Choy sum is a Chinese phrase meaning “stems and flowers.” It is used to refer to a flowering vegetable closely related to bok choy which produces thick, crunchy stalks, yellow flowers, and lush green leaves. It is used in a variety of Chinese foods, usually stir fries and soups, and many Asian markets carry it, usually with other vegetables such as bok choy and Chinese broccoli. It is also possible to grow this vegetable at home in temperate climates, for cooks who have difficulty obtaining it in the marketplace.
Unlike other leafy green relatives, choy sum is cultivated specifically for the tender, crunchy stems and flavorful flowers, which have a hint of a mustard-like bite. Green choy sum produces greenish to yellow stalks and leaves with yellow flowers, while white choy sum has white stalks, and strongly resembles bok choy except that the bunches also include the bright yellow flowers. All of the parts of the plant are edible.
White choy sum is sometimes called bok choy sum, while the green type is also known as yu choy sum. Yu choy sum more closely resembles mustard greens or rabe, and has a slightly different flavor. Both plants are milder and more sweet than their more pungent relatives, especially when cooked. Since the vegetables are delicate, they are usually added to recipes at the last minute, so that the flavor is not lost.
When selecting this vegetable in the store, consumers should look for crisp specimens with upright leaves. It should not have any discolorations or soft spots, and smaller vegetables are actually better, since they will be more tender and flavorful. Once selected, the vegetables can be kept wrapped in the produce drawer for several days, after which it should be used. To use this vegetable quickly, try steaming it and serving it over rice with peanut sauce and stir fried tofu or chicken to make a version of pra ram, a popular Thai dish.
To grow this vegetable at home, start seedlings in a greenhouse or purchase them from a garden store. Plant them out in fertile soil well worked with mulch with excellent drainage. Keep the soil moist, but not water logged, and trim greens as needed. The plant will usually produce several sets of leaves and flowers before being exhausted. It can also be grown in tandem with bok choy, cabbage, chard, and similar vegetables.