The culinary world uses the word “nut” broadly, applying it to nearly any edible kernel encased in a hard shell, which often leads to confusion about what a true nut is and where it grows. This general terminology includes a wide variety of plant products, from seeds and legumes to certain types of fruit. To determine which varieties are true tree nuts, it is necessary to move past the common culinary definition and look closely at the botanical structures of these foods. The diversity of nuts, both true and culinary, represents an important food group that provides healthy fats, protein, and micronutrients to global diets.
Defining the “Tree Nut”
A tree nut, in the most accurate botanical sense, is a specific type of fruit that grows on a tree. A “true nut” is a dry, single-seeded fruit that develops a hard, woody ovary wall (the pericarp) that does not naturally open to release the seed upon maturity (indehiscent). This strict definition is the classification framework used by botanists, which only includes a small number of the items we commonly call nuts.
Many popular edible kernels that grow on trees are not true nuts but are instead classified as seeds or drupes. A drupe is a type of fleshy fruit, such as a peach or cherry, where the fruit wall is divided into three layers, and the seed is contained within a hard, stony pit called the endocarp. In the case of many tree-grown items consumed as nuts, we eat the seed found inside this stony pit, not the fruit itself.
Common Nuts That Grow on Trees
Some of the most familiar tree-grown products fit the botanical definition of a nut. Hazelnuts are considered true nuts because their hard shell is the indehiscent fruit wall surrounding a single seed. Chestnuts are also true nuts, developing inside a spiny outer casing called a burr that splits open at maturity to release the nuts.
Walnuts and pecans, while commonly accepted as tree nuts, are botanically classified as drupes. Walnuts are initially encased in a thick, green, fleshy husk that eventually dries and splits open to reveal the familiar hard, wrinkled shell. Pecans develop in a similar manner, contained within a four-valved outer husk that separates to expose the thin-shelled nut. These examples demonstrate that many consumed tree nuts are dry drupes where the outer fruit layers are removed, leaving the hard pit with the edible seed inside.
Tree Nuts with Unique Growth Structures
Many popular items grown on trees are botanically distinct due to their unique fruit structures. Almonds are not true nuts but are the seeds of a drupe. The tree produces a fruit similar to a small, unripe peach; the outer fleshy layer is discarded, and the hard pit (which we call the shell) is cracked open to retrieve the edible kernel.
Pistachios are also classified as the seed of a drupe. The fruit begins with a reddish-yellow outer hull, and the characteristic shell is the endocarp, or stony pit, that surrounds the seed. Cashews exhibit unusual growth patterns. They develop as a kidney-shaped structure attached to the bottom of a fleshy, pear-shaped accessory fruit called the cashew apple. The cashew itself is the seed of a drupe, and its shell contains caustic oil that must be removed through heat treatment before consumption.
Common “Nuts” That Do Not Grow on Trees
Some of the most frequently consumed “nuts” grow on small plants or underground. The peanut, for example, is not a tree nut but is a legume, placing it in the same botanical family as beans and peas. Peanuts are unique because after the flower is fertilized above ground, the plant stem elongates and pushes the developing pod underground, a process known as geocarpy, where the edible seed matures.
Tiger nuts, sometimes called “earth almonds” or “ground almonds,” are frequently mistaken for true nuts but are actually tiny tubers. These edible tubers, produced by the plant Cyperus esculentus, grow in the soil much like potatoes. The fact that these foods grow below the surface or on small, non-woody plants highlights the vast difference between the common culinary understanding of a nut and its specific botanical definition.