The word “nut” is a common term for any hard-shelled, edible seed or fruit, but its culinary meaning is much broader than its scientific one. This loose application causes confusion about where these popular foods originate. While many nuts grow on trees, others develop in entirely different ways, such as underground. Understanding the growth process for each type reveals why their classifications are so varied.
Nuts That Develop on Trees
The majority of commercially consumed nuts grow on trees. Walnuts and pecans, for example, grow on large, deciduous trees inside a thick, protective outer casing. The hard shell is actually a stone, or pit, surrounded by a fleshy green husk. This husk dries out and splits open when the fruit is mature, allowing the inner shell to be collected.
Almonds also develop on trees, but they are the seed inside a fuzzy fruit called a drupe, which resembles a small peach. The outer hull splits open to reveal the almond shell, which is cracked to access the seed. Hazelnuts, or filberts, grow in clusters on a shrub or small tree and are encased in a leafy husk that separates easily when ripe.
Cashews have an unusual tree-borne origin, growing on tropical evergreen trees native to Brazil. The cashew develops inside a hard shell at the bottom of a swollen, pear-shaped structure called a cashew apple. The cashew apple is an edible fruit, and the single cashew shell hangs beneath it.
Nuts That Develop Underground
The peanut is the most prominent example of a common “nut” that develops its fruit beneath the soil. The peanut plant is an annual herbaceous plant that grows a small, leafy bush above ground. Its life cycle includes a unique reproductive phase known as geocarpy, or “pegging.”
After the yellow flowers are pollinated above ground, a structure called a peg forms from the fertilized ovary. This peg is a stalk that elongates and curves downward until it pushes into the soil. Once beneath the surface, the tip of the peg swells and matures into the familiar peanut pod. The soil provides the dark, moist environment necessary for the fruit to fully develop.
The Botanical Definition of a Nut
The scientific classification of nuts clarifies why their growing environments differ. Botanically, a “true nut” is defined as a dry fruit with a single seed that does not open at maturity to release that seed. This means the hard shell is the entire fruit wall. Examples that fit this strict definition include chestnuts and hazelnuts.
Many popular culinary nuts are actually seeds or other types of fruit. Almonds, walnuts, and pecans are classified as seeds of drupes, which are fruits characterized by a fleshy exterior surrounding a single shell, or stone, with a seed inside. We consume the seed from these drupes, while with fruits like cherries or peaches, we eat the fleshy exterior and discard the stone.
The peanut is not a nut at all, but a legume, which is the same botanical family as beans and peas. Legumes are seeds that grow inside a pod. The peanut’s underground pod is a clear indicator of its identity as a member of this family. This difference in classification is important for understanding the plant’s structure and its unique process of pushing the fruit into the earth to mature.