Peanuts, also known as groundnuts, are often mistakenly believed to grow on trees like walnuts or almonds. The common belief that peanuts sprout from trees like walnuts or almonds is a widespread misunderstanding based on their name. However, they are unique in the agricultural world because they do not grow above ground. Their reproductive process involves a downward journey into the soil where the edible part matures in darkness. This unusual method of development sets them apart from true nuts and highlights the difference between culinary and botanical classifications.
Peanuts Are Legumes Not Tree Nuts
Peanuts, scientifically named Arachis hypogaea, are legumes, not nuts in the botanical sense. This classification places them in the Fabaceae family, making them close relatives of peas, beans, lentils, and soybeans. True nuts, like pecans or hazelnuts, are hard-shelled fruits that grow on trees.
The confusion arises because peanuts share a similar high-fat profile and are used in cooking much like tree nuts. Unlike tree nuts, the peanut plant is an annual herbaceous plant, meaning it completes its life cycle within one growing season. Like other legumes, it forms a symbiotic relationship with nitrogen-fixing bacteria in its root nodules, a trait absent in true tree nuts.
The Phenomenon of Subterranean Fruiting (The Pegging Process)
The unique growth method of the peanut is called geocarpy, which involves the development of fruit beneath the soil. The plant is a low-growing annual shrub, typically reaching 12 to 20 inches tall. After the plant produces self-pollinating yellow flowers above ground, fertilization occurs.
Following the wilting of the petals, the fertilized ovary elongates rapidly into a specialized structure known as a “peg” or gynophore. This peg is a tube-like stalk that exhibits positive gravitropism, meaning it is programmed to grow downward toward the force of gravity. The peg elongates, pushing through the air and then penetrating the soil to a depth of one to three inches.
The tip of the peg, which contains the arrested embryo, is pointed and hardened to facilitate burrowing. Once underground, the peg stalk stops growing, and the embryo resumes development. The tip then swells to form the mature peanut pod, a shell containing one to four seeds. If the peg fails to penetrate the soil, often due to hard or dry conditions, the developing embryo aborts.
Bringing the Peanut to the Surface
The time for harvest is signaled when the leaves of the plant begin to yellow and the internal color of the pods indicates maturity. Because the crop is underground, harvesting requires a specialized two-stage mechanical process.
Stage 1: Digging and Inverting
The first stage involves a machine called a digger-shaker-inverter. This equipment uses a horizontal blade to cut the taproot just below the pods, lifting the entire plant from the soil. The machine gently shakes the vines to remove excess soil before flipping the plant upside down. The inverted plants are deposited in long rows called windrows, with the pods exposed to the air.
Stage 2: Curing and Combining
The purpose of inverting the plants is to allow the peanuts to “cure” or dry naturally in the field for two to four days. This step reduces the moisture content of the pods from around 35-50% to an optimal range of 18-24%. Once the moisture content is lowered, a peanut combine follows. The combine threshes the windrowed plants, separating the dried pods from the vines and collecting the harvested peanuts.