Why do peanuts grow underground?
Peanuts grow on a plant with yellow flowers — so why do you have to dig them up? The answer is a flower that plants its own seeds, and it comes with a surprise: a peanut isn't really a nut.
The peanut plant flowers above ground but makes its peanuts below ground. Its yellow flowers pollinate themselves, then each one sends out a “peg” that grows down, away from the plant, and pushes into the soil. The pod with the peanuts inside develops there, underground. Scientists call this geocarpy, and the peanut's species name, hypogaea, means “under the earth.”
Who says so: North Carolina Extension Gardener Plant Toolbox (NC State University) (opens in a new tab)
Here's the surprise: a peanut isn't really a nut. It belongs to the same group of plants as beans and peas. A U.S. Department of Agriculture bulletin from 1911 put it this way: properly speaking, the peanut is a pea, and the word “nut” was added because it tastes like many true nuts. Other names people have used for it include “ground pea” and “groundnut.”
The peg has to make it into the dirt. If the peg does not get into the soil or get covered, no peanuts form. That's why gardeners pile soil around the plants like potatoes: the mounds give the underground pods room to grow. Growing underground has one downside — mice and other animals that live in the ground sometimes eat the peanuts before anyone digs them up.
Who says so: Utah State University Extension (opens in a new tab)
Something else happens underground, too. Peanut roots can form nodules where helpful bacteria (rhizobium) live. The bacteria turn nitrogen from the air into a form the plant can use. So when the seeds are treated with these bacteria before planting, peanut plants add nitrogen to the soil like peas and beans.
Who says so: Utah State University Extension (opens in a new tab)
About a hundred years ago, peanuts got a big job in the South. A 1920 USDA bulletin called peanuts one of the best money crops to grow taking turns with cotton in the fight against the boll weevil.
Who says so: USDA National Agricultural Library (quoting the 1920 USDA bulletin “Peanut Growing for Profit”) (opens in a new tab)
Note: a “theory”, “folklore” or “outlook” is not settled fact. Always check who is saying it.
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