How to Grow Kohl Rabi in Australia

Kohl rabi (Brassica oleracea Gongylodes Group), of the cabbage family, Brassicaceae, is grown for the round, swollen stem that sits on top of the soil. It looks like a turnip but is a stem, crisp and mild, and often ready about two months after sowing in the cool months. Most gardeners who go wrong leave it in the ground until the stem turns woody.

Three mistakes that matter

1. Picking too late. Past about 8 cm across, the Vienna types turn tough and fibrous, and nothing softens them again. Start checking at 5 cm and pick every stem by tennis-ball size. 'Superschmelz' is the exception, as set out in Thinning.
2. Letting the soil dry out. The roots are shallow, so even a short dry spell checks growth and leaves the stem woody and tough. Mulch the bed and water deeply whenever the top few centimetres feel dry.
3. Leaving the row crowded. Seedlings left too close together grow thin stems that never swell. Thin in stages to 15 to 20 cm apart, or 30 cm for 'Superschmelz', as set out in Thinning.

When to plant

Kohl rabi seed germinates in soil from about 8°C to 30°C, and fastest between 15°C and 25°C. Heat, rather than cold, closes most windows, because stems that swell in hot weather turn woody sooner. Sow a short row every two to three weeks through your window for a steady supply.

Give sowings at the warm edge of a window some afternoon shade. At the cold edge, raise seedlings under cover and plant them out once days reach about 10°C and nights stay above -2°C.

Site, soil and setup

Pick a sunny spot with loose, fertile soil. Leave at least four years between brassica crops in the same bed, so clubroot (Plasmodiophora brassicae) cannot build up.

Test before you lime: a home kit gives a reading in minutes. The soil pH article explains how.

Container growing

Kohl rabi grows well in a large pot or trough of at least 40 L and 30 cm deep. A 50 cm pot holds about 52 L, enough for four plants of 'White Vienna' or 'Purple Vienna'. Fill it with premium potting mix blended with compost. 'Superschmelz' grows too large for most pots.

• Water: feel the mix every day and water once the top is dry; pots lose moisture quickly on hot or windy days.
• Feed: fish and seaweed blend at 25 mL per 9 L, fortnightly, from the first true leaves.
• Position: full sun through the cool months, moved into afternoon shade on hot days.

Sowing Seeds

Kohl rabi does best sown straight into the bed, though it can also be raised in cell trays and planted out young.

Sowing direct

• Make shallow drills 30 cm apart and sow thinly, 5 mm deep.
• Cover, press the soil down gently and water with a fine rose.
• Keep the surface damp; seedlings usually appear in 3 to 10 days.
• Thin in stages to the final spacing, as set out in Thinning.

Gaps in the row usually trace back to seed sown too deep, or a surface that dried out before the seedlings emerged.

Starting under cover

• Fill cell trays with seed-raising mix and drop two or three seeds into each cell, 5 mm deep.
• When true leaves appear, keep the sturdiest seedling and snip the others off at the mix.
• Transplant at three or four true leaves, usually 3 to 4 weeks from sowing, disturbing the roots as little as possible.
• Harden off first (give seedlings a little more time outdoors each day for about a week).

Settle each transplant with seaweed solution at half strength, 15 mL per 9 L. Then mulch the bed to hold moisture and smother weeds.

Care by growth stage

Steady, unbroken growth decides kohl rabi. Even moisture and a moderate feed swell a tender stem, while too much nitrogen builds leaves at the stem's expense. Weeks count from sowing.

Netting has a cost: the ladybirds, lacewings and small birds that clean up sap suckers are kept out too. Aphids, thrips and whitefly can multiply under a cover left on for weeks, so check beneath it every week. Pull weeds by hand near the plants, because a hoe can sever the shallow roots.

Thinning

Thinning decides whether the stems swell at all, and full spacing lets air move between plants, which slows downy mildew.

• First pass: once the first true leaves show, thin seedlings to about 10 cm apart.
• Second pass: when the leaves of neighbouring plants touch, pull every second plant and cook its young leaves as greens.
• Final spacing: 15 to 20 cm for the Vienna types and 30 cm for 'Superschmelz'.

Companion planting

Low, quick crops that like the same steady watering sit well between kohl rabi rows, and flowering herbs feed the insects that hunt aphids.

Grows well with

• Onions and spring onions, slim enough to fill the space between rows.
• Beetroot, a low crop that shares the same even moisture.
• Dill left to flower, whose blooms feed adult hoverflies; their larvae eat aphids.
• Celery, which thrives in the same rich, moist soil.

Keep apart

• A solid block of broccoli, cabbage, cauliflower, kale, bok choy or Chinese cabbage, which invites the caterpillars, aphids and clubroot they all share.
• Mustard, a clubroot host that draws the same caterpillars.
• Climbing beans on the sunny side, which shade the low plants.

Harvesting Stems

Pick when the stem is 5 to 8 cm across, about the size of a tennis ball, usually 8 to 10 weeks from sowing.

• Start checking at 5 cm across and pick the largest stems first.
• Cut the stem just below the swelling with a sharp knife or secateurs.
• Pull the whole plant instead if you want the leaves as well.
• Pick sooner in warm weather, when stems harden faster.
• Let 'Superschmelz' grow on well past tennis-ball size for a bigger stem.

Cut the leaves off before storing, as they draw moisture from the stem. Kept in the fridge, stems usually stay crisp for up to two weeks, and peeled, diced stems freeze well. Peel older stems, grate them raw into slaws, stir-fry or roast them, and cook the young leaves like kale.

Common problems

Split stems. Irregular watering, such as a heavy soak or a downpour after a dry spell, can crack the swelling stem, so keep moisture even.

Stems that stay thin. Beyond crowding, covered in Three mistakes that matter, too much heat, freezing weather or too little water can stop a stem swelling.

Running to flower. Several weeks of cold after planting out can trigger bolting (sending up a flower stem), so avoid planting young seedlings into the coldest weeks.

Seedlings keeling over in the tray. Damping off (a rot caused by soil fungi and water moulds) thrives in wet, crowded trays with little air movement. Read more

Long, weak seedlings. Raised without enough light, seedlings stretch, then flop when moved outside. Read more

Ragged holes and silvery trails. Snails and slugs feed at night, chewing seedlings, leaves and even the stems. Go out with a torch about two hours after sunset, especially after rain, or set shallow beer traps. Read more

Pests and diseases

Caterpillars and aphids cause most of the damage, and the diseases follow cool, wet weather or acid, poorly drained soil.

Cabbage white butterfly (Pieris rapae). Accidentally introduced to Melbourne in 1929, it has since spread all over Australia, and each female can lay up to 800 eggs. The green caterpillars eat ragged holes and leave droppings behind. Pick them off, or spray Bacillus thuringiensis, a bacterial insecticide specific to caterpillars, while they are small, following the label. Read more

Diamondback moth (Plutella xylostella). Also called cabbage moth, its small caterpillars graze thin windows in the leaves that tear into holes as they grow. Caterpillars are abundant from spring to autumn, especially in dry summers. Netting and the same bacterial spray work against it. Read more

Cabbage aphid (Brevicoryne brassicae). These grey-green insects, no more than 3 mm long, form dense colonies coated in a whitish powder, and they can spread turnip mosaic virus. Knock them off with a jet of water, then treat the leaf undersides with insecticidal soap, following the label. Read more

Downy mildew (Hyaloperonospora parasitica). Yellowing, spotted patches on the upper leaf carry white, fluffy growth beneath, mostly in cool, rainy weather. Full spacing, covered in Thinning, keeps air moving and leaves drying. Read more

Clubroot (Plasmodiophora brassicae). Knobbly galls form on the roots, and the tops yellow, wilt on warm days and stop growing. Its spores stay alive in the soil for up to 20 years. Lift sick plants, roots and all, and put them in the rubbish rather than the compost, then wash the soil off tools.

Prevention first

• Check under a few leaves every week for eggs, caterpillars and aphid colonies.
• Remove shepherd's purse, wild mustard and other brassica weeds, which carry clubroot and downy mildew.
• Water the soil rather than the foliage, so leaves stay dry.
• Pull out finished plants promptly, as leaf diseases carry over on old crops.

Tips

1. Never hill soil or pile mulch against the swelling stem. It is meant to sit on the surface, and a buried stem can rot.
2. In frosty districts, time a sowing to size up in the cold months, since frost turns some starch to sugar and sweetens the stems.
3. Every kohl rabi in the range is an heirloom, so seed can be saved. It crosses with cabbage, broccoli, cauliflower and kale. Let only one of them flower within 250 m, and leave at least five plants to flower together.