How Do Peanuts Grow Underground?

The peanut plant (Arachis hypogaea) is an annual with edible seeds that grow in hard-shelled seedpods beneath the soil surface. The plant has oval-shaped leaves and grows to a height of about 18 inches. Before setting seed, it blooms with small yellow, orange, white or cream-colored flowers.

The peanut's distinctive fruiting behavior begins after the plant's flowers are fertilized. A flower's developing ovary enlarges into a downward-growing, stemlike structure called a peg. The peg grows toward the soil surface and eventually pushes its tip below the surface. Beneath the surface, the peg's growth changes from a vertical to a horizontal direction, and the embryonic seedpod begins to develop at the tip of the peg. Because the developing pods need to be able to push through the soil as they grow, peanuts do best when the soil has a loose texture and has been well tilled before planting.

Each peanut plant may produce more than 40 pods, and each pod contains one to five edible, nutlike seeds.

Peanuts require a relatively long growing season to reach maturity, and they are frost-tender. So they grow best in warm climates, such as those of U.S. Department of Agriculture plant hardiness zones 7 through 10, where the time between the last frost in spring and the first frost in fall is relatively long. Commonly grown varieties require four to five months, or 120 to 140 days, of frost-free temperatures to reach maturity.

In locations with shorter growing seasons, peanut plants may be started indoors four to five weeks before they're moved to an outdoor garden. Also, some varieties, such as those in the "Virginia" group, mature more quickly than other varieties and may be ready for harvest after 100 to 120 days. Varieties in the "runner" group may take as long as 170 days to mature.

Peanuts usually are harvested after their seedpods have reached maturity and the first frost has killed the plants, typically about 120 to 160 days after planting. At that point, it's time to dig up each entire plant and expose the seedpods to air for drying; the seeds have a moisture content of up to 50 percent and must dry significantly before they're stored.

To dry the seedpods, separate them from the plants, and spread them in a room with good air circulation. An easier alternative is to leave the seedpods on the plants and hang each entire plant in a well-ventilated place for several days.

Once the moisture content of the seeds reduces to about 10 percent, they can be stored for several months.