Indigofera tinctoria aka Dye Indigo

Indigofera tinctoria is also known as true indigo. It is part of the bean family. People have long grown it for its blue dye. After centuries of growing it, its first home is unclear. Useful Tropical Plants names Malaysia as its likely first home. Other sources list Africa, Asia, Mexico and South America. It now grows across the tropics and subtropics. How long it lives depends on where it grows. The same holds for how long it keeps its leaves. It may live for one year or for many years. It may also grow as a shrub that stays green.

This shrub has many branches, a broad shape and arching stems. Sources give garden sizes that differ by a large amount. NC State gives both height and width as 2–3 feet [0.6–0.9 metres]. Other accounts give heights of 3.3–6.6 feet [1–2 metres]. RHS lists height and spread reaching 8.2 feet [2.5 metres]. Young stems are green and smooth. Older branches turn woody. The pale green leaves are pinnate, with paired oval leaflets. Each leaflet has a smooth edge with no teeth.

Small pink to purple flowers grow in short racemes, or clusters. Missouri Botanical Garden gives the bloom time as July to August. The flowers have two lips and draw pollinators. Slim legume pods grow after the flowers. They measure about 0.17 feet [0.05 metres] long. Both flowers and fruit add interest to the garden. Uses include borders, low hedges and groups of plants. It also suits pots and beds by walls that offer shelter. It can serve as ground cover too.

Give this plant sun and rich soil that drains well. NC State also allows part sun and shade on hot afternoons. It likes moist soil but can bear dry spells at times. Sources accept many soil types and pH levels. Advice on cold tolerance differs between sources. American sources list USDA zones 10–12. RHS gives the UK rating H5, a much colder range. Raise new plants from seeds or semi-hardwood cuttings. These cuttings come from stems that have begun to turn woody. RHS allows hard pruning in spring when needed. Most accounts report few serious pest or disease problems. RHS warns of honey fungus where it is present.

Growers soak and ferment the leaves to get blue dye. Some use this dye to change the colour of icing sugar. Sources note long-standing uses of leaves, roots and seeds as medicine. These records do not show that such treatments work. PFAF and Useful Tropical Plants report no known hazards. Soil bacteria in root nodules help the plant fix nitrogen. Farmers use it as a cover crop and green manure. Waste from making dye can also serve as manure. Plants that escape gardens can become weeds and harm native habitats.