• Indigofera tinctoria, I. suffruticosa
• (In-dih-GOFF-er-uh tink-TOR-ee-uh/ suh-froo-tih-KO-suh)
• Family Leguminosae
• Tender subshrubs
Indigo, best known to Americans as the traditional colorant of blue jeans, has been esteemed and used for centuries throughout the world. Long before synthetic dyes became available, this blue pigment was being extracted from a variety of species belonging to several families of plants.
The Japanese preferred dyer’s knotweed (Polygonum tinctorium), a member of the buckwheat family; Europeans favored woad (Isatis tinctoria), a mustard native to the Middle East; while West Africans obtained their indigo dye from Lonchocarpus cyanescens, a vine of the pea family.
However, the most commercially significant indigo sources throughout history have been Indigofera tinctoria, pictured above, which is native to Southeast Asia, and I. suffruticosa, from the New World Tropics. Both are known by the common name indigo.
The genus Indigofera (the name is Latin for “indigo-bearing”) comprises about 700 species of trees, shrubs, and annual or perennial herbs of the pea family native to tropical and subtropical regions. The dye plants I. tinctoria (tinctoria is Latin for “dyer’s”) and I. suffruticosa (suffruticosa refers to having a woody base and herbaceous top) are both perennial deciduous subshrubs that are grown as annuals, at least in cooler climates. Both grow 3 to 6 feet tall, have compound leaves 2 to 3 inches long with many pairs of rounded leaflets. If you tear a leaflet, the cut edges will turn blue. Clusters of tiny coppery pink pealike flowers are produced in late summer, followed by small seedpods. Those of I. tinctoria are straight, those of I. suffruticosa, curved.
The chemistry of indigo is complex. In living plants of the genus Indigofera, indigo occurs as a colorless, water- soluble compound known as indican. When the leaves are placed in water, the indican leaches out and is transformed into an insoluble compound called indoxyl. With the addition of oxygen (as by stirring up the water), the blue dye indigo is formed, but it, too, is insoluble. To dye yarn or fabric with it, you have to add a reducing agent (which removes oxygen) to convert it to a yellowish water-soluble form called indigo white. In olden times, stale urine—vats of it—was used as the reducing agent, but today, Spectralite and Rit Color Remover serve as convenient substitutes. You soak your yarn or fabric in a warm bath of indigo white for about twenty minutes. As you remove it from the dyebath, the dye will react with the oxygen in the air and turn blue—at last. Repeated soakings and airings deepen the color. Because indigo coats the fibers rather than chemically bonding with them as other dyes do, no mordant is needed to ensure colorfastness.
The name “indigo” comes from Greek and Latin words meaning “from India”: The ancient Greeks and Romans imported cakes of indigo from India, where, as in Southeast Asia, the dye from I. tinctoria had been used for hundreds of years. When indigo was introduced into Europe in the seventeenth century, it was at first banned because it competed with a thriving woad industry. Later, after the ban had been lifted, both dyes were used, sometimes in combination. Dutch and Portuguese merchants made a bundle of money importing indigo from India. After the Spanish discovered New World natives using I. suffruticosa as a dye, they established plantations of this species in Guatemala. The French and British raised indigo in the Caribbean, the latter also in India.
In 1744, a young woman in South Carolina, Eliza Lucas Pinckney, became the first person to grow indigo successfully in colonial America, using seeds that her father had sent from the West Indies. Thanks to her formidable efforts, American growers were exporting more than 1,000,000 pounds of indigo to England just before the American Revolution. The advent of cotton as a cash crop after the war contributed to the demise of indigo production in the United States. The discovery, late in the nineteenth century, of synthetic indigo, which offered a purer color and was cheaper and easier to produce, spelled the end of commercial natural indigo production worldwide. Considering natural indigo’s variability an asset, some of today’s fiber artists are again growing the plants, extracting the pigment, and dyeing silk, wool, and cotton in myriad shades from pale sky blue to nearly black.
Ingesting I. tinctoria is said to produce nausea and vomiting, and other species of Indigofera have been used as emetics. Practitioners of Traditional Chinese Medicine have used I. tinctoria to combat pain, fever, and inflammation as well as to purify the liver and blood. Ancient Egyptians, Greeks, and Romans applied it to wounds and sores. The powdered root of the South African species I. patens has been packed into decayed teeth to treat toothache. Scientific validation of any of these uses is lacking, but two species, both called creeping indigo, have been shown to be toxic when taken internally: I. endecaphylla has reportedly poisoned livestock in the Tropics, and indospicine, an amino acid from I. spicata, can cause birth defects.
Full sun and a long, hot growing season are essential for growing both I. suffruticosa and I. tinctoria. The first seems to tolerate cooler climates better and blooms earlier than the latter. Outside the Deep South, hot desert Southwest, or Hawaii, start indigo indoors in individual pots about eight weeks before the last expected frost. Soak the seeds in warm water overnight and sow them 1/2 inch deep. Don’t sow seeds outdoors or transplant seedlings to the garden until the weather and soil are warm. Set or thin seedlings to a foot apart. The type of soil isn’t important, but it should be fertile and well drained. Keep the plants well watered and protected from rabbits and deer. Disease and insect pests shouldn’t be a problem. Harvest the leaves when the plants are just coming into bloom, starting with the lower leaves. You can make several pickings at two-week intervals, weather permitting.
If summers are too cool to raise a crop of indigo in the garden, you can grow a few plants in 6-inch pots set in a sunny window.
• Companion Plants, 7247 N. Coolville Ridge Rd., Athens, OH 45701. Catalog $3. Plants of I. suffruticosa and I. tinctoria.
• Native Seeds/SEARCH, 2509 N. Campbell Ave. #325, Tucson, AZ 85719. Catalog $1. Seeds of I. suffruticosa.
• Richters, Goodwood, ON, Canada L0C 1A0. Catalog free. Seeds of I. tinctoria.