Choy sum (also spelled choi sum) is a leafy vegetable commonly used in Chinese cuisine. It is a member of the Brassica genus and the Brassicaceae (mustard) family (Brassica rapa var. parachinensis or Brassica chinensis var. parachinensis). Choy sum is a transliteration of the Cantonese name (Chinese: 菜心), which can be literally translated as "vegetable stem". It is also known as Chinese Flowering Cabbage.
Choy sum is a green leafy vegetable similar to kai-lan, and can be characterized by the distinct yellow flowers which it bears. Each flower has four yellow, oval to round petals with six stamens on fleshy, erect stems which are 0.5 to 1 centimetre (0.2 to 0.4 in) in diameter and 15 to 20 centimetres (6 to 8 in) tall with light to dark green, and are oval (becomes acuminate shaped, or basal-shaped near the flowering stage) with slightly serrated margins leaves, which never forms compact heads like the cabbage. Fruits can develop out of cross-pollination or self-pollination, and are silique structured, that opens at maturity through dehiscence or drying to bare open to brown or black seeds that are small and round in shape. A single pod can bear up to 4 to 46 seeds.
The height of the plant varies greatly, ranging from 10 to 40 centimetres (4 to 16 in) depending on the growing conditions and the variety. Flowering usually appears when there are about 7 to 8 leaves on the plant or about 20 centimetres (8 in) tall. The bulk of the root system is found within a depth of 12 centimetres (5 in) and is confined to a radius of 12 centimetres (5 in).
The whole plant is overall an annual, herbaceous plant, rarely perennial, rarely growing into subshrubs. The whole plant consists of a simple or branched (when it is near the flowering stage), leafy structure. It grows best in soil with a pH range of minimum pH level at 5.6, maximum pH level at 7.5.