It is actually not a mustard plant, nor a spinach, but it is a member of the larger Brassica family. There are many cultivars.
The leaves can be harvested at any stage of growth. The more mature the plant, the more pungent the flavour. You can treat it as a cut and come-again plant. If you harvest it leaving about 1 inch (2 1/2 cm) of stalk, it will re-sprout.
The plant is mature 35 to 40 days after planting, but young leaves can be harvested before that. The plant grows up to 8 to 14 inches (20 to 35 cm) tall. Mature mature leaves are dark-green, glossy and oblong leaves, about 7 inches (18 cm) wide by 12 inches (30 cm) long. It has thin, light-green stalks.
The plant tolerates dry weather and cool temperatures, and is frost-hardy down to 10 F (-12 C) for a short period. It resists bolting in hot weather.
Japanese Mustard Spinach is grown mostly in Korea, Taiwan and of course in Japan, where it is grown year-round. In fact, plants grown during the winter are considered to have the best taste. It is particularly popular at the New Year in Japan.
When cooked, the leaves don’t wilt, and the stalk retains its water.
Young leaves can be used in salads or stir-fries. Leaves from older plants can be simmered, braised, steamed, stir-fried or pickled in salt.
Store in refrigerator.
Introduced into North America by the 1930s.
The name, “Komatsuna” comes from the Komatsugawa district in Japan, which includes Edogawa, Katsushika and Adachi. Edogawa is a major centre of production of Japanese Mustard Spinach.