How to Grow Bok Choy in Australia

Bok choy or pak choi (Brassica rapa subsp. chinensis) is a leafy crop of the cabbage family, Brassicaceae, grown for crisp stems and tender leaves. Often ready six to eight weeks from sowing, it fits between bigger crops in every Australian zone. The usual mistake is sowing it into warming, lengthening spring days, where it runs to seed before it fills out.

Three mistakes that matter

1. Sowing into heat and long days. Hot weather and fast-lengthening days push bok choy into bolting (running to flower early), and the leaves toughen once a flower stem rises. Sow inside your zone's window and shade sowings made at its warm edge.
2. Letting the soil dry out. The roots are shallow, so a few dry days stress plants into bolting and leave the leaves bitter. Mulch the bed and water whenever the top few centimetres feel dry.
3. Leaving plants uncovered. Caterpillars of cabbage white (Pieris rapae) and diamondback moth (Plutella xylostella) chew the soft leaves to lace, and bok choy has little time to recover. Stretch fine insect mesh over hoops on sowing day and weigh the edges down.

When to plant

Bok choy seed germinates in soil from 10°C to 30°C, so cold soil rarely stops it. Heat and lengthening days set the calendar instead. Sow a short row about every four weeks through your window for a steady supply.

At the warm edge of a window, sow where plants get afternoon shade, or rig shadecloth over the row. In frosty districts, a clear plastic tunnel over hoops, its ends opened on warm days, shelters seedlings and extends the season.

Site, soil and setup

Pick a sunny, sheltered bed of rich, free-draining soil where no brassica has grown for four years, which keeps clubroot (Plasmodiophora brassicae) from building up.

A home pH kit tells you in minutes whether the bed needs lime. The soil pH article covers testing and adjusting.

Container growing

A pot at least 25 cm across and 20 cm deep, about 8 L, holds one full-size plant or two or three dwarf ones. Check it has drainage holes, then fill it with premium potting mix. The most compact types in the range are 'Extra Dwarf', 'TaiSai Ching Chang' and 'Choko Green'.

• Water: check daily, and water whenever the top of the mix feels dry; shallow roots in a pot dry out fast in wind and heat.
• Feed: fish and seaweed blend at 25 mL per 9 L, fortnightly, from the first true leaves.
• Position: a sunny balcony or veranda, moved into afternoon shade in warm weather.

Sowing Seeds

Sow bok choy where it will grow, or raise it in cell trays and plant it out.

Sowing direct

• Sow 5 mm deep in rows 25 cm apart, dropping seed thinly along the row.
• Firm the soil lightly and water with a fine rose.
• Keep the surface damp; seedlings appear in 4 to 10 days.
• Thin in stages to the final spacing, as set out in Thinning.

Most packets in the range hold seed coated with a fungicide, thiram. Never eat the seed or feed it to animals, and keep packets away from children and pets. Wear gloves to sow and wash your hands afterwards. The crop grown from treated seed is edible.

Starting under cover

• Sow two or three seeds per cell of seed-raising mix, 5 mm deep.
• Snip out all but the strongest seedling once true leaves show.
• Plant out at three or four true leaves, usually 3 to 4 weeks from sowing.
• Harden off (move seedlings outdoors for a little longer each day over about a week) before planting.

Water transplants in with seaweed solution at half strength, 15 mL per 9 L, then mulch around them.

Care by growth stage

Fast, unbroken growth decides this crop: steady water and a nitrogen-led feed keep leaves tender and hold off flowering. Weeks count from sowing.

Mesh keeps butterflies off, but it also shuts out the birds and wasps that hunt sap suckers. Aphids, thrips and whitefly can then build up underneath. Treat all three as for cabbage aphid (see Pests and diseases). White butterfly decoys on sticks generally do not stop egg laying.

Thinning

Thinning sets how large each plant grows and lets air move between them, which slows downy mildew.

• First pass: once true leaves show, thin seedlings to 5 to 10 cm apart.
• Second pass: at about 10 cm tall, roughly four weeks from sowing, pull every second plant and eat it.
• Final spacing: 15 cm for the dwarf type and 25 to 30 cm for full-size plants.
• Yellowing lower leaves: strip them off and bin them, so the base of each plant stays dry and airy.

Companion planting

Flowering herbs among the rows feed the wasps and hoverflies that hunt caterpillars and aphids.

Grows well with

• Onions and spring onions, which fill the gaps between rows without shading them.
• Dill and coriander left to flower, whose blooms feed parasitic wasps and hoverflies.
• Lettuce and beetroot, low, quick crops that share the same steady watering.
• Sweet corn on the sunny side in hot districts, as living shade for a late sowing.

Keep apart

• Broccoli, cabbage, cauliflower, kale and Chinese cabbage massed in one block, as they share caterpillars, aphids and clubroot.
• Mustard and other fast brassica greens crowded in the same row, which compete for water and draw the same pests.

Harvesting Leaves

Full-size plants are usually ready 6 to 8 weeks from sowing, once the stems are thick and the heart is full. Dwarf and baby-leaf cuts come sooner.

• Cut whole plants at the base, or pull them and slice off the root.
• For a longer harvest, snap off a few outer leaves at a time from about six weeks.
• Pick in the cool of the morning, when leaves are crisp.
• Harvest at once if a flower stem starts to rise from the centre.

Kept in a plastic bag in the fridge below 5°C, bok choy stays fresh for up to a week. In stir-fries and soups, add the stems before the leaves, since they take longer to cook. Baby and red leaves suit salads.

Common problems

Young plants running to seed in spring. Heat and dry soil are covered in Three mistakes that matter. A week or more of cold below about 10°C can also set young plants up to flower. Cover spring seedlings with a plastic tunnel on cold nights.

Patchy or no emergence. Bok choy seed is tiny, so seed buried too deep or a surface left to dry after sowing are the usual causes. Read more

Bitter or stringy leaves. Uneven watering and plants left past their best turn leaves bitter and stems fibrous, so pick on time and water steadily.

Seedlings collapsing at soil level. Damping off thrives in wet, crowded, airless trays. Read more

Tall, floppy seedlings. Seedlings raised in poor light stretch towards it and topple when planted out. Read more

Ragged holes and silvery trails. Snails and slugs rank as the biggest pest of Asian greens, grazing seedlings on damp nights. Hunt them by torchlight a couple of hours after dark, or set a beer trap. Read more

Pests and diseases

Caterpillars and sap suckers do most of the damage to bok choy, while its diseases follow cool, damp weather or acid, wet soil.

Cabbage white butterfly (Pieris rapae). Its velvety green caterpillars eat irregular holes and foul the heart with droppings, and a single female can lay up to 800 eggs. Pick caterpillars off, or spray Bacillus thuringiensis, a bacterial insecticide that affects only caterpillars, while they are small and as the label directs. Read more

Diamondback moth (Plutella xylostella). Plump, bright green caterpillars about 10 mm long graze the undersides, leaving thin windows that tear into holes as the leaf grows. They are most active from spring to autumn, especially in dry summers. Mesh and Bacillus thuringiensis control it as they do cabbage white. Read more

Cabbage aphid (Brevicoryne brassicae). Dull grey-green aphids up to 3 mm long gather in powdery, blue-grey colonies and can carry turnip mosaic virus. Hose them off, then spray insecticidal soap under the leaves as the label directs. Read more

Rutherglen bug (Nysius vinitor). This native grey-brown bug, about 4 mm long, breeds on weeds and moves into crops in spring and early summer. Its feeding makes small holes in young leaves that widen as they grow. Keep weeds down around the bed, where it breeds. Read more

Downy mildew (Hyaloperonospora parasitica). Yellowish, often angular spots on the upper leaf carry a grey-brown down beneath, and it is the main disease of commercial baby-leaf pak choy. Full spacing, covered in Thinning, and leaves that dry quickly keep it in check. Read more

Clubroot (Plasmodiophora brassicae). Roots swell into knobbly galls, so plants yellow, wilt and stunt, and its spores can survive in soil for up to 20 years. Bin affected plants with their roots and never compost them. Clean soil off tools and boots after working an affected bed. 'Miyako' carries intermediate resistance.

Prevention first

• Once a week, lift the mesh and turn a few leaves to look for eggs, caterpillars and sap suckers.
• Pull brassica weeds such as shepherd's purse and wild mustard, which harbour clubroot, downy mildew and aphids.
• Water at soil level early in the day so leaves dry quickly.
• Clear crop remains straight after harvest, as white blister and other diseases carry over on debris.

Tips

1. Save seed only from heirloom types such as 'White Stem (Cantong)', 'TaiSai Ching Chang' and 'Green (Bok Choy)'. The F1 hybrids do not come true from saved seed.
2. Bok choy crosses with Chinese cabbage, turnip, mizuna and tatsoi, all Brassica rapa, so let only one of them flower at a time. Leave at least five plants to flower together for good seed set.
3. Time red and purple types to mature in autumn, when cooler weather deepens their colour.