Yu Choy Sum Seeds - Kosaitai - Hon Tsai Tai. Heirloom, Non-GMO. Brassica rapa var purpurea. 50 days. Cool season annual. This flowering purple stem choy sum, also known as kosaitai or hon tsai tai, is a popular Chinese specialty vegetable. The leaf stalks and veins are purple while the leaves are dark green and the flowers are yellow. ~ 10,200 seeds / oz.