Tephrosia

What is Tephrosia?

Tephrosia is a plant that is grown for what it leaves behind. It is a legume, a relative of the pea, and a fast, soft-wooded shrub whose leaves are rich in nitrogen. Cut it back and it sprouts again; the cuttings stay on the ground and feed the soil. In the tropics it has been used this way for generations, as green manure, as temporary shade over young coffee and cocoa, and as a hedge along contour lines on slopes.

The genus is large. Two species carry most of the practical experience: Tephrosia vogelii from tropical Africa and Tephrosia candida from South Asia. Both form root nodules with nitrogen-fixing bacteria, both tolerate hard pruning, and both have leaves that are poisonous. Among nitrogen-fixing plants Tephrosia belongs to the quick, short-lived helpers that set a planting in motion rather than to the trees that stay.

Origin and growth

Tephrosia vogelii grows across tropical Africa, from savannas and forest margins to fallow fields, and has been planted in Asia and Latin America. It is a branching shrub or small tree of 0.5 to 4 metres, densely leafy, with silky hairs and white to violet flowers. Tephrosia candida reaches about 3.5 metres, with silvery leaves and white flowers. Both are deep-rooted and resprout readily, even after fire.

Requirements

Tephrosia is a tropical plant. The World Agroforestry database places T. vogelii in areas with a mean annual temperature of 12 to 27 °C and 850 to 2650 mm of rain; T. candida in areas of 18 to 28 °C and 700 to 2500 mm. Neither tolerates frost, and T. candida tolerates no waterlogging. Both cope with poor, acid soils; T. vogelii prefers well-drained loams with a pH of 5 to 6.5. They want full sun. T. vogelii is described as tolerant of drought, wind and grazing, with one restriction: in drought it often stops resprouting after a cut.

Role in the soil: nitrogen, biomass and mulch

Tephrosia fixes nitrogen through bacteria in its root nodules. For T. candida the literature names Bradyrhizobium as the partner; T. vogelii is reported to nodulate on acid soils where leucaena does not. How rhizobia behave in a soil that has never seen the plant is a separate question, and nobody can answer it in advance.

The value of Tephrosia lies in its leaf mass. Young growth is richest: T. vogelii contains 3.7 g of nitrogen per 100 g of dry matter at two to three months, 1.2 g at ten months. That argues for frequent, light cuts rather than rare, heavy ones. On fertile soil T. candida gives 25 to 30 tonnes of green matter per hectare a year in three cuts. In fallow trials at Msekera in eastern Zambia, Madagascan provenances of T. candida produced about twice the biomass of T. vogelii after one and a half years, and maize after T. candida yielded more.

The soft leaves break down quickly. As mulch they feed the soil fast but cover it only briefly; the woodier stems last longer. This makes Tephrosia a typical biomass plant of the early years: a supplier of nitrogen-rich material, less a permanent ground cover.

In the food forest

In a food forest Tephrosia belongs to the shrub layer, and to the beginning. In syntropic agriculture it fills the role of a short-lived support plant: planted densely between young trees, cut back several times a year, its cuttings laid around the trees it is meant to feed. After a few years, as the trees close over, it loses its light and makes way. That is its intended place in succession: a pioneer that prepares the ground and leaves.

Because cattle and goats will not eat T. candida, it is used in the tropics as a hedge around gardens. The same property means it offers no fodder.

In the Mediterranean climate

Documented experience from Mediterranean Europe is scarce. The climate data sketch the limits. Frost damages it, so a sheltered, frost-free spot matters. The summer drought of five months and more lies outside its natural range, which is shaped by rainy seasons; deep roots help, but resprouting after a summer cut can stall. A sensible rhythm in a Mediterranean garden is to cut in the rainy months, from autumn to spring, and to leave the plants unpruned through summer so that they shade the ground.

Tephrosia is not on the EU list of invasive alien species. T. candida is naturalised in Hawaii, and a weed risk assessment there names viable seed and a persistent seed bank among its warning traits. Cutting before the pods ripen limits self-seeding.

Propagation and care

Tephrosia is sown directly or raised in pots. Seed of T. vogelii stores well; the database recommends letting fresh seed rest for about two months before sowing. For a green manure stand a spacing of 40 by 40 cm with two or three seeds per hole is given, for hedges 1.5 m between rows. Weakly branched plants are tipped to encourage branching. Cut material must stay away from ponds and cisterns, because the leaves are toxic to fish.

How Terra Humana understands it

At Sa Medada Tephrosia stands for the first years of a planting: a plant that does not stay, but whose work remains in the soil. It is meant to supply nitrogen-rich prunings while the trees are still small, and to give way when they no longer need it. That it has become established without inoculation is a modest but instructive observation in itself.