Horticulturalists think peanut plants originated in Brazil or Peru and migrated with Spanish explorers, but regardless of their origin, they remain a high-protein staple for many households. According to the Peanut Institute, based in Albany, Georgia, Americans annually consume about 2.4 billion lbs. of these annual legumes, and half of that total constitutes peanut butter. Peanuts, as opposed to other nuts that grow in trees, are known as ground nuts because they mature under the soil's surface. They are primarily commercially cultivated in eight southern U.S. states and in other tropical and subtropical regions throughout the world.
Peanut seeds are planted in the spring after all danger of frost has passed and when the soil temperature has reached between 65 and 70 degrees Fahrenheit. They are sowed up to 2 inches deep in loose, fertile soil that drains well. Germination and sprouting occurs within 10 to 14 days.
Peanut plants grow up to 18 inches tall and develop an extensive tap root system. Each leaf blade includes four oblong leaflet pairs that employ nyctinastic movement, or movement that responds to light. During darkness and hot, sunny days, the leaves remain closed, and on favorable days, they spread open horizontally.
The flowers start to appear from four to six weeks after the seeds are planted. They are small, bright yellow and pea-like, according to the Iowa State University Extension. They include 10 stamens, most of which do not fully develop, and a pistol containing an ovary, style and stigma. Pollination takes place at dawn within the closed flower. Because the flowers bloom at different times over the growing season, the fruit does not develop simultaneously.
After each flower fertilizes itself, the stalk below the ovary elongates, reaching toward the ground. In about 10 days, the stalk, or peg, penetrates the soil. One week after it taps into the soil, the peg's tip grows larger and pod and seed development begin. The fruit matures within nine to 10 weeks if the weather remains warm and the soil stays moist.
The plants are ready to be harvested when their foliage begins to turn yellow in the late summer and early fall. These warm-season legumes require at least 120 days to reach maturity, with some types requiring up to 200 days. Home gardeners are advised to dig up the plants with a spading fork, shake off the loose soil and cure the plants by hanging them in a warm, dry place for one to two weeks.