Kohlrabi: The Alien Vegetable Worth Growing in Australia

Introduction to Kohlrabi: Australia's Quirky Vegetable Gem

Kohlrabi grows fast and tastes like a cross between broccoli and apple. Learn harvest timing (the most critical factor), succession sowing, and eating raw vs cooked. Australian gardeners have unique climate conditions. This guide covers everything from planting timing in your specific zone through to harvest, with troubleshooting for the most common problems.

If you've wandered through a farmers' market or scrolled through seed catalogues and wondered what that peculiar, bulbous vegetable with protruding stems could possibly be, you've likely spotted kohlrabi. Often described as an "alien vegetable" due to its otherworldly appearance, kohlrabi (Brassica oleracea var. gongylodes) is a brassica crop that deserves a place in every Australian vegetable garden. Whether you're a seasoned gardener or just beginning your growing journey, kohlrabi offers rewards that far outweigh the minimal effort required to cultivate it.

What is Kohlrabi and Why Grow It?

Kohlrabi is a cool-season brassica that forms a swollen, bulbous stem just above soil level. The name itself comes from the German words "kohl" (cabbage) and "rabi" (turnip), which perfectly describes its unique growing habit. Unlike most vegetables where we harvest leaves or underground bulbs, kohlrabi's edible portion is this distinctive stem bulb, which can range from pale green to deep purple depending on the variety.

The flavour profile is one of kohlrabi's greatest assets. When harvested at the optimal size, it delivers a subtle sweetness reminiscent of a crisp apple, combined with the mild, slightly earthy notes of broccoli. This makes it incredibly versatile in the kitchen—equally suited to raw slaws and salads as it is to stir-fries, roasts, and curries. The leaves are equally edible and nutritious, tasting somewhat like kale, so you're essentially getting two vegetables from one plant.

Beyond flavour, kohlrabi offers genuine practical advantages for Australian gardeners. It matures remarkably quickly—most varieties are ready to harvest within 45–60 days from transplanting. It requires minimal space, making it perfect for small gardens or container growing. It's relatively pest-resistant compared to other brassicas, and it stores reasonably well, extending your harvest season.

Climate Considerations for Australian Gardeners

Understanding Australia's diverse climate zones is essential for kohlrabi success. This cool-season crop thrives in temperatures between 15–20°C and becomes stressed in heat above 25°C. Unlike many vegetables that suit Australia's warm climate, kohlrabi demands strategic planning around temperature patterns.

Timing by Australian State and Zone

Victoria and Tasmania: These cooler states are kohlrabi paradise. In Victoria (particularly around Melbourne), sow seeds from late August through October for spring harvests, and again from February to April for autumn crops. Tasmania, with its mild summers and cool winters, can even accommodate summer sowings (December to February) in higher altitude areas. Gardeners here should aim for harvest during the coolest months possible.

New South Wales: The coastal regions and tablelands of NSW suit kohlrabi well. Sydney gardeners should focus on autumn (March–May) and spring (September–November) plantings. The Blue Mountains and highlands offer extended growing seasons, making these regions ideal for this crop. Avoid the December–February period unless you're at significant elevation.

South Australia: Adelaide and the Adelaide Hills have a Mediterranean climate with mild winters. Plant from February to April for winter and spring harvests, and again from August to September for late spring crops. The cooler Adelaide Hills are particularly suited to extended kohlrabi seasons.

Queensland: In this warmer state, kohlrabi is primarily a cool-season crop. Brisbane gardeners should target April to June planting (for autumn/winter harvest) and August to September planting (for spring harvest). The Darling Downs and Atherton Tablelands, with cooler temperatures, can support more frequent plantings. Avoid December through February entirely in lowland areas.

Western Australia: Perth's mild winters are perfect for kohlrabi. Plant from February to April for winter and spring crops, and again from August to September. The cooler southern regions around Margaret River can support more extensive growing seasons.

Northern Territory: The dry season (May–September) is when kohlrabi can thrive in Darwin and Katherine. The intense heat during other months makes growing challenging unless you're at altitude or can provide significant shade.

Planting Kohlrabi: Timing and Method

Starting from Seeds vs Seedlings

Kohlrabi can be grown from seed or seedlings. For Australian gardeners, both methods work well. Direct seeding is economical and works perfectly if you have protected seed-raising space, but transplanting seedlings gives you more control over timing and spacing. Most gardeners purchase seedlings from nurseries during appropriate seasons, which eliminates guesswork about germination.

If starting from seed, sow indoors 4–6 weeks before your intended transplant date. Seeds germinate in 7–10 days at temperatures around 18–20°C. Provide bright light once sprouted and maintain consistent moisture. Harden off seedlings gradually before transplanting.

Site Selection and Soil Preparation

Kohlrabi prefers full sun (at least 6–8 hours daily) and well-draining, fertile soil rich in organic matter. Before planting, work in compost or well-rotted manure to improve soil structure and nutrient content. This brassica is a moderate feeder and responds well to balanced fertiliser application.

Soil pH should ideally sit between 6.0 and 7.5. If your soil is acidic, consider lime application several weeks before planting. Ensure drainage is adequate—waterlogging causes root rot and bolting.

Spacing and Transplanting

Space plants 20–25 cm apart in rows 30–40 cm apart, or adjust for your garden configuration. When transplanting seedlings, plant at the same depth they were growing in their containers. Water immediately after planting and keep soil consistently moist for the first two weeks while plants establish.

In container growing (excellent for small spaces), use 5–litre pots minimum, one plant per pot, filled with quality potting mix. Containers dry out faster than garden beds, so monitor moisture carefully.

Succession Sowing for Extended Harvests

One of kohlrabi's greatest strengths is its suitability for succession planting. Rather than planting everything at once, sow new seeds or transplants every 2–3 weeks throughout your suitable growing season. This approach ensures a continuous supply of young, tender kohlrabi rather than a glut followed by nothing.

Within your state or region's optimal growing window, plan 3–4 successive plantings spaced 2–3 weeks apart. For example, a Sydney gardener might plant in early September, late September, early October, and late October, resulting in successive harvests from mid-October through December.

Fertilising and Nutrient Management

Kohlrabi requires consistent nutrients for optimal bulb development. Apply a balanced, slow-release fertiliser at planting time, or use a liquid fertiliser every 2–3 weeks once growth is established. Nitrogen promotes leaf development, whilst phosphorus and potassium support bulb formation.

Organic gardeners should apply compost mulch or use seaweed-based fertilisers. Avoid excessive nitrogen late in the season, as this encourages excessive leaf growth at the expense of bulb development.

Watering Requirements

Consistent moisture is critical for kohlrabi success, particularly once bulb development begins. Aim for 25–30 mm of water weekly through rainfall or irrigation. Inconsistent watering causes stress, leading to bolting, splitting, or woody texture in the bulb.

In Australian conditions, particularly during warm spring periods, water deeply but less frequently to encourage deep root development. Morning watering minimises disease risk. Mulching around plants (keeping mulch away from the stem) helps retain soil moisture and regulates temperature.

Common Australian Pests and Disease Management

Cabbage White Butterfly

The cabbage white butterfly (Pieris rapae) lays eggs on brassicas, producing larvae that devour foliage. In Australia, this is one of the most common kohlrabi pests. Use fine mesh netting over young plants to prevent egg-laying, or monitor closely and hand-remove eggs and caterpillars. Organic insecticidal soaps are effective on young larvae.

Flea Beetles

These tiny beetles create numerous small holes in leaves. Young seedlings are most vulnerable. Protect young plants with netting or row covers. Kaolin clay spray deters adults. Once plants are established and vigorous, they typically outgrow flea beetle damage.

Cabbage Moth

Another common Australian brassica pest, cabbage moths cause similar damage to white butterflies. The same protective measures—netting and regular inspection—apply. Bacillus thuringiensis (Bt) is an organic option for managing caterpillars.

Clubroot

This soil-borne fungal disease causes roots to swell and distort, resulting in stunted, wilting plants. It's a significant concern in Australian brassica growing regions, particularly where soil is acidic and wet. Prevent clubroot by: raising soil pH to above 7.0 if possible; practising crop rotation (avoid growing brassicas in the same spot more frequently than every three years); and purchasing disease-free seedlings from reputable nurseries.

Powdery Mildew

This fungal disease appears as white coating on leaves, particularly in cool, humid conditions. Improve air circulation, avoid overhead watering, and apply sulphur-based fungicides if needed. Remove affected leaves promptly.

Harvest Timing: The Most Critical Factor

Harvest timing is absolutely paramount for kohlrabi quality. This is where most Australian gardeners make their critical mistake.

Kohlrabi should be harvested when bulbs reach 5–7 cm diameter—roughly the size of a golf ball to tennis ball. At this size, the flesh is tender, flavourful, and has that prized apple-like crispness. Continue harvesting regularly; once bulbs reach 10 cm or larger, the texture becomes woody and fibrous, and the flavour becomes stronger and less pleasant.

From transplanting, expect harvest timing at roughly 45–60 days, depending on variety and conditions. Warmer weather accelerates growth, sometimes reaching harvest-ready size in as little as 40 days. Keep checking plants regularly once they're about 30 days old; the difference between perfect and past-prime can be just a few days.

Harvest by cutting bulbs at soil level with a sharp knife. Trim away any damaged outer leaves. The remaining stalk will sometimes produce smaller side shoots that can be harvested later, extending your yield from each plant.

Eating Kohlrabi: Raw vs Cooked

Raw Kohlrabi

Young, tender kohlrabi is absolutely delicious eaten raw. Peel the bulb (the skin is tough and inedible), and slice or dice the flesh into salads, coleslaws, or as a crunchy snack. Raw kohlrabi has a mild, slightly sweet flavour with satisfying crunch. The leaves, when young and tender, can be eaten raw in salads, though older leaves are better cooked.

Cooked Kohlrabi

Roasting brings out kohlrabi's natural sweetness beautifully. Cut into wedges, toss with oil and seasonings