nut [ nuht ] noun: a dry, indehiscent fruit with a hard shell enclosing typically one seed; the usually edible seed itself
Just like the culinary use of the words fruit and vegetable, what we call a nut in the kitchen or supermarket may be different than what is considered a nut botanically. A nut is a fruit. In botany, a fruit is the ripened ovary and the seeds within. The wall of the ripened ovary is called the pericarp, which has three layers: the outer exocarp, the middle mesocarp, and the inner endocarp. In a nut, the exocarp is always hard. A nut does not dehisce to release the typically one seed (usually edible and sometimes referred to as a nutmeat) inside. Some nuts also have outer protective covers called husks.
Almonds, cashews, chestnuts, hazelnuts, hickory nuts, peanuts, pine nuts, and walnuts are commonly referred to as nuts,* but are they all nuts botanically? Almonds and cashews are actually edible seeds within drupes, which have thin-skinned exocarps, sometimes fleshy mesocarps, and a hard endocarp that contains the seed. Despite their common name, peanuts and pine nuts also are not nuts. Peanuts belong to the Fabaceae (the bean family) and are indehiscent legumes with multiple seeds. Pine nuts are the edible seeds produced by some pine trees and are contained in pine seed cones or strobili, which are not fruits. However, chestnuts and hazelnuts are true nuts, as are acorns—fruit of oak trees—which, although rare in today’s grocery stores, were an important food** for the early indigenous peoples of North America.
Sweet chestnuts (Castanea sativa) have “tassels” at their tips and are encased in prickly husks called burrs. The bases of the pericarps of hazelnuts (Corylus spp.) and acorns (Quercus spp.) are covered by cupules, modified woody involucres that are a whorl of bracts hardened and coalesced into a cup. Within each of these nuts is the edible seed.
Left to right: Castanea sativa developing fruit and dehisced burr displaying nuts; Corylus americana developing nuts hidden in cupules; Corylus avellana ‘Contorta’ nuts with cupules at their base; Quercus alba developing nuts and cupules; and Quercus phellos cupules with and without nuts.
Hickory (Carya spp.) and walnut (Juglans spp.) are both in the Juglandaceae (the walnut family). Those who have collected black walnuts know that fruits are ripe when fingers can leave an impression in the soft green husks. [They also know that broken husks will stain anything they touch.] Some categorize the walnut as an accessory fruit where the husk develops from the involucre and the shell is the pericarp (Zhao et al., 2019). Other sources liken walnut to a drupe with a fleshy outer husk and a hard inner shell (Britannica, 2023). Spjut calls walnut a pseudodrupe and hickory a tryma. The Flora of Virginia describes the fruit of both walnut and hickory as nuts “enclosed in a ripened, fibrous-fleshy or woody involucre and thus drupe-like.” The husk/involucre of walnut is indehiscent, while that of hickory dehisces into four segments.
From top row, left to right: Carya ovata fruit—growing on a tree and fallen and dehisced husks/involucres displaying a nut inside. Juglans nigra fruit—growing on a tree, ripe and decomposing husks/involucres and a nut, and held by an eastern gray squirrel.
Nuts or not, for the gourmand in each of us, it is the taste that counts. Nuts are important hard mast crops. Besides being of economic and nutritional value to humans, nuts are an essential food for wildlife from passerines to game birds to water fowl to a variety of small and large mammals. The fact that they can be stored for months or even years is especially important for animals during the winter season. Nuts are a source of protein, healthy fats, fiber, vitamins and minerals. They also pack more calories in proportion to their size than many forms of animal protein. This is valuable for wildlife especially during times of food scarcity, but for people watching their weight, it is something to keep in mind.
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NOTE: Plants in the wild should not be eaten without consulting an expert or authoritative field guides for information on identification and food preparation. It is easy to confuse plants in the wild, so you should be 100% sure they are edible before consuming them.
*Peanuts and culinary “tree nuts” can cause serious allergic reactions in some people.
**Sometimes plant parts are only edible at a certain time in their life cycle or when prepared in a certain way. For example, raw acorns are not safe to eat because they contain tannins, which can be toxic in large enough amounts. The tannins must be leached from acorns before consumption.
References
Acorns a Superfood? 2015. University of Utah Health.
Britannica, T. Editors of Encyclopaedia (2023, November 11). fruit. Encyclopedia Britannica.
Classification of Fruit Types. Department of Botany. University of Wisconsin-Madison.
Geiger M. 2021. Cashews, Not Really a Nut. Iowa State University and Outreach.
Lizotte E. Updated by Corp S. October 9, 2019. What’s the difference between horse chestnuts and sweet chestnuts? Michigan State University Extension.
Michaels T, Clark M, Hoover E, Irish L, Smith A, Tepe E. 2022. 8.1 Fruit Morphology. The Science of Plants. University of Minnesota Libraries Publishing. EBOOK ISBN: 978-1-946135-87-2
Milliken, William. 2009. Neotropical Juglandaceae. In: Milliken W, Klitgaard B, Baracat A. 2009 onwards. Neotropikey – Interactive key and information resources for flowering plants of the Neotropics.
Nuts. US Forest Service. USDA. (accessed December 4, 2023)
Spjut RW. 2003–2015. A Systematic Treatment of Fruit Types. ©The World Botanical Associates.
Weakley AS, Ludwig JC, Townsend JF. 2012. Caulophyllum. Flora of Virginia. Botanical Research Institute of Texas. page 651.
WebMD Editorial Contributors, Reviewed by Brennan D, MD. December 2022. Are Acorns Safe to Eat? WebMD.
WebMD Editorial Contributors, Reviewed by Ambardekar N, MD, MD. September 2022. Nut Allergy. WebMD.
Zhao S, Niu J, Yun L, Liu K, Wang S, Wen J, Wang H, Zhang Z. (2019). The Relationship among the Structural, Cellular, and Physical Properties of Walnut Shells. HortScience horts, 54(2), 275-281. Retrieved Dec 4, 2023.