The peanut, scientifically named Arachis hypogaea, is often misunderstood as a nut that grows on a tree. This plant is actually an annual herbaceous species that hugs the ground, not a woody perennial. It is a member of the legume family, which includes peas and beans. The truly unique aspect of the peanut plant is its reproductive strategy, where the edible part develops beneath the soil in a process that sets it apart from nearly all other food crops.
The True Appearance of the Peanut Plant
The mature peanut plant presents as a low-growing, bushy annual that rarely exceeds 12 to 18 inches in height. Depending on the variety, the plant may have an upright, “bunch” growth habit or a more sprawling, “runner” habit that spreads across the field. Its foliage is a rich green color, with leaves that are pinnately compound, with multiple leaflets attached to a central stalk. Each peanut leaf consists of four distinct leaflets, which help the plant efficiently capture sunlight for growth.
The flowers of the peanut plant are small and yellow, resembling those of a sweet pea. These flowers appear along the lower portion of the stems, starting about 40 days after the plant emerges from the soil. The flowers are self-pollinating. Once fertilized, the flower will wither and begin the transition that leads to the underground fruit.
How Peanut Fruits Develop Underground
The development of the peanut fruit is a biological process known as geocarpy, which translates to “earth-fruit.” After the aerial flower is fertilized, the ovary at the base of the flower begins to elongate, forming a specialized stalk called a gynophore, or peg. This peg exhibits positive gravitropism, meaning it is genetically programmed to grow downward toward the earth.
The slender peg pushes away from the main stem, elongating until its sharp tip penetrates the soil surface. This penetration is essential because the actual peanut pod will not develop if the peg fails to reach the soil. Once beneath the ground, the tip of the peg swells, and the fertilized ovules within it begin to mature into the familiar peanut pod.
Botanical Identity and Growth Timeline
The peanut is a member of the Fabaceae family. Like other legumes, the peanut plant forms a symbiotic relationship with nitrogen-fixing bacteria in nodules on its roots, enriching the soil. This nitrogen-fixing ability makes it a valuable crop in rotation systems, improving fertility for subsequent plantings.
The growth cycle of the peanut plant requires a warm climate. Planting occurs after the last spring frost when soil temperatures are reliably warm, typically in the range of 65° to 70°F. The continuous process of pegging and pod development continues for much of the growing season.
Harvesting occurs between 120 and 150 days after planting, once the leaves begin to yellow and the interior of the shells shows signs of maturity. The entire plant is lifted from the soil by a machine, exposing the clusters of peanut pods still attached to the roots. After being dug, the plants are inverted and left in the field for several days to “cure,” a drying process that reduces the moisture content and concentrates the flavor before the pods are separated from the rest of the plant.