How do you grow choy sum from seed? Direct- seed choy sum into well-tilled garden beds that have been amended with a 2-inch layer of compost. Thin seedlings to about 8 inches apart in rows 18 inches apart. Water the choy sum patch at least once a week, checking during the week to ensure that the soil remains moist. Yu Choy Sum Flowering Brassica Jung Green Seeds for Planting, Chinese Broccoli Vegetable, Flowering Pak Choy, Chinese Flowering Cabbage, Brassica Rapa Asian Green Seed, Gunsho Asian Green. Heat-tolerant choi sum. Very similar to Gunsho, just a few days later to mature. Choi sums are among the most popular vegetables in Hong Kong, Taiwan, and China. Choy sum is a leafy vegetable commonly used in Chinese cuisine. It is a member of the mustard family and has green stems and leaves with yellow flowers. Choy sum is a small, quick-growing, annual herbaceous plant reaching about 7 to 15 inches in height. Very popular 'Asian Green', closely related to Pak Choi with similar flavour.