Choy sum

Choy sum, also called choi sum or choi sam in Cantonese and caixin in Standard Mandarin, is a leafy green vegetable common in Chinese cooking. It belongs to the genus Brassica in the mustard family, Brassicaceae, and is classified as either Brassica rapa var. parachinensis or Brassica chinensis var. parachinensis. The name comes from Cantonese and translates literally to "heart of the vegetable." It is also known as Chinese flowering cabbage or yu choy (you cai in Standard Mandarin).

This plant resembles gai lan and is notable for its bright yellow flowers. Each flower has four yellow, oval to round petals and six stamens, growing on fleshy, upright stems that are 0.5 to 1 cm thick and 15 to 20 cm tall.

Its leaves are light to dark green, oval (becoming pointed or basal-shaped near flowering), with slightly serrated edges, and never form a compact head like cabbage. Fruits develop from cross- or self-pollination and are siliques that split open when mature, releasing small, round brown or black seeds. A single pod holds 4 to 46 seeds.

Plant height varies widely, from 10 to 40 cm, depending on growing conditions and variety. Flowering typically starts when the plant has 7 to 8 leaves or reaches about 20 cm tall.

Most roots grow within 12 cm of the surface and spread no more than 12 cm outward. The plant is generally annual and herbaceous, rarely perennial or shrubby, with a simple or branched leafy structure that branches near flowering. It grows best in soil with a pH between 5.6 and 7.5.

Choy sum is highly prized as a vegetable in China and Japan. It is often eaten in soups, blanched, or stir-fried.