The peanut plant, Arachis hypogaea, appears as a typical leafy annual plant above the ground. It is botanically classified as a legume within the Fabaceae family, grouping it with beans and peas. Unlike its relatives, the peanut has evolved a unique reproductive strategy known as geocarpy, meaning its fruit develops entirely below the soil surface. This subterranean fruit development is the most distinguishing feature of the plant, setting it apart from nearly all other cultivated crops.
The Visible Canopy and Leaf Structure
Above the ground, the peanut plant forms a low-growing, herbaceous canopy that typically reaches a height of 30 to 50 centimeters (1 to 1.5 feet). Its overall shape varies based on the cultivated variety, falling into two main growth habits. Upright, or “bunch,” types have a more erect structure, while “runner” varieties spread outward with long, trailing branches close to the soil line.
The leaves are compound and pinnate, differentiating them from the simple leaves of many garden plants. Each leaf is composed of four oval-shaped leaflets, a configuration known as tetrafoliolate, attached to a central stalk in two opposite pairs.
The plant exhibits a circadian rhythm called nyctinasty, where the leaflets fold together in the evening. This closing motion, often described as “sleeping,” helps the plant conserve moisture. The dense, green foliage provides effective ground cover, but gives no visual indication of the fruit production occurring beneath the soil.
The Unique Flowering and Pegging Process
The first sign of the reproductive cycle is the appearance of small, showy flowers above the ground, usually 30 to 40 days after the seedling emerges. These blooms are a bright golden-yellow color and possess the pea-like structure characteristic of the legume family. They emerge in the axils of the leaves near the base of the plant.
Most peanut flowers are self-pollinating, often occurring before the flower fully opens. Once fertilization is complete, the petals quickly wither, and the plant begins the specialized process known as pegging.
A stalk-like structure called a gynophore, or “peg,” grows downward from the base of the fertilized ovary. The peg is thin, stiff, and sharp-pointed, elongating rapidly toward the soil surface. Within one to two weeks, this structure pushes forcefully into the ground, carrying the developing embryo at its tip. This penetration into the soil is required for the fruit to mature; if the peg fails to enter the soil, the fruit will not develop.
The Appearance of the Underground Fruit
Fruit development begins only after the peg has successfully penetrated the soil. Once the tip of the peg is situated a few centimeters underground, it turns horizontal, and the ovary begins to swell and mature into the familiar peanut pod. The mature pods are typically oblong, measuring between 25 and 50 millimeters (one to two inches) long.
The shell of the peanut pod has a characteristic thin, netted, and somewhat spongy texture. Inside, the pod usually contains one to four seeds (kernels), which are often constricted between the seeds, giving the shell a wrinkled appearance. The mature pod’s inner lining changes from white to darker shades of orange or brown as the fruit reaches full maturity.
As a member of the legume family, the peanut plant possesses a robust root system featuring a prominent taproot. Along the lateral roots, small nodules are visible, which are home to symbiotic bacteria. These bacteria take nitrogen gas from the atmosphere and convert it into a usable form, a process that enriches the soil and confirms the plant’s classification as a nitrogen-fixing crop.