The peanut, or groundnut, is often mistaken for a true nut, but it is actually a legume, belonging to the same family as peas and beans. This distinction is important because the peanut has a unique growth habit known as geocarpy, meaning it produces its fruit underground. Understanding the plant’s appearance above the soil and the unusual mechanism it uses to bury its fruit is central to appreciating how this popular crop grows.
The Above-Ground Plant Appearance
The peanut plant is a low-growing annual herb, typically reaching 1 to 2 feet in height and spreading across the ground. Its stems are sturdy and slightly hairy, supporting distinctive leaves. Each leaf is pinnately compound, divided into two pairs of oval-shaped leaflets. The overall appearance is bushy and green, resembling a small, low sweet pea plant.
The plant begins producing flowers about 25 to 40 days after germination, with peak flowering occurring later in the cycle. These blossoms are small, delicate, and bright golden-yellow, sometimes featuring reddish veins. They are usually self-pollinating and short-lived, appearing close to the base of the plant in the leaf axils. The flowers do not directly turn into the peanut pod; they initiate a transformation process.
The Unique Process of Pegging
After the flower is pollinated, its petals wither and fall away, marking the start of the process known as pegging. From the base of the former flower, the fertilized ovary begins to elongate rapidly, forming a specialized stalk called a peg, or gynophore. This peg is a tube-like structure containing the developing embryo at its tip.
The peg exhibits positive gravitropism, meaning it grows downward, actively bending and pushing toward the soil surface. This structure can grow several inches long, appearing rigid as it probes the ground. Once the tip successfully penetrates the soil, the embryo is carried down to a depth of about 1 to 2 inches. Pod development begins only after the peg tip is surrounded by darkness and moisture in the soil.
Underground Pod Development and Harvest
Once the peg has buried its tip underground, the embryo’s growth resumes, and the pod begins to swell into the familiar peanut fruit. The developing pod requires sufficient calcium directly from the surrounding soil to mature properly; this nutrient is absorbed through the peg itself. The resulting peanut pod has a thin, netted, spongy outer shell that typically contains two or three seeds.
The plant continues to flower and set new pegs throughout the growing season, meaning pods of varying maturity can be found simultaneously underground. As the crop approaches harvest, typically 120 to 150 days after planting, the above-ground appearance begins to change. The leaves often start to yellow and dry back, signaling that energy is shifting away from foliage production and into the subterranean fruit. Harvesting involves lifting the entire plant out of the ground, revealing clusters of mature, shell-encased peanuts clinging to the pegs that extend down from the main stems.