At distribution today, a number of people were mystified as to what mizuna actually is. My readings indicate that mizuna (Brassica juncea var. japonica) is a Japanese mustard green, also known as Xiu Cai, Kyona, Japanese Mustard, Potherb Mustard, Japanese Greens and California Peppergrass.

Mizuna has dandelion-like jagged edge green leaves with a mild, sweet earthy flavor. It has been culitaved in Japan since ancient times, but most likely originated in China. Mizuna makes an excellent salad green, and is frequently found in mesclun. It is also used in stir-frys and soups.

Source: foodreference.com