How to Grow Choy Sum in Australia

Choy sum (Brassica rapa var. parachinensis) is a leafy member of the cabbage family, Brassicaceae. It is grown for slender flowering shoots, cut while the yellow buds are still closed. Yu choy sum and the purple-stemmed 'Hon Tsai Tai' are grown the same way. A crop usually takes four to seven weeks and fits a pot. The common mistake is picking too late, once the flowers have opened and the stems have turned woody.

Three mistakes that matter

1. Waiting for flowers. Once most of the buds open, the stem toughens and the plant turns to making seed. Cut each shoot when the buds are plump and tight, or when the first one or two have just opened.
2. Letting young plants stall. Heat, dry soil and a run of cold nights all push small plants into flower on thin, leafless stems. Sow inside your zone's window, mulch the bed and water before the surface dries out.
3. Sowing without a cover. Caterpillars of cabbage white butterfly (Pieris rapae) and diamondback moth (Plutella xylostella) can strip young plants to bare stalks. Fix fine insect mesh over hoops on sowing day and seal the edges with soil.

When to plant

Choy sum seed comes up best in soil of about 15°C to 26°C, and the stems grow sweetest in days of 15°C to 25°C. Yu choy sum can flower early after nights below about 10°C to 12°C, so cold snaps matter as much as heat. For a steady supply, sow the next short row once the last one is half grown.

At the hot end of a window, choose a yu choy sum and give the row afternoon shade. At the cold end, 'Hon Tsai Tai' copes with light frost better than the green types.

Site, soil and setup

Choy sum roots reach only about 13 cm down, which is why rich topsoil and steady moisture decide the crop. Rotate beds so that no cabbage-family crop grows in the same spot for four years, which starves clubroot of hosts.

Test before adding lime: the soil pH article explains how to read a kit and adjust gently.

Container growing

Shallow roots make choy sum a good pot crop for a balcony or courtyard. A pot at least 30 cm across holds about 13 L, room for three green plants or one or two of 'Hon Tsai Tai'. Fill it with premium potting mix and check that it drains freely. Of the compact types, 'Early Green' has short, stocky stems, and the upright hybrids 'Karate' and 'Taboo' sit neatly in a trough.

• Water: check every day and water once the top of the mix feels dry; pots in sun and wind dry out quickly.
• Feed: fish and seaweed blend at 25 mL per 9 L, fortnightly, from the first true leaves. Or mix a controlled release fertiliser for containers into the potting mix at sowing, at the label rate.
• Position: a sunny spot, moved into afternoon shade in warm spells.

Sowing Seeds

Sow choy sum where it is to grow, or raise it in cell trays for planting out.

Sowing direct

• Sow 5 to 10 mm deep in rows 25 cm apart, dribbling seed thinly along the drill.
• Cover lightly, firm the soil and water with a fine rose.
• Keep the surface damp until seedlings appear, usually in 4 to 14 days.
• Thin to the spacing set out in Thinning.

Seed of 'Jung Green', 'Early Green', 'Hon Tsai Tai' and 'Taboo' is coated with the fungicide thiram, while 'Karate' is sold untreated. Do not eat treated seed or feed it to animals, and store packets away from children and pets. Wear gloves to sow, then wash your hands. The crop grown from it is edible.

Starting under cover

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

Water each transplant in with seaweed solution at half strength, 15 mL per 9 L, and mulch around it.

Care by growth stage

Choy sum is judged by its stems, and tender stems come from fast growth without a check. Keep the soil evenly moist and the nitrogen steady from sowing to the last cut. Weeks count from sowing.

Fine mesh keeps butterflies off, but it also shuts out the birds, wasps and ladybirds that eat aphids. Lift it only in the early morning or late afternoon, when butterflies are resting. Overfeeding with nitrogen draws sap suckers too, so keep to the rates in the table.

Thinning

How far apart you leave the plants depends on the type, because each is cut in a different way.

• First pass: once true leaves show, thin so seedlings no longer touch.
• Second pass: as leaves meet, thin to the final spacing in the table and cook the thinnings whole.
• Lower leaves: strip off any that yellow and bin them, so air moves at soil level.

Companion planting

A patch of mixed crops makes it harder for caterpillars to travel from plant to plant, and flowering herbs feed the wasps that parasitise them.

Grows well with

• Lettuce and celery, which want the same steady moisture.
• Beetroot and carrots, whose roots feed deeper than choy sum.
• Dill and coriander allowed to flower, as nectar for parasitic wasps and hoverflies.
• Sweet corn on the sunny side, as living shade in hot districts.

Keep apart

• Bok choy, Chinese cabbage, komatsuna, kale and broccoli massed in one block, which share caterpillars, aphids and clubroot.
• Wild radish, wild turnip and shepherd's purse, brassica weeds that carry aphids, downy mildew and clubroot.

Harvesting Stems

Cut choy sum when the flower buds are fully formed but still closed, or the first one or two have just opened.

• Take the main stem at about 15 to 25 cm long, cutting just above a healthy leaf.
• Side shoots then grow from the leaf joints below; pick them every few days.
• Many plants give two or three cuts in all before they tire.
• Snap 'Hon Tsai Tai' shoots by hand; one that bends rather than snaps is past its best.
• Cut 'Karate' and 'Taboo' whole at soil level as a bunch.
• Pick in the cool of the morning, when stems are crisp.

Bunches keep for up to a week in a plastic bag in the fridge. Leaves, stems and buds are all eaten. Steam or stir-fry them briefly, add them to soups, or toss young 'Hon Tsai Tai' shoots raw through a salad.

Common problems

Gaps along the row. The seed is small, so sowing too deep or letting the surface dry before emergence leaves bare stretches. Read more

Woody or stringy stems. Shoots left until the flowers open, and dry soil during budding, both toughen the stem; the timing is covered in Three mistakes that matter.

Pale leaves and slow growth. Choy sum is sensitive to a shortage of nitrogen and to waterlogged soil. Feed at the rates in Care by growth stage, and raise the bed if water lies after rain.

Seedlings toppling at the base. Damping off thrives in cold, wet, crowded trays with still air. Read more

Seedlings grazed overnight. Snails, and slugs to a lesser extent, are the biggest pest of Asian greens; look for shiny trails. Hunt with a torch about two hours after dark, especially after rain. Read more

Pests and diseases

Caterpillars and aphids do most of the damage, and most leaf diseases follow damp weather.

Cabbage white butterfly (Pieris rapae). The white butterflies lay on the leaves, and their velvety green caterpillars chew ragged holes and leave droppings behind. Pick caterpillars off, or spray small ones with Bacillus thuringiensis, a bacterial insecticide that harms only caterpillars, as the label directs. Read more

Diamondback moth (Plutella xylostella). Bright green caterpillars up to about 10 mm long feed under the leaves, leaving clear windows that tear into holes. It is among the most damaging brassica pests, and around Stanthorpe mid to late summer crops suffer most. Use mesh and Bacillus thuringiensis as for cabbage white. Read more

Cabbage aphid (Brevicoryne brassicae). Grey-green aphids up to 3 mm long form blue-grey, powdery colonies under leaves and in the buds, and can spread turnip mosaic virus. Hose them off and follow with insecticidal soap as the label directs. Thrips, which multiply in dry, warm spells and leave silvery streaks, take the same treatment. Read more

White blister (Albugo candida). Small white blisters form under older leaves and on stems and flowers, then burst into white powder. It favours cool weather with heavy dew, so remove affected leaves and bin them.

Downy mildew (Hyaloperonospora parasitica). Yellow to pale brown patches on the upper leaf sit over a greyish-white growth beneath, and crowded seedlings are hit hardest. Thin on time and water early in the day. Read more

Clubroot (Plasmodiophora brassicae). Swollen, distorted roots leave plants stunted, yellowing and wilting on warm afternoons. Spores can last up to 20 years in soil, so pull affected plants, roots and all, and bin them.

Prevention first

• Clean soil off tools and boots before working another bed, so clubroot spores stay put.
• Cut with a clean knife when plants are dry, as soft rot enters through wounds.
• Water at the base early in the day, so foliage dries quickly.
• Clear spent plants straight after the last cut, as leaf diseases carry over on debris.

Tips

1. 'Karate' and 'Taboo' are F1 hybrids, so saved seed will not come true. Choy sum also crosses with turnip, bok choy and komatsuna, so seed savers isolate it by 250 metres or more. Let at least five plants of one variety flower together for good seed set.
2. 'Hon Tsai Tai' deepens to a richer purple in cold weather, so an autumn sowing often gives the best colour.
3. Leave one plant to flower after the last cut; its open blooms feed the parasitic wasps and hoverflies that help keep caterpillars and aphids down.
4. Leave one unfed brassica outside the mesh as a sacrificial plant to draw egg-laying butterflies away.