Whoever holds a peanut in their hand rarely thinks of a tree. The plant doesn't have one either. Peanuts don't grow on branches but in the soil, hidden beneath the leaves, where you only find them when you dig.
A flower that bends downward
The peanut plant (Arachis hypogaea) grows as a low bush and bears small yellow flowers that last only one day. After pollination the flower wilts, and then something unusual happens: a thin stalk grows from the ovary, bends downward and pushes the young fruit into the ground like a finger. Only there, in the dark, moist soil, does the pod ripen. Botanists call this geocarpy, fruit formation in the earth. The species name gives it away: hypogaea means “under the earth”.
Not a nut at all
Botanically, the peanut is not a nut. It belongs to the legumes, the same family as peas, beans and lentils. The “shell” is a pod, and the kernels inside are seeds, usually two per pod. Its roots also hint at the relationship: nitrogen-fixing bacteria live in nodules on them, taking nitrogen from the air and fertilizing the soil. That is why legumes are considered a good preceding crop for other plants in crop rotation.
Why in the ground?
There is no definitive answer. Botanists suspect that seeds underground are better protected than those hanging openly on a twig: from heat, from drying out and from hungry animals. The plant hides its future instead of displaying it. Its home is South America; from there it traveled with seafarers to Africa, Asia and Europe.
Digging instead of picking
This has consequences for the harvest. The whole plant is lifted out of the ground, the pods hanging from the roots. Afterwards they dry for a few days before being separated from the plant. You pull at a green bush, and suddenly a cluster of pale pods hangs from the roots, still damp and covered in soil.
So the next handful of peanuts has come a long way that began in the dark.