Choy sum
Commercial production

Department of Primary Industries and Fishereies, Queensland.

Figure 1. The harvest from several choy sum plants

Choy sum (choi sum) (Brassica rapa var. parachinensis or Brassica chinensis var. parachinensis) is sometimes referred to as a flowering pak choy or Chinese flowering cabbage. The flower shoots of choy sum and younger leaves are used in salads or stir-fried, lightly boiled or steamed and added to meat.

Choy sum is characterised by yellow flowers borne on slightly fleshy stalks 0.5 to 1 cm in diameter and 15 to 20 cm long. Leaves are light or dark green, generally oval or egg-shaped with slightly-serrated margins (Figure 1).

Climate and soil

Choy sum is a cool season crop that prefers uniform conditions, moderate moisture levels and reasonable sunlight. It can be grown year round when seasons are not too extreme but higher temperatures may result in thinner, tougher and less sweet shoots, or lead to bolting. Choy sum is not frost tolerant although there is a purple-flowered variety which can withstand temperatures below 0° C. The optimum temperature for minimum time to harvest is between 15 and 25°C. Long photoperiods shorten the growth period of the plant by up to 10 days.

Choy sum will grow on a wide range of soil types as long as they are fertile, high in organic matter and have good drainage. The ideal soil pH is between 6.0 and 7.0. Do not let the soil pH fall below 5.0.

Cultivation

Choy sum plants can be grown in raised beds, at a spacing of 10 cm between plants. Plants raised from seeds greater than 0.118 mm in diameter produce more vigorously growing seedlings that mature earlier. Discard seed smaller than 0.118 mm in diameter.

Apply frequently but light irrigations, usually daily or twice daily. Liberal nitrogen application can improve yields.

Pests and disease

Aphids, green looper caterpillar and white butterfly caterpillar are the common pests that can be managed using registered insecticides. White rust, black leg disease, Alternaria and Mycosphaerella may all cause extensive damage to crops.

Harvesting, storage and marketing

Choy sum is harvested 30 to 50 days after sowing. Plants are harvested as the first flower buds begin to open by cutting with a sharp knife at the base and tying 10 to 12 plants together. Seasonal yields with two to three harvests are commonly around 11 to 18 t/ha. A high quality product has a white and tender stalk with developed but unopened yellow flower buds. The main stem should be long (at least 10 cm) and thick at the base (1.5 to 2.5 cm in diameter), there should be no roots. Common postharvest defects include open or deteriorating flowers and yellowed or decayed leaves.

Choy sum is best stored between 1 and 5ºC at a relative humidity of 90 to 95%. At 1°C, choy sum can last more than 20 days while at 10°C it will only store for 3 to 4 days. Choy sum is ethylene sensitive.

Additional information sources

• Recommended Reading: Larkcom, J. (1991) Oriental Vegetables. John Murray (Publishers) Ltd, London.

DPI&F information and services

• To access DPI&F's information and services, Queensland residents can contact the DPI&F Business Information Centre on 13 25 23 for the cost of a local call, from 8 am to 6 pm Monday to Friday (excluding public holidays). E-mail callweb@dpi.qld.gov.au. Non-Queensland residents phone (07) 3404 6999.

• Current national information on agricultural chemicals registered for use on all crops is available on the Infopest CD-ROM. Write to DPI&F, GPO Box 46, Brisbane, Qld 4001, E-mail infopest@dpi.qld.gov.au, visit the Infopest web page, or phone (07) 3239 3967 for further information.

Industry links

• Industry links is a page of links to various sites of interest to horticultural growers.

Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries and Fisheries, Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.

Last reviewed 22 October 2007