Two varieties of hazelnuts, sometimes called filberts, are grown in the United States: Corylus Americana in the East and C. cornuta in the Pacific Northwest. Both species belong to the hazel genus Corylus of the birch family (Betulaceae). The chestnut (Castanea dentate) is a member of the beech family (Fagaceae). The smaller hazelnuts or filberts and the larger chestnuts superficially resemble one another in appearance, but they are unrelated, and "chestnut filbert" is a misnomer.
A fibrous husk surrounds the smooth shell of the hazelnut or filbert that is usually about ½ inch in diameter and from ½ to 1 inch long. Botanists call this husk an involucre, meaning a cupule or cup-shaped growth that surrounds a nut. When the nut is mature, it drops from its husk. The common hazel (C. avellana) is a shrub or small tree that usually grows 10 to 25 feet tall but can grow up to 50 feet tall. Edible American chestnuts (Castanea dentate) are slightly larger than filberts. They grow in spiny involucres that look like burs, each of which contains two or three chestnuts. Huge, sprawling chestnut trees grow from 2 to 5 feet in diameter and from 100 to 150 feet tall.
Hazelnuts and Filberts
Much confusion surrounds the correct name of the nut produced by the common hazel tree (Corylus avellana). People commonly call filberts hazelnuts and hazelnuts filberts. The filbert is technically the nut yielded from the C. maxima, a hazel tree that is native from the Balkans to Turkey. The filbert nut is more fully enclosed in the surrounding involucres than nuts yielded by other species of hazel trees. The European filbert (C. avellana pontica) is grown commercially in Turkey. Nearly all hazelnuts produced commercially in the United States are C. cornut grown in Oregon and Washington. A smaller amount of C. Americana hazelnuts are grown in the Northeast.
A fungal blight killed most American chestnut trees (Castanea dentate) that are native to North America. The inedible horse chestnut (Aesculus hippocastanum), native to Europe, is sometimes planted as an ornamental. The most common chestnut in the United States is the inedible Ohio buckeye (Aesculus glabra). Most chestnuts sold commercially come from Asian species of chestnut trees.
C. avellana grandis is a variety of hazelnut commonly called the cob nut in the United Kingdom, where it is widely grown. The cob nut is larger and rounder than hazelnut varieties grown in the U.S. and could conceivably be called a chestnut filbert owing to the superficial resemblance between chestnuts and European filberts.