Mustard Seeds - Early Mibuna. Heirloom, Brassica rapa var. japonica. Also known as mibuna. 21-40 days to maturity. NonGMO. Cool season annual. This traditional (heirloom) Japanese green vegetable, dento yasai, is cultivated in Mibu, Kyoto prefecture. An early open pollinated variety, this vigorous grower produces a dense cluster of long, narrow, rounded, dark green leaves. Approx. 11,100 seeds / oz.