Plant 372

Indigofera tinctoria L. (Fabaceae)

True indigo

Indigofera (which means ‘bearing indigo’) is a cosmopolitan genus of some 700 legume species found in subtropical and tropical areas. However, since Indigofera tinctoria has been grown, moved and used by people in Asia, Africa and the Middle East for millennia, the species’ native range cannot be determined with precision.

Indigofera tinctoria is a shrub which can be managed as an annual or perennial depending on where and how it is cultivated. As a legume it can be used to fix nitrogen from the atmosphere and improve soils. However, historically, the plant’s most important use has been as a dye plant, a source of the blue pigment indigo. For example, in India, indigo has been extracted from the plant for at least 5,000 years.

To extract the dye, leaves are fermented in water. The solution is then aerated and treated with alkalis to convert the colourless, water-soluble indican, found inside the plant’s cells, into blue, insoluble indigotin. However, if this highly skilled process of dye production is to be profitable, it is essential to pay attention to the sources of the raw material and the timing of each of the many steps.

In addition to Indigofera tinctoria, other members of the genus produce indigotin. For example, the tropical American shrub Indigofera suffruticosa was an important source of indigo for pre-Colombian Mesoamerican cultures, and became widely cultivated in pre-Revolutionary North America. Indigotin is also produced from other non-leguminous plants, of which the most famous is woad (Isatis tinctoria), a member of the cabbage family.

Until the eighteenth century, woad was the primary source of indigotin in western Europe, where the fertile areas needed for its cultivation frequently competed with areas needed for food production. Eventually, the local interests of British and European woad producers were sacrificed on the ebb and flow of imperial priorities that demanded high-value crops for cultivation in colonies. In Victorian India, the European conflict between woad and food production was repeated when famine hit areas of indigo cultivation.

The interests of both woad and indigo producers were usurped in 1883 when the German chemist Adolph von Baeyer discovered how to synthesise indigotin artificially. In 1897, about 19,000 tonnes of plant-derive indigotin was produced annually – most of this came from Indigofera tinctoria; a century later a similar amount of indigotin was produced using Baeyer’s industrial process. Major users of indigo were manufacturers of the practical, hard-wearing cloth, denim.

Further reading

Balfour-Paul J 2011. Indigo: Egyptian mummies to blue jeans. The British Museum.

Harris SA 2015. What have plants ever done for us? Bodleian Publishing, pp. 103-106.

Stephen Harris