Bambara Groundnut

What is Bambara groundnut?

Bambara groundnut hides its harvest. After flowering, the short flower stalks bend towards the ground and press the young pods into the soil, where they ripen like peanuts. Above ground there remains a low, leafy tuft, below it the round pods, each usually with one or two hard seeds. The plant is a legume, Vigna subterranea, a close relative of the cowpea.

In Africa it is one of the old crops of smallholders, grown mostly from local landraces. Its reputation rests on a simple quality: it still yields on poor, sandy soil and in dry years when groundnut fails. For a garden that wants to build its soil it has a second quality. Like most legumes it fixes nitrogen and needs no nitrogen fertiliser. Among the nitrogen-fixing plants it is one of the few that also feed people directly.

Origin and growth

Bambara groundnut comes from West Africa; its origin is placed in the region of today's Nigeria, Cameroon and the Central African Republic. The wild form forms long runners and smaller seeds; the cultivated form is compact, with more erect leaf stalks and larger seeds, 11 to 15 millimetres long. The leaves have three leaflets on long stalks rising from a short stem. The plant stays low and covers the ground around itself.

Requirements

It is a warm-season crop. Frost kills it, so in a Mediterranean climate it is sown in late spring and harvested in autumn. It tolerates heat well, but not without limit: in Ghana temperatures above 38 °C with dry air reduced pod yield, apparently because pollen tubes dried out. It is considered drought-tolerant, but drought also reduces leaf area, pod number and yield, to a different degree in each landrace. Light, sandy, well-drained soils suit it best, since the pods ripen in the ground; heavy, crusting clay makes pod formation and harvest difficult.

The decisive factor in southern Europe is day length. In many landraces long days of more than twelve hours delay or reduce pod set, while vegetative growth increases. In trials by a Ghanaian researcher, plants formed significantly more pods at 12 hours of light than at 14.

Role in the soil: nitrogen, biomass and mulch

Bambara groundnut forms root nodules with Bradyrhizobium. How much nitrogen it fixes varies widely, from about 4 to 200 kilograms per hectare according to a 2023 study in Frontiers in Microbiology. In the trials of that study in Nigeria the best landraces reached around 62 to 67 kilograms after inoculation with strains of Bradyrhizobium japonicum, the group from which soybean inoculants are made. More on the partners is in the article on rhizobia.

Its biomass is modest. Most of the nitrogen ends up in the seeds that are harvested; what remains are the leaves and roots. They are best left on the bed as a thin mulch. Bambara groundnut is therefore no biomass plant in the strict sense. Its strength is that it covers the ground in summer and supplies food without drawing nitrogen from the soil.

In the food forest

In a food forest Bambara groundnut belongs to the ground layer of the early years, when the trees are small and there is still a lot of light between them. In syntropic agriculture such short-cycle crops make up the first stage of a planting: sown together with the trees, harvested after a season, their residues left on the ground. As the canopy closes they move to the remaining sunny gaps. As a low living ground cover it fits alongside taller summer crops in a polyculture.

In the Mediterranean climate

There is hardly any documented field experience from southern Europe; research has concentrated on Africa and Southeast Asia. What is known points in two directions. The Mediterranean summer suits it in heat and drought tolerance, and a frost-free season from late spring to autumn is long enough. Day length is the open point: in June the days at Sardinian latitude last far longer than twelve hours, so pods are likely to set mainly from late summer, when the days shorten. Sowing time and choice of landrace therefore decide more than watering.

Bambara groundnut is not on the EU list of invasive species. As an annual whose pods stay in the ground, it hardly spreads on its own.

Propagation and care

It is sown directly from seed once the soil is warm, a few centimetres deep. The seeds come from the previous harvest; landraces differ strongly, so it is worth saving seed from the plants that did best. Earthing up the plants, as is common with peanuts, is also practised. The harvest comes when the leaves turn yellow; the plants are lifted whole. The hard seeds are cooked before eating.

How Terra Humana understands it

At Sa Medada Bambara groundnut stands for the ground layer of the first years: a plant that feeds people and leaves the soil richer, not poorer. It also embodies an idea that runs through the whole place: that crops from dry regions of the world can find a home in a Mediterranean that is becoming drier. Whether it sets pods in the Sardinian summer is something the land itself will answer.