Interviews with 193 informants in Hainan (144 Miao and 49 Li) showed that five indigo-yielding plants were used historically and four are still used today: Strobilanthes cusia, Wrightia laevis, Indigofera suffruticosa and Indigofera tinctoria. Persicaria tinctoria is no longer used and its seeds have not been kept.

Strobilanthes cusia is the main raw material for indigo. The two peoples make indigo paste in basically the same way, with minor differences in detail; the Hainan Miao consider S. cusia the best indigo plant. The study reported for the first time the detailed extraction process from Wrightia laevis and recorded for the first time the use of castor seeds to remove foam and improve the quality of the indigo paste, although the mechanism is not yet clear (see the traditional indigo-making process below).