Start from seed: February-March and August-November
Optimum soil temperature for germination: 15-25°C
Plant spacing: 20-30 cm
Days to maturity: 55-70 days
Introduction
Kohlrabi (Brassica oleracea var. gongylodes) is a member of the Brassicaceae family which also includes broccoli, cauliflower, kale, kohl rabi, Brussels sprouts, turnips, radish, rocket, collards and mustard greens.
The vegetable traces its origins to Northern Europe, particularly Germany, where it has been cultivated since at least the 14th century. The name “kohlrabi” derives from two German words: “kohl” meaning cabbage and “rabi” meaning turnip, accurately describing its flavour profile. There is some evidence that similar vegetables were grown in ancient Rome, with Pliny the Elder making reference to such a plant.
Despite its turnip-like appearance, kohlrabi is not a root vegetable but an enlarged, bulbous stem that grows predominantly above the ground with leaves growing from the sides. The bulbous stem can be green or purple, depending on the variety, while the interior flesh is always crisp and white. It offers a mild, sweet flavour with notes of broccoli stem and mild turnip. Both the stem and younger leaves are edible. The vegetable is versatile in the kitchen and can be enjoyed raw in salads, cooked in stir-fries, steamed, or roasted.
Site and soil
Kohlrabi grows well in Canberra’s climate, particularly during the cooler seasons of spring and autumn. The plant prefers full sun but will tolerate partial shade. Being relatively shallow-rooted, kohlrabi can succeed in a variety of garden settings, including raised beds and containers.
Prior to planting, prepare your soil by incorporating compost or well-rotted manure to create a fertile, well-draining growing medium. Kohlrabi thrives in soil with a pH between 6.0 and 7.5, similar to other brassicas. It is commonly thought that they prefer alkaline soil. This is because clubroot disease can be problematic in acidic soils. If this is the case liming to raise the pH above 7.0, ideally to 7.8, is beneficial. The top layer of soil should be loose and aerated to accommodate the shallow root system and allow the bulbous stem to develop properly.
For container growing, choose pots at least 30 cm deep with good drainage holes. Fill with premium potting mix enhanced with compost and a slow-release organic fertilizer. Containers should be at least 40 litres in capacity for kohlrabi to develop properly. One advantage of container growing is the ability to move plants to optimal conditions as the season progresses.
For details on preparing soil and pots, including the use of lime, dolomite, gypsum and fertilizers, please refer to the COGS guide to garden bed preparation.
Planting Seeds and Seedlings
In Canberra, kohlrabi can be direct sown or started in seedling trays during two main periods: February to March for autumn/winter harvest, and August to November for spring/summer harvest. Sow seeds direct into the soil to a depth twice the size of the seed, in rows 25-30cm apart, and thin to 10cm apart once they have their first true leaves.
If starting in cell or seedling trays, transplant when seedlings have developed 3-4 true leaves, usually 3-4 weeks after sowing. Handle seedlings carefully during transplanting to minimize root disturbance.
For a continuous harvest, consider succession planting every 2-3 weeks during the growing seasons. This ensures a steady supply of fresh kohlrabi over a longer period.
After planting, water thoroughly with seaweed solution and mulch to retain soil moisture and suppress weeds. Be careful not to cover the developing bulb with mulch, as this can encourage rot.
Cultivation
Consistent moisture is key to growing quality kohlrabi. Maintain even soil moisture throughout the growing period, as dry conditions can lead to woody, tough bulbs. Water at the base of plants rather than overhead to reduce disease risk, especially in cooler weather.
Avoid excess soluble nitrogen, which can lead to lush foliage growth at the expense of bulb development. A monthly foliar spray with seaweed solution; a mix of worm juice, seaweed and compost tea; or trace elements will help boost growth and resilience to pests and diseases.
Weed control is important, especially when plants are young, as kohlrabi doesn’t compete well with aggressive weeds. Remove weeds carefully to avoid damaging the shallow root system, and maintain mulch to suppress weed growth.
Unlike some other vegetables, kohlrabi should not have soil mounded around the developing bulb. The bulbous stem naturally grows above ground and mounding can promote rot. The plants grow quickly and don’t generally require staking or other supports.
Kohlrabi is quite cold-tolerant and can withstand light frosts, making it an excellent cool-season crop for Canberra gardens. In fact, frost improves the flavour because it stimulates the conversion of starches to sugars, that function as anti-freeze, resulting in a sweeter taste.
Harvesting
Kohlrabi is typically ready to harvest 55-70 days after sowing, depending on the variety and growing conditions. For the best flavour and texture, harvest when bulbs are between 5-7.5cm in diameter. Larger bulbs may become tough and woody, particularly in warmer weather. Some of the newer hybrids stay tender until they are larger.
To harvest, cut the stem just below the bulb with a sharp knife or garden shears. The entire plant can be pulled if you also wish to use the leaves, which are edible and nutritious.
The leaves can be cooked like other greens, such as kale or collards. Young leaves are more tender and have a milder flavour compared to older ones.
Kohlrabi can store in the refrigerator for up to two weeks. To extend shelf life, wash just before use rather than before storage. For the best quality, remove the leaves if storing for more than a few days, as they will draw moisture from the bulb. The bulbs can also be peeled, diced, and frozen for later use.
Problems
The best approach is to have a reliable Australian reference book to identify problems. Garden Pests, Disease and Good Bugs by Denis Crawford is recommended.
A general strategy to follow with any problem is to observe the plant carefully before taking action. Identify the problem then carefully consider the course of action. For example, with pest insects, you often can see a predator nearby. If you watch and wait the predator may deal with the problem for you. Beneficial predatory insects which feed on pest insects can be purchased from suppliers such as Bugs for Bugs. If the problem needs further action the above recommended book has suggestions.
Practicing good garden hygiene, crop rotation, and maintaining plant vigour by providing adequate nutrients are key strategies for prevention.
While kohlrabi is generally a trouble-free crop, it can face several challenges in Canberra gardens:
Cabbage White Butterfly (Pieris rapae): This common pest lays tiny white eggs on the undersides of leaves. The green caterpillars that hatch are perfectly camouflaged and feed on leaves, creating irregular holes and contaminating plants with their droppings. Exclusion netting is the most reliable method to prevent damage from cabbage white butterfly. Keep in mind that the netting also excludes beneficial insects and birds that glean plants cleaning up other pests. Thus, leaving the netting on for too long can result in populations of sap suckers such as aphids, thrips and white fly building up inside the netting.
Control methods include:
- Covering plants with fine insect mesh or exclusion netting from the time of planting
- Regularly inspecting and manually removing eggs and caterpillars
- Using biological controls such as Bacillus thuringiensis (also known as Bt and sold as Dipel) for significant infestations
- Creating butterfly decoys by placing white plastic pieces around plants
- Avoiding brassica weeds that act as hosts
Clubroot (Plasmodiophora brassicae): This soil-borne disease causes swollen, deformed roots and stunted growth. Plants may wilt during warm weather and show yellowing leaves. The disease thrives in acidic soils (below pH 7.0) and can persist in soil for many years. Management includes:
- Maintaining soil pH above 7.0, ideally 7.8, through liming
- Practicing crop rotation (avoid planting brassicas in the same area for at least 4 years)
- Improving drainage in wet areas
- Removing and disposing of infected plants (do not compost)
- Using disease-free transplants and clean tools
Aphids, thrips and white fly: These small sap-sucking insects can transmit celery mosaic virus. They often cluster on the undersides of leaves, causing distortion and stunting. Small populations can be controlled by washing them off with a strong stream of water or using insecticidal soap. Alternatively, neem oil or pyrethrin can be used. Beneficial predators like ladybugs and lacewings should be encouraged and can be purchased for release on the crop. Look for aphid mummies which are evidence that the aphid has been parasitised. Eggs can also be parasitised and will appear bronze in colour. Generally speaking, plants that are high in soluble nitrogen (nitrates and ammonia) are prone to attack by sap sucking insects. Ideally avoid using soluble nitrogen fertilisers and ensure plants have adequate trace elements. This can be achieved by using foliar sprays.
Slugs and Snails: These pests can damage young seedlings and create holes in leaves and bulbs. Control using beer traps, copper tape barriers, organic (iron EDTA) snail baits, or evening patrols to manually remove them.
Recommended Varieties
Several kohlrabi varieties perform well in Canberra’s climate:
Purple Vienna: A popular heirloom variety with purple skin and white flesh. It has excellent flavour and is ready to harvest in approximately 60 days. This variety is particularly good for cool-season planting and can withstand light frosts.
White Vienna: Similar to Purple Vienna but with pale green skin. It has a mild, sweet flavour and tender texture when harvested young. Also matures in about 60 days and is good for both spring and fall planting.
Grand Duke: An F1 hybrid with excellent disease resistance and uniform growth. Produces consistent, high-quality bulbs with good resistance to splitting. Matures in approximately 50 days.
Superschmelz: A large-growing variety that can reach the size of a bowling ball without becoming woody. Good for late-season planting, as it maintains tenderness even at larger sizes.
Korridor: A reliable, quick-maturing variety that produces uniform, tender bulbs with light green skin. Good resistance to bolting in warmer weather.
For Canberra gardens, both Purple Vienna and White Vienna are excellent choices for beginners due to their reliability and adaptability to local conditions. For those wanting to extend the harvest season, consider planting a combination of early and late-maturing varieties.
References
- ABC Organic Gardener Magazine. (2021). How to grow kohlrabi.
- Bubel, Nancy. The New Seed-Starters Handbook. Rodale Books, 2018.
- Crawford D. Garden Pests, Disease and Good Bugs. Australia: ABC Books; 2015.
- Dove, Helena. Botany of the Kitchen Garden, Kew Publishing, Royal Botanic Gardens Kew, UK, 2023.
- Fanton, Michel and Jude. The Seed Savers Handbook. The Seed Savers Network, Australia, 1993.
- The Seed Collection. (2024). How to Grow Kohlrabi Seeds. https://www.theseedcollection.com.au/growguide/kohlrabi-seeds
- Yates Australia. (2024). How to Grow Kohlrabi. https://www.yates.com.au/how-to-grow/kohlrabi/
