Many people encounter peanuts daily, often wondering about their origin. A common question is whether they grow on trees like other familiar nuts. The truth about how peanuts grow reveals a unique botanical process that sets them apart from typical tree nuts, hinting at a fascinating underground development.
The Truth About Peanut Growth
Contrary to common assumption, peanuts do not grow on trees. Instead, their edible pods develop beneath the soil surface.
The peanut plant begins its life cycle above ground, sprouting from a planted seed and growing into a green, oval-leafed plant typically reaching about 18 inches tall. Around 40 days after planting, the plant produces yellow, pea-like flowers above ground. Once these flowers self-pollinate, their petals fall off. The fertilized ovary then extends into a specialized structure called a “peg.” This peg grows downward, actively pushing into the soil to allow the peanut to mature in its subterranean environment.
Peanut’s Unique Underground Development
The development of peanuts underground is a distinctive botanical phenomenon known as geocarpy. After the peanut flower is pollinated, a structure called a “peg” emerges from the base of the withered flower. This peg, an elongated ovary stalk, grows downwards, exhibiting a strong attraction to gravity. It elongates, sometimes reaching 4 inches or more, and actively pushes itself into the surrounding soil.
Once the tip of this peg penetrates the soil, the peanut embryo begins to develop horizontally. The tip then swells, and the peanut pod starts to form and mature underground.
This subterranean environment is essential for proper development of the peanut, providing darkness, consistent moisture, and specific soil conditions. For instance, the pod directly absorbs calcium from the soil in its immediate vicinity, which is vital for producing well-filled kernels. If the soil lacks adequate calcium in this “podding zone,” the peanuts may not develop properly.
Peanuts: A Legume, Not a Tree Nut
Beyond their unique growth habit, peanuts are commonly misunderstood in their botanical classification. Despite their name and culinary use, peanuts are not true nuts; they are botanically classified as legumes. They belong to the Fabaceae family, which also includes peas, beans, and lentils. A true nut, like a walnut or almond, is a hard-shelled fruit that typically grows on trees and does not split open to release its seed.
Legumes, including peanuts, are characterized by their ability to grow in pods. Another distinguishing feature of legumes is their symbiotic relationship with specific bacteria in their root nodules. These bacteria convert atmospheric nitrogen into a form usable by the plant, a process known as nitrogen fixation. This natural ability enriches the soil, reducing the need for external nitrogen fertilizers and making peanuts a valuable crop in agricultural rotation practices.