Arachis hypogaea

roots & peanuts


Arachis hypogaea

L. 1753

pronounced: uh-RAK-iss hy-poh-JEE-uh

(Fabaceae — the pea family)

subfamily: Faboideae - the bean subfamily


common name: peanut

Arachis comes from the Greek word αρακις (arakis), a legume, and hypogea from 'υπο (hypo), under and γη (gé) the earth – a legume under the ground.

The peanut is a fascinating plant. Native to South America, Mexico and central America, the plant grows up to about 50 cm tall. The leaves are opposite, pinnate with four leaflets (two opposite pairs, with no terminal leaflet). Its yellow flower, which looks like any other pea family flower, is borne above the ground, and, after self-pollination, it withers, the stalk elongates, bends down, and forces the ovary down to about 10 cm underground to complete the development. The pods act in nutrient absorption.

The fruits have wrinkled shells that are constricted between the two to three seeds. The mature seeds resemble other legume seeds, such as beans, but they have paper-thin seed coats, as opposed to the usual hard legume seed coats.

Peanuts grow best in light, sandy loam. They need five months of warm weather, and an annual rainfall of between 500 and 1000 mm, or the equivalent in irrigation water. The pods ripen 120 to 150 days after the seeds are planted. If the crop is harvested too early, the pods will be unripe; if too late, they will snap off at the stalk, and stay in the ground. Harvesting occurs in two stages. A machine cuts off the main root of the plant just below the level of the pods, lifts the whole plant from the ground and shakes off the soil. Then the machine inverts the plant, leaving it upside down on the ground to keep the peanuts out of the dirt. This allows the peanuts to dry to a bit below a third of their original moisture content for two to three weeks exposed in the field. After drying, they are threshed to remove the pods from the rest of the plant. Hay is made from the plant tops.

It is fairly certain than peanuts were domesticated in prehistoric times, at least 7,600 years ago, in Peru. Cultivation had spread to central America by the time of the Spanish conquistadors, who found them for sale in the market-place of what is now Mexico City. The plant was later spread worldwide by European traders. It was brought to North America by slaves from Africa, where it had become popular after being imported there from Brazil by the Portuguese around about 1800. Portuguese traders also introduced it into China in the 1600s, where it soon became an ingredient of many Chinese dishes. Peanuts are also widely used in south-east Asian cuisine, especially in Indonesia. They came there via the Philippines, from Mexico in the times of Spanish colonization.

Soon after a legume begins to grow, special nitrogen-fixing bacteria that live in the soil invade the tiny root hairs and multiply in large numbers. The legume roots react to this infection by forming swellings called nodules on the surface of the root. Bacteria inside the nodules absorb air from the soil and convert the nitrogen from the air into ammonia, the nitrogen form which can be used by the legume and other nearby plants. An enlarged photograph of the nodules on peanut roots is shown.

The plant is a food source for several Lepidoptera larvae, including those of the Cotton Cutworm Spodoptera litura, and the moths Athetis reclusa and Hydrillodes dimissalis.


Photographed in Picnic Bay, 2008, 2012
Page last updated 11th October 2018