White Vienna Kohlrabi Seeds Heirloom Vegetable. This rapidly growing member of the cabbage family produces a greenish-white enlarged stem with short tops and few leaves. Slow to bolt and early producing, White Vienna is best harvested when the stem reaches 5-7.5cm (2-3") in diameter. For best growth, irrigation is essential through dry periods. Flesh is white with a nutty flavour when eaten rawand a mild cabbage taste when cooked.