The present invention provides an in vitro flavonoid-rich rhizome tissue of Neomarica gracilis, which is obtained from a tissue culture preparation of an N. gracilis tissue capable of proliferating, such as a root, a leaf, a basal portion of a leaf, and/or a rhizome. The in vitro flavonoid-rich rhizome tissue of N. gracilis contains tectorigenin, which is distinctively different from the naturally grown rhizome of N. gracilis which contains no tectorigenin. The present invention further provides a method for cultivating the in vitro flavonoid-rich rhizome tissue, a method for extracting the tectorigenin from the flavonoid-rich rhizome tissue, and quantitative methods for determining the amount of tectorigenin in the in vitro flavonoid-rich rhizome tissue.