The seedlings of choy sum or chinese flowering cabbage plantation. Choy sum (also spelled choi sum, choi sam in Cantonese; cai xin, caixin in Standard Mandarin) is a leafy vegetable commonly used in Chinese cuisine. It is a member of the genus Brassica of the mustard family, Brassicaceae (Brassica rapa var. parachinensis or Brassica chinensis var. parachinensis).