The classification of the common walnut, or English walnut (Juglans regia), often causes confusion between kitchen terminology and plant science. Many people wonder if this popular snack is a fruit, a nut, or something entirely different. The answer depends on whether one uses everyday language or the precise definitions used in botany. To resolve this ambiguity, one must first establish the scientific criteria for what constitutes a botanical fruit.
The Botanical Definition of a Fruit
Botanically, a fruit is a structure that develops from the matured ovary of a flowering plant, serving to protect the seeds inside. This structure is defined by its wall, the pericarp, which consists of three distinct layers.
The outermost layer is the exocarp, forming the skin or peel. Beneath this is the mesocarp, the middle layer that often becomes the fleshy, edible portion in fruits like peaches or tomatoes. The innermost layer, which directly surrounds the seed, is the endocarp. All fruits originate from a flower’s ovary and contain seeds.
The True Walnut Fruit Structure
The entire structure that grows on the walnut tree is botanically classified as a fruit. It shares characteristics with a drupe—a fleshy fruit with a hard inner stone, like a peach or olive. The structure is composed of a thick, green, outer husk which is often mistakenly called the hull.
This green husk is actually the exocarp and mesocarp, the outer and middle layers of the fruit wall. When the fruit matures, this fleshy husk dries out and splits open, revealing the familiar, hard, wrinkled brown shell underneath. This shell is the innermost layer of the fruit wall, the endocarp, which has become stony and hardened to protect the contents.
Culinary Nut Versus Botanical Seed
Confusion arises because the edible part of the walnut is not the fruit wall, but what is inside the hard endocarp. The wrinkled kernel we consume is the seed of the plant. A botanical seed is the mature ovule of a plant, containing the embryonic plant and a food supply.
The term “nut,” in a strict botanical sense, refers to a specific type of dry, indehiscent fruit—meaning it does not split open to release the seed at maturity. These true nuts, such as an acorn or a hazelnut, possess a woody pericarp. In contrast, the culinary use of the word “nut” is far broader, referring to any large, oily kernel or seed encased in a hard shell used for cooking and snacking.
Other Misclassified Nuts and Seeds
The walnut is not unique in having its identity blurred by common usage, as botanical and culinary naming conventions often differ. For instance, the popular peanut is not a nut but a legume, belonging to the same family as peas and beans because it develops in an underground pod.
Similarly, the almond is the seed found inside the pit of a drupe fruit, related to the peach and cherry. The cashew is also the seed of a drupe, growing attached to a fleshy structure called the cashew apple.