Brassica rapa var. parachinensis
Choi sum is one of the Asian greens, a type of Chinese cabbage. This quick growing cabbage doesn’t build a firm head but long slender crunchy stems with oblong green leaves. The stems are 2 cm wide and up to 30 cm long. The leaves are up to 25 cm long and about 10 cm wide and slightly serrated. Leaves and stems are the vegetable. Choi sum is is grown as and annual plant, if it goes to flower it shows the typical cross-shaped flower of the cabbage family with four petals.
How to grow:
Sow directly 3 mm deep in well drained and lightly fertilised soil where it’s supposed to grow. Sow in spots 30 cm apart. Put in 3 seeds per spot, 2 cm apart. Firm down and keep moist. Choose a sunny warm place. Plants appear within 10 days. Thin the plants to 2 and finally one plant per spot, 30 cm apart and mulch between the plants. Plants need to be watered consistently.
Harvest in 8 to 10 weeks.
Growing in the neighbourhood:
Likes to grow with lettuce, kale, endive, tatsoi, mizuna.
Pests and other problems and how we deal with them:
Caterpillars of butterflies, especially the white cabbage butterfly are pests on choi sum. We keep them out by netting the bed with a 5-10 mm net as long as the plants are small. A bit of leaf loss on mature plants is tolerable, if caterpillars take away too much leaf material we spray the plants with a bacterial solution that controls them.
All members of the cabbage family can be affected by a single-celled organism deforming the roots and making it hard for the plants to take up water. The problem is called club root and requires serious soil treatment. We try to avoid this by strictly rotating crops.
Autumn to spring
Choi sum cross pollinates with other plants of the cabbage family. In order to collect useful seeds the mother-plant would need to be kept isolated.
How to harvest and use:
Pick single outer leaves or harvest whole plant. Never only pick the leaves and leave the stems on the plant. It makes the plant vulnerable. The leaves and stems are kept to be eaten, the roots at the bottom are taken to the compost.
Choi sum leaves and stems are a great addition to stir fries. They are also great quickly blanched, served with a sauce or added to soups. They work well with garlic, ginger, mushrooms, soy sauce, tofu, pork, white fish, chicken.