Choy sum

Choy sum (Brassica rapa subsp. chinensis var. parachinensis) is a short-cycle herb of the mustard family (Brassicaceae), 20–40 cm tall, with a succulent, tender, tapering stem. Leaves are ovate-elongate, green or purple-green, with thick long stalks in alternate arrangement; young leaves are soft and sweet. As the plant bolts, the tip forms a cluster of buds and four-petalled yellow mustard flowers — this is when the vegetable tastes best. Eaten parts include the stem, stalks, young leaves and flower buds. In Vietnam choy sum is an everyday family vegetable, grown year-round, mainly in the cool season, from the lowlands to the Central Highlands.

Choy sum originates from southern China, especially Guangdong, and is a staple green of Cantonese and many Asian cuisines, widely used in Singaporean and Hong Kong dishes. In Vietnam it is grown and eaten in all three regions; the cycle is short (30–40 days), allowing multiple crops a year. It is delicious blanched with soy sauce or chili soy, garlic stir-fried, in minced-pork soup, noodles or hotpot. Choy sum is rich in vitamin C, vitamin K, beta-carotene, calcium and fibre, low in calories; folk regard it as cooling and diuretic, aiding digestion and suiting dieters; young leaves are also used in fresh-wrapped rolls.