Lithospermum erythrorhizon

It has been cultivated in Japan since the Nara period for its root, which can be used for herbal medicine and to make dyes. Before the introduction of synthetic dyes to Meiji period Japan, the roots were commonly used as a dyestuff for luxury textiles, typically high-end kimono and obi. The process of extracting purple dyestuff from the roots was an exceedingly long, complex and time-consuming process, necessitating its relatively high expense:

The roots [of the plant] are collected and dried for two to three months to mature the color. While this occurs, the silk is pre-mordanted [...] the mordanting process involves repeated immersion of the cloth or yarn in the alum bath and drying over a two- or three-month period. To prepare the dyestuff, the gromwell roots are softened overnight in 60° [Celsius] water and then pounded to release the dye. The silk is then repeatedly immersed in the bath, aired to allow more oxygen to penetrate the cloth and then steeped in the dye until the desired color is achieved. When the dyeing is completed the cloth is placed in dark storage for as long as a year while the color continues to mature. For a deep purple, up to 50 dips could be needed. …