There is a slightly bitter tang to the stalks and leaves, and an almost sweet mustardy taste to the flowers (which are edible), so it gives a combined sweet and sour taste which the Chinese still value. In fact, it’s considered one of the more desirable types of Bok Choy in china.
There are three sub-varieties: Tsai Shim, Tsoi-Sim and Hon Tsai Tai. All three have dark green leaves. Hon Tsai Tai’s leaves also have purply veins in them, and sometimes instead of producing small yellow flowers, the flowers will be purple. It is very tender. Hon Tsai Tai is ready to harvest from seed in 37 to 50 days, depending on where you are.
Tends to hold onto dirt and grit more than other Bok Choys, so needs a thorough washing with two changes of water.
“Sum” in Chinese means flower stalk. “Yu choy”, one of its synonyms, means “oil vegetable”, because the seeds of the flowers can be used to make oil from.