The term “nut” is used loosely in everyday language, often leading to confusion when compared to its specific botanical definition. Many items labeled as nuts in grocery stores are scientifically classified as a different type of fruit or simply a seed. The distinction lies in the precise structure and development of the plant’s reproductive part. The majority of the items we call nuts are actually seeds or a type of fruit known as a drupe, rather than a true botanical nut.
Defining a True Botanical Nut
Botanists define a true nut as a specific type of dry fruit that possesses distinct characteristics. The defining feature is that the fruit is indehiscent, meaning it does not naturally split open at maturity to release the seed inside. This contrasts with fruits like beans or peas that open along seams.
A true nut consists of a single seed protected by a hard, woody outer layer called the pericarp, which is the ripened ovary wall of the flower. This hard shell must enclose both the fruit and the seed. A true nut typically develops from a compound ovary and contains only one seed, or sometimes two, with the pericarp hardening completely during the ripening process.
Common Tree Nuts That Are Not Botanically Nuts
Many popular tree-grown products we consume are grouped as nuts but are scientifically something else entirely. Almonds, walnuts, and pecans, for example, are all classified as seeds from drupes. A drupe is a fleshy fruit, like a peach or a cherry, where the seed is encased in a hard, stone-like inner layer.
In the case of almonds, the leathery outer covering, or hull, splits open as the fruit ripens, discharging the hard shell that contains the edible seed. Similarly, the edible portion of a walnut or pecan is the seed found inside a pit. This means the hard “shell” you crack open on a walnut or almond is actually the stone or pit layer of the original fruit.
Other items are simply the seeds of a larger, non-nut fruit or capsule. Brazil nuts, for instance, are seeds tightly packed inside a large, woody capsule. Macadamia nuts are also seeds that are removed from a hard, protective outer fruit capsule.
Cashews are botanically unique as they are seeds that develop outside of the main fruit on the cashew apple, an accessory fruit. The cashew seed is contained within a double shell layer that holds an irritating resin. It must be carefully roasted or steamed to remove the caustic substances before the edible portion is extracted.
The True Tree Nuts
Only a few commonly eaten tree products meet the strict botanical criteria of a true nut. Hazelnuts, also known as filberts, are classic examples of true nuts. The hazelnut features a very hard shell, which is the indehiscent pericarp, and is often partially enclosed by a leafy husk called an involucre.
Chestnuts are another clear example, as they are dry, single-seeded fruits that grow inside a spiny, protective husk. These nuts are distinct from most others because they are high in starch and sugar rather than fat. The acorn, the fruit of the oak tree, is also a true botanical nut, characterized by its hard shell and the cup-shaped structure, or cupule, at its base.
Acorns typically require anywhere from six months to two years to mature, depending on the specific oak species. The tough outer shell and cupule serve to protect the single kernel inside, which contains the embryonic plant and stored food.
Nuts That Don’t Grow on Trees
The peanut, one of the world’s most popular “nuts,” is not a tree nut at all and is not a true botanical nut. It is a legume, belonging to the same family as beans and peas (Fabaceae).
Peanuts grow in pods beneath the soil after the plant’s self-pollinated flowers on the stems push themselves into the earth. This unusual growth pattern is called geocarpy. Like other legumes, peanuts have the ability to host symbiotic nitrogen-fixing bacteria in their root nodules, which helps to improve soil fertility.