Chinese flowering cabbage (Choy sum)
Note Number: AG0924
Published: October 2001
Updated: November 2009
Reviewed: August 2013
Chinese Flowering Cabbage (Brassica rapa var. parachinensis or Brassica chinensis var. parachinensis) is also referred to as a flowering pak choy or choy sum. Other names include tsoi sum and cai xin (Chinese), cai ngot (Vietnamese), pakauyai or pakaukeo (Thai), saishin (Japanese), Chinese soup green, white flowering cabbage and mosk pak choy (English).
Choy sum is native to China and is one of the most popular vegetables among the Chinese. It is probably the most popular vegetable in Hong Kong and also widely used in the western world.
The flowering shoots and younger leaves of choy sum are used in salads or stir-fried, lightly boiled or steamed and added to meat. Rich in carotene (pro-vitamin A), calcium and dietary fibre, it also supplies potassium and folic acid.
Choy sum is characterised by yellow flowers borne on slightly fleshy stems 0.5 to 1 cm in diameter and 15 to 20 cm long. The height of the plant varies from 20 – 30 cm. Leaves are light or dark green and are oval or egg-shaped with slightly serrated margins. Flowering starts when there are about 7 - 8 leaves on the plant. The bulk of the root system is found within a depth of 12 cm and is confined to a radius of 12 cm. The average spread of the plant varies between 15 – 45 cm.
Choy sum is a cool season crop that prefers uniform conditions, moderate moisture levels and reasonable sunlight. It can be grown all year round when seasons are not too extreme, but higher temperatures may result in thinner, tougher and less sweet shoots or may lead to bolting. Choy sum is not frost tolerant, although there is a purple-flowered variety that can withstand temperatures below 0 o C the optimum temperature for minimum time to harvest can be between 15 o and 25 oC.
In China, the cultivars of Chinese flowering cabbage are classified by the number of days from sowing to harvest and their susceptibility to premature seed stalk formation. However not many cultivars are available outside China. Cultivars used generally include: Choy sum (Brassicarapa var parachinenis) (sometimes B. chinensis var parachinensis), Purple-flowered choy sum (Brassica rapa var purpurea) and Flowering pak choy (Brassica rapa var chinensis).
Choy sum grows in a wide range of soil types from light sands to clay loams but prefers fertile, well-drained soils. It does not tolerate dry conditions or water logging. If drainage is an issue and/or on heavier soils, plants should be grown on raised beds. The ideal soil pH should not fall below 5.0 and is between 6.0 to 7.0, although pH readings greater than 7.0 are desirable for club root control.
Choy sum can be grown at plant spacing varying from 2.5 – 20 cm and row spacing of between 7 – 30 cm but more commonly, 10 - 20 cm between plants and 30 cm between rows are used. Seeds with a diameter greater than 0.118 mm produce more vigorously growing seedlings that mature earlier. In temperate climates, the best time of planting would be from spring to late summer, after the risk of frost passed. In warmer climates, planting times may be from mid to late spring and early autumn. As the plant does not tolerate frost autumn, planting should preferably occur 6 weeks prior to the first frost.
Choy sum prefers a very fertile soil, rich in organic matter. Selection of a fertiliser program depends on the soil type and the fertility of the soil. As a general guide, the fertiliser program below may be followed but cropping history needs to be taken into account and a soil analysis should be used to decide the correct nutrient requirements of the crop.
NPK 6:6:6 700 to 1200 kg/ha
NPK 8:11:10 500 to 900 kg/ha
If fowl manure (deep litter) is available that can be added at a rate of 50 m 3/ha with superphosphate at 1.25 t/ha as a basal dressing, or at lower rates the amount of base fertiliser can be reduced.
Side dressing is typically applied one week after emergence and again three weeks after emergence, using 125 kg/ha of CaNO3 or NPK 34:0:0 and muriate of potash (each at 150 kg/ha) should be applied.
Important: There is a danger of introducing high cadmium levels to soils and crops with phosphate fertilisers, which may contain heavy metal cadmium as a contaminant. As there are legal maximum levels of allowable cadmium in vegetables, growers should consult fertiliser suppliers or manufacturers for advice on this. Fertilisers containing cadmium in excess of 1mg/kg are required to state the following warning:
"WARNING––this product contains cadmium. Continued use of this product in agricultural situations may lead to residue levels in plant and animal products in excess of the maximum level specified by the Australia New Zealand Food Standards Code and the accumulation of cadmium in soils."
Pests and diseases
There are many pests and diseases that damage the brassica family. The following pest and diseases are of major concern when growing Chinese flowering cabbage:
Aphids can feed on a large number of plant hosts. They have piercing and sucking mouthparts, which they use to get sap from plants. The aphids like to feed on new shoots and buds, but can be found feeding underneath mature leaves. Parts of the plant may wilt, look distorted and curled. The aphids can act as vectors for a number of viruses causing even greater damage.
There are several different types of caterpillars that attack brassicas. For example large and small cabbage white butterflies, diamond back moth and the green looper are common.
Snails and slugs
Both snails and slugs are attracted to the delicate and crisp leaves of the brassicas. They are of major concern to growers, because they eat whole young plants and can severely damage older plants. The greatest damage occurs during mild and damp weather, which is when the snails and slugs are most active.
This is caused by a fungus (Plasmodiophora brassicae) that lives in the soil. Club root is the most serious of all brassica diseases and very common in areas with long history of cruciferous crop cultivations. Fungal infection occurs on roots at any stage of growth. Infected roots show characteristic swellings or knots. The aboveground symptom is wilting of plants particularly during hot-dry weather. Severely infected plants are generally stunted with different colour foliage and in these plants normal root growth does not occur. With the proper management practices clubroot can be controlled, using a combination of plant rotation, adding lime to maintain soil pH above 7 and ensuring a high organic content in the soil.
White rust or white blister (Albugo candida)
White rust is a minor disease of brassicas, radish and other mustards. Symptoms are small circular spots raised on both sides of the leaves. Underneath the leaf a mass of white powdery spores develop and on top of the leaf spots are yellowish to green in colour.
White rust can be controlled by removing weeds in the area susceptible to the disease and before sowing to ensure that all the plant residues are completely decomposed.
Young plants are more susceptible to Downy mildew than mature plants. Downey mildew is caused by a fungus (Peronospora parasitica) mainly affecting leaves. Initially it looks as a fluffy or powdery-white mass of spores on the under surface of leaves. This is followed by a black speckling and puckering of the upper surface. Leaves prematurely become yellow and fall.
Control of this disease can be achieved by integrating various management practices such as seed treatments, disease free seedlings, preventative fungicide spraying, maintaining proper ventilation and lower relative humidity (in green house conditions), balanced fertiliser application and crop rotation.
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 2 to 3 harvests can commonly be around 6 to 10 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 25cm) and thick at the base (1.5 to 2.5 cm in diameter) and there should be no roots. Common post-harvest defects include open or deteriorating flowers and yellowed or decayed leaves.
Choy sum is best stored between 1o and 5oC at a relative humidity of 90 to 95%. At 1oC, Choy sum can last longer than 20 days, while at 10oC it will only store for 3 to 4 days.
For information relating to the safe and appropriate use of chemicals, including management of chemical residues and licensing requirements, contact us and ask to speak to your local chemical standards officer or visit our Chemical use page.
This Agnote was developed by Farm Services in October 2001.
It was reviewed by Farm Services in November 2009 and reviewed in 2013
Published and Authorised by:
Department of Environment and Primary Industries
1 Spring Street
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