Choy sum is a transliteration of the Cantonese name (Chinese: 菜心), which can be literally translated as "heart of the vegetable".

[1] Choy sum is a green leafy vegetable similar to gai 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 (1⁄4 to 1⁄2 inch) in diameter and 15 to 20 cm (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.

The height of the plant varies greatly, ranging from 10 to 40 cm (4 to 16 in) depending on the growing conditions and the variety.

The whole plant consists of a simple or branched (when it is near the flowering stage), leafy structure.