Many items commonly referred to as “nuts” in daily conversation are, from a botanical perspective, not true nuts at all. The culinary use of the term “nut” broadly applies to various dry, edible seeds or fruits enclosed in a hard shell. However, the precise scientific classification reveals a more intricate picture, distinguishing between true nuts, which are a specific type of fruit, and other botanical categories like drupes or seeds that share similar characteristics.
Defining What a Nut Is
Botanically, a true nut is a specific type of dry fruit characterized by a single seed and a hard, woody outer layer called a pericarp or shell. A defining feature is that this shell does not naturally split open at maturity to release the seed, a characteristic known as indehiscence. This strict definition contrasts sharply with the broader culinary understanding, where the term “nut” is often applied to any edible kernel encased in a hard shell, regardless of its botanical origin.
Many items we consume and label as nuts are actually seeds from other types of fruits, or they are classified as drupes. Drupes are fruits with a fleshy outer layer surrounding a hard inner shell, or “pit,” which contains the seed. Familiar examples of drupes include peaches and cherries, where the fleshy part is typically eaten and the pit discarded.
True Nuts That Grow on Trees
Several tree-grown foods align with the botanical definition of a true nut. These include hazelnuts, chestnuts, and acorns, each possessing the characteristic hard, indehiscent shell and single seed.
Hazelnuts, also known as filberts, are a prime example of a true nut, growing on trees in the genus Corylus and enclosed in a leafy husk. Chestnuts, belonging to the genus Castanea within the beech family, are another type of true nut, typically found encased in a spiny, burr-like fruit that splits open to reveal the edible nut inside. Acorns, the fruit of oak trees (Quercus species), also fit the botanical criteria for true nuts, recognized by their distinct cup-shaped cap, which is a modified involucre.
Tree-Grown Fruits Often Called Nuts
While many tree-grown items are colloquially called nuts, most are botanically classified as fruits, specifically drupes, or are the seeds of drupes. This includes popular choices like almonds, walnuts, pecans, cashews, and pistachios. The edible part of these items is typically the seed, rather than the entire fruit as defined botanically.
Almonds are the edible seeds of a drupe, with the almond fruit having an outer hull and a hard shell enclosing the seed. Walnuts and pecans are also botanically considered drupes, where the edible “nut” is the seed contained within a hard inner shell. The outer fleshy husk of these drupes is typically removed before the hard shell is cracked to access the seed.
Cashews are another example of a drupe seed, not a true nut. They grow at the base of a “cashew apple,” which is an accessory fruit of the cashew tree. The cashew “nut” is the kidney-shaped seed found within a hard shell beneath this apple. Similarly, pistachios are the seeds of a drupe, where the fleshy fruit surrounds a hard shell containing the green edible seed. Coconuts, often debated in their classification, are also botanically classified as large fibrous drupes; their outer fibrous husk and hard inner shell enclose the edible meat and liquid, which constitute the seed.