Introduction

Over a dozen Oriental vegetables are grown commercially on a limited scale in southern Ontario. One of the most important is Chinese cabbage (Brassica rapa, Pekinensis group). Chinese cabbage, also known as celery cabbage and napa, was the first brassica cultivated in North America. Today it is increasing in popularity in western kitchens. The large savoyed leaves with thick succulent midribs possess a sweet taste and crisp texture when eaten raw. It has a flavour somewhat milder than cabbage when cooked. Although traditionally favoured for pickling, soups and stir-fry medleys, it can substitute adequately for cabbage in many western dishes.

Figure 1. Head type of Chinese cabbage with outer leaves removed: Wong Bok.

Figure 2. Head type of Chinese cabbage with outer leaves removed: Michihli.

Type and Cultivars

Chinese cabbage may be divided into two types: the loose-leaf forms and the head forms, the latter being the most common type grown in Ontario. The cylindrical head of Chinese cabbage resembles that of Cos lettuce, but is usually more firm at maturity. At maturity the outer foliage and wrapper leaves are characteristically pale green, whereas the inner leaves are blanched a creamy colour.

There are two head shapes of Chinese cabbage. The Wong Bok types produce a barrel-shaped head which is typically short and broad, and about 20 to 25 cm in length and 15 to 20 cm in diameter
(Figure 1). The Michihli types yield long tapering heads which may reach 38 to 46 cm in length and 10 to 15 cm in diameter (Figure 2).

Cultivars of Chinese cabbage differ in plant and head size, time of maturity, disease tolerances and certain head and foliage characteristics. Based on local experience, growers have chosen the cultivars most suitable for their production operations and clientele. The cultivars grown commercially in Ontario are not restricted to the numerous entries currently handled by North American seed companies. A number of growers obtain seed from reliable sources in the Orient.

Climatic and Soil Requirements

Chinese cabbage thrives best during the cooler periods of the growing season. Although the optimal temperature range of Chinese cabbage development is between 13 and 15 °C, certain cultivars tolerate the higher temperatures of midseason providing there is ample soil moisture. Other cultivars which mature during midseason may readily bolt. Late-growing Chinese cabbage can withstand light frosts in the fall, although alternate freezing and thawing may damage leaf tissue.

Soils which possess good structure, fertility and water-holding capacity usually produce a satisfactory crop of Chinese cabbage. Chinese cabbage will grow on soils which have pH values from 5.5 to 7.6, although the ideal pH for growth is near neutral (7.0). In southern Ontario, Chinese cabbage is grown successfully on different soil types, but sandy or porous soils are the least suitable for production due to their low water-holding and nutrient-retention capacities. Also, Chinese cabbage is not grown commercially on the muck soils of the Holland Marsh.

Nutrition

A rich fertile soil is necessary for the rapid growth of Chinese cabbage. Nitrogen is typically applied in southern Ontario at 80 to 130 kg per hectare and may be applied alone or in combination with phosphorus and potash. The higher nitrogen application rates are usually necessary on lighter soil types. Best results are obtained when the nitrogen is broadcast before planting, and also sidedressed in one or more applications 10 days apart following thinning or within one month of transplanting, thereby ensuring an ample supply of this element during plant development. Conservative usage of nitrogen is recommended since excessive applications may increase disease susceptibility and delay maturity. Nitrogen fertilization near maturity should be avoided since heads of larger size but lesser density may result.

Satisfactory growth of Chinese cabbage may be expected when the phosphorus and potassium levels in the soil are sufficient to support the production of cabbage (see OMAF publication 363, Vegetable Production Recommendations).

Planting

Early summer harvest of Chinese cabbage is possible by planting locally grown transplants. In early May plants are started in the greenhouse, hardened off, then set in the field. It is essential that the setting-out of transplants commences within a month after seeding and under favourable soil and climatic conditions to insure that a check in growth does not occur which may cause plants to bolt. Generally, the seedlings are handled in a manner similar to cabbage transplants, and may be hand or mechanically set in the field. A key to the success of this operation is soil moisture, which should not become limiting following transplanting.

Direct seeding in the field also occurs during late April and beyond. To accomplish this, some larger growers use precision seeders to reduce labour and seed costs.

A continuous supply of Chinese cabbage through the season is achieved by successive plantings every 10 to 14 days (Figure 3), remembering that ample time must be allowed for plant development before the anticipated date of the first frost. To safeguard against total crop failure, growers may also plant more than one cultivar at a time.

Plants are spaced 25 to 60 cm apart within rows and 40 to 82 cm between rows. The shorter row spacings are used for the narrow upright Michihli types and the wider spacings for the larger, spreading Wong Bok types.

Figure 3. A typical field of Chinese cabbage planted on successive dates.

Irrigation

Ample and uniform soil moisture for plant development is essential for the successful production of Chinese cabbage. Because rainfall is unpredictable in Ontario, growers should closely monitor soil moisture levels and be prepared to irrigate when necessary. Sprinkler irrigation should be used in the morning so plants will dry before evening. Moist soil conditions will facilitate lower field temperature, giving additional benefit during midsummer.

Weeds

Nightshades, mustards and hairy galinsoga (Galinsoga quadriradiata) are a few of the most troublesome weeds. Presently there are no herbicides registered in Ontario for use in Chinese cabbage for eradicating these and other noxious weeds. Hand or mechanical cultivation may be employed and should commence before weeds become established.

Diseases

A number of crucifer diseases have been reported to infect Chinese cabbage. The importance of these diseases will depend on locality and year.

Some cultivars of Chinese cabbage are susceptible to the viral diseases of turnip mosaic and turnip yellow mosaic. Unfortunately, information is lacking on the distribution and severity of these viruses in Chinese cabbage in Ontario and on the availability of resistant cultivars. Young plants infected with turnip mosaic virus are stunted with markedly crinkled foliage and may never reach harvest maturity. As the leaves expand, areas of green tissue will become interspersed with yellow tissue, giving a characteristic mottled appearance (Figure 4 and Figure 5). The virus is transmitted solely by aphids, but insecticides will not protect fields against migrating aphids carrying turnip mosaic virus (see OMAF Factsheet, Virus Diseases of Rutabaga).

In certain areas in Ontario clubroot may be a major factor limiting production. This disease is caused by a simple soil-borne fungus (Plasmodiophora brassicae) which thrives in acid soils of high moisture content and invades the roots of plants, thereby adversely affecting water uptake. Plants in early stages of infection will exhibit wilting on sunny days and, in advanced stages, roots of infected plants become swollen and malformed. Control is achieved by raising the soil pH of fields above 7.2, thereby minimizing the activity of the fungus. Fields with a past history of clubroot infection should be avoided and planting of resistant cultivars should be used when possible.

Downy and powdery mildews caused by the fungi Peronospora parasitica and Erisyphe polygoni attack Chinese cabbage at any stage of development. On mature plants the head may become unfit for sale, especially if the infection occurs deep within its interior. These diseases may be followed by secondary infection of bacterial soft rots caused by Erwinia carotovora or species of Pseudomonas.

Leaf spots caused by Alternaria brassicae and related species, blackleg (Phoma lingam) and white rot (Sclerotinia sclerotiorum) may also affect production in certain years.

Insects and Pests

Insects which feed on cole crops may also damage Chinese cabbage. The most common insects include several species of aphids, and the larvae of the diamond-back moth (Plutella xylostella), imported cabbage worm (Artogia rapae) and cabbage lopper (Trichoplusia ni). The larvae of the latter three insects will eat holes in the plants' foliage and may burrow into maturing heads making them unsuitable for sale.

Chinese cabbage seedlings are susceptible to several species of flea beetles and the cabbage root maggot (Hylemya brassicae). Other insects which have been reported to feed on Chinese cabbage in southern Ontario include the tarnished plant bug (Lygus lineolaris) and various types of slugs and snails.

Figure 4. Turnip mosaic virus on Chinese cabbage. Note the characteristic mottled appearance of an infected plant.

Figure 5. Turnip mosaic virus on Chinese cabbage. Note the characteristic mottled appearance of a single leaf.

Physiological Disorders

Chinese cabbage normally completes its life cycle during the first year of growth. Bolting refers to the premature production of seeds on plants. When plants of Chinese cabbage bolt, heads of undesirable size and/or quality normally result. Studies have shown that the bolting response in Chinese cabbage is under genetic control, with certain cultivars being more prone to bolt than others. Several environmental factors have been associated with bolting in Chinese cabbage. For example, young plants exposed to low temperatures for extended periods during early spring have a high probability of bolting during the higher temperatures and longer day lengths of midsummer. Short days with near optimal growing temperatures will tend to keep plants in the vegetative phase of development. Factors which cause a check in plant growth, such as nutrient deficiencies, may also induce bolting.

It is not certain that tipburn in Chinese cabbage is similar to the tipburn observed in lettuce and cabbage. The symptoms are brown and black necrotic areas on the leaf margins of external and internal foliage. In lettuce this disorder has been attributed to factors which affect the water status and the calcium and/or boron nutrition of the plant. Chinese cabbage cultivars which appear to possess tolerance to tipburn are available.

Harvesting

The maturity date for Chinese cabbage varies among cultivars, but it is usually 60 to 95 days. In southern Ontario harvesting commences in mid-July and continues through November. Chinese cabbage is harvested by hand when the heads are fully developed. Plants are cut at the based and the outer leaves removed. If the heads are to be stored some of the outer leaves, providing they are free from disease, may be left on to protect the head. These outer leaves are removed after the heads are taken out of storage. Harvested heads should be firm and free from discolouration, pests and disease injury. Heads intended for long-term storage should be handled with care to avoid bruising, cuts and abrasions, which lead to accelerated deterioration.

Chinese cabbage is sold directly from the grower to local specialty stores or restaurants, or distributed through central outlets such as Farmer Markets or the Ontario Food Terminal. A few growers have diversified their marketing operations to include "Pick Your Own" sales.

Storage

Studies at the University of Guelph have shown that the Wong Bok types of Chinese cabbage store for longer than two months when held at 1-2 °C, a high relative humidity exceeding 90% and minimal concentrations of ethylene gas. Chinese cabbage subjected to high levels of ethylene gas during storage exhibit leaf abscission. Other studies suggest that optimal post-harvest head quality can be achieved in controlled atmospheres at 1-2% 02< and 2-5% CO2.

The appropriate storage conditions should be established quickly following harvest to assure maximum storage length. When placed in cold storage, the heads should be packed loosely in crates to allow for proper ventilation around the heads. Heads of certain cultivars should be stored in an upright position to safeguard them against becoming misshapen.