The hairy indigo (Indigofera hirsuta Linnaeus) lingers in pastures and ruderal (disturbed) places in Florida, Georgia, South Carolina, and Puerto Rico. Native to northern Australia, southern Asia, Africa, and Madagascar, hairy indigo has naturalized in subtropics and tropics worldwide. Its introduction to the American South is uncertain—perhaps intentional, a plant for the indigo dye industry, or merely an incidental traveler with the slave trade.

Farmers and ranchers found the plant useful; green cropping (cultivated for plowing under) with the legume improved soil for crops like melons and strawberries and cattle gained more weight when it was part of their forage. Now hairy indigo is distained, a nuisance plant, seldom recognized like its more famous cousins, Indigofera tinctoria and Indigofera suffruticosa.

Indigo dye can be manufactured from numerous plants. In Europe, the famous blue coloring came from woad (Isatis tinctoria), a plant in the Brassicaceae or mustard family. (See Plants that Changed History, April 8, 2003) In the tropics, indigo was extracted from species of Indigofera [in de gof' er a], plants in the Fabaceae or legume family. But extracting indigo is neither easy nor obvious. Indigo does not exist in plants.

Indican (1H-Indol-3-yl-β-D-glucopyranoside) is thought a byproduct of plant metabolism; a similar "metabolic indican" is present in the urine of mammals. Plant indican is an indole molecule with a sugar moiety (glucose) attached via a β bond. Indican is colorless.

During the height of the indigo industry, indican was extracted by immersing bundles of Indigofera in large shallow vats of water. Bacteria present on the plants ferment the sugars including breaking the β bond thus stripping the sugar moiety from the indican. The fermentation was quick, about 15 to 20 hours later the sugar moiety was replaced with a hydroxyl (-OH). The intermediate molecule or indoxyl is amber to yellowish green and chemically unstable.

The water from the fermentation vat was drained into a beater vat. Beater vats were large enough for several beaters, usually slaves with paddles or split rods, to stand in. It was their job to beat the smelly yellow water. As beating commenced, the water turned green then blue. The blue coalesced into specks, the specks into flakes which drifted to the bottom of the vat.

Beating introduced air and hence oxygen, the next player in the chemical reaction. The oxygen strips two hydrogen atoms from the indoxyl; these molecules, indoles, 'face' each other and double bond. These double-bonded indoles are indigotin or indigo. Indigo is not soluble in water which is why it settles out of solution during the beating process.

For indigo to be useful, it must be water-soluble. It is reduced (made acidic) by adding a hydrogen atom to the indigotin molecule. The reduced indigo goes into solution; the solution is yellow or slightly green. The reduced indigo bonds to fibers soaked in the solution. When lifted from the bath, the fibers (or fabric) are the color of the solution. But back in the presence of oxygen, the reduced indigo loses the hydrogen and reforms as the double-bonded indigo. The cloth changes from yellow to blue.

Most indigo today is synthesized in chemical factories. It is homogeneous and produces a predictable blue or purple. For a mass fabric dyer, it will have standard results.

But naturally dyed indigo cloth can range from deep green to purple depending upon the duration and number of baths. Textile artists, mostly cottage industry artisans, use hairy indigo as well as its famous cousins to attain beautiful and subtle shades not found in the chemically-synthesized indigo.

(Compiled from: The Merck Index, 11th Edition, Susan Budavari, ed., Merck and Co., Inc., Rahway, NJ, 1989; A Modern Herbal, Mrs. Maude Grieve, 1931, reprinted 1971, Dover Publications, New York; "Indigofera hirsuta" Pacific Island Ecosystems at Risk; "Indigo", Chemistry of Natural and Synthetic Dyes, C.J. Cooksey; "Indigo" Maiwa Handprints; "Indigo (Indigofera spp.)" Bello Marini: Tie-dyeing in Indigo—dyes; "Indigo in the Early Modern World" Anne Mattson, James Ford Bell Library, University of Minnesota)