Indigofera Tinctoria - True Indigo - Rare Tropical Plant Seeds (10) You are purchasing 10 of these rare seeds: Indigofera Tinctoria - True Indigo Best known as True Indigo and one of the oldest plants in cultivation, it is one of the original sources of natural indigo dye. This shrub can be annual, biennial or perennial, depending on its climate. The flowers, themselves, are lovely. The plant is a legume used to for crop soil improvement. Purple and blue dyes are obtained by fermenting the leaves. When in bloom, the indigo plant blooms profusely.