohlrabi, wash and peel the bulbs, then blanch them in boiling water for 2 to 3 minutes. Immediately cool them in ice water, drain well, and pack them in airtight containers or freezer bags. Frozen kohlrabi can be stored for up to 12 months. Kohlrabi can also be stored in a cool, dark place with high humidity, such as a root cellar, for several months.
Nutritional Content of Edible Parts
Nutritional Breakdown
Kohlrabi is highly nutritious, offering a good source of vitamins C and B6, as well as potassium, fiber, and antioxidants. It is low in calories and contains compounds such as glucosinolates, which have been shown to have cancer-preventive properties. The leaves of kohlrabi are also rich in vitamins A and C, making them a valuable addition to the diet.
Health Benefits and Comparisons
In addition to its high nutrient content, kohlrabi offers several health benefits. The high levels of vitamin C in kohlrabi support immune health and skin integrity. The dietary fiber in kohlrabi aids in digestion and helps maintain healthy blood sugar levels. Compared to other brassicas, kohlrabi is particularly valued for its unique flavor and crisp texture, making it a popular choice for those seeking to add variety to their vegetable intake.
