Corylus is a small but widely recognised genus in the birch family (Betulaceae), comprising the hazels and filberts. Members of the genus are deciduous shrubs or small to medium-sized trees, typically multi-stemmed when growing from coppice but capable of reaching 20 m or more in tree forms such as Turkish hazel. The simple, alternate leaves are rounded to broadly ovate with double-serrate margins and softly hairy undersides, casting the dappled, light shade that suits hazel to the understorey of temperate woodlands.
Hazels are among the earliest woody plants to flower each year, often blooming weeks before their leaves emerge. They are monoecious: long, pale-yellow male catkins 5–12 cm in length release pollen on late winter winds, while the female flowers are tiny clustered buds whose only outward sign is a tuft of bright-red styles a few millimetres long. The fertilised flowers ripen by autumn into the genus's defining fruit — a hard-shelled nut 1–2.5 cm long held in a leafy, lobed or tubular husk (the involucre), which gives species such as the beaked hazel their common names.
The genus is native to the temperate Northern Hemisphere. Its range stretches across eastern North America from Alabama to Wyoming, through Europe from the British Isles to Türkiye, and across Asia from Assam to Japan, reaching Primorye and Manchuria. POWO recognises sixteen accepted species, with treatments in other databases putting the figure higher when infraspecific taxa or contested eastern Asian segregates are counted. Hazels are ecologically generous plants: they host more than twenty mutualistic fungi (including Lactarius pyrogalus, which grows almost exclusively under hazel), feed specialist moth larvae, and supply nuts to woodpeckers, jays, squirrels, deer, and the eponymous hazel dormouse, which depends on the crop to fatten for hibernation.
Humans have leaned on the genus for at least as long. Common hazel (C. avellana) and the closely related filbert (C. maxima) anchor the world's hazelnut industry, while American hazel (C. americana) and Turkish hazel (C. colurna) supply temperate landscapes with hardy ornamentals and rootstocks. Beyond the nut, hazel's straight, pliable rods have been coppiced on roughly seven-year cycles for millennia to make wattle, hurdles, basketry, thatching spars, and divining wands — a craft tradition that survives today in British conservation woodlands. Folklore is just as deeply rooted: Celtic legend placed nine hazels around a sacred pool whose nuts fed wisdom-bearing salmon, and Grimms' tales cast hazel branches as protectors against evil and granters of wishes.
Etymology
The genus name Corylus was adopted by Carl Linnaeus in Species Plantarum (1753) from the classical Latin word for the hazel tree, itself thought to descend from the Greek korys ("helmet" or "hood"), in reference to the leafy husk that wraps the developing nut. Linnaeus chose Corylus avellana — the common hazel of Europe, named for the town of Avella in southern Italy — as the type species, anchoring all subsequent additions to the genus around that familiar Mediterranean hazel.
Distribution
Corylus is a temperate Northern Hemisphere genus with a near-continuous range across three continents. Its native distribution covers eastern and central North America from Alabama and Georgia north and west to Wyoming, much of Europe from Great Britain to Türkiye, and a broad swath of Asia from Assam and the Himalayas east through China and Japan, reaching Primorye and Manchuria. Outside this native range POWO records Corylus as introduced only on the Azores and Crete (Kriti). In Switzerland, regional botany authority Info Flora documents three taxa — C. avellana, C. colurna, and C. maxima — illustrating the typical European mix of native common hazel alongside cultivated Turkish hazel and filbert.
Ecology
Hazels function as understorey and woodland-edge plants throughout their range, doing equally well in lowland oak, ash, and birch woodland, in scrub, and along hedgerows. Their early flowering — often weeks before leaf-out — supplies one of the first significant pollen sources of the year for honey bees and early-flying solitary bees, and the autumn nut crop is a keystone food resource for a long list of vertebrates including woodpeckers, nuthatches, tits, wood pigeons, jays, red squirrels, red deer, and the hazel dormouse, which fattens on the nuts for hibernation and depends on hazel-feeding caterpillars in spring. At least twenty-one fungi form mutualistic associations with the genus, including Lactarius pyrogalus, which is essentially restricted to hazel. Hazels are long-lived for their size: trees that reach about 80 years naturally can persist for several centuries when worked as coppice. Specialist insects such as the hazel aphid (Myzocallis coryli) can become economic pests where hazels are grown as a monoculture, as in Oregon's filbert orchards.
Cultivation
Hazels are obliging plants in cultivation. They tolerate most soils, including chalk, but reward moderate-fertility loams with the heaviest nut set, and the principal species sit comfortably in roughly USDA zones 4–8. Corylus avellana (common hazel) and C. maxima (filbert), rated 5/5 for edibility in pomological references, dominate commercial hazelnut production worldwide, with cultivars such as the corkscrew-stemmed 'Contorta', weeping forms, and purple-leaved selections grown widely as ornamentals. Other species — American hazel (C. americana), Turkish hazel (C. colurna), beaked hazel (C. cornuta), and Siberian filbert (C. heterophylla) — are useful where heavier nuts are not the goal: C. colurna makes a single-trunked specimen tree to 20 m, while C. cornuta stays a shrub around 3 m. Hazel responds vigorously to coppicing on a roughly seven-year cycle, producing the straight, pliable rods that have underpinned British rural crafts for centuries and that today supply pea sticks, bean poles, and woodland-conservation material.
Propagation
Hazels are typically propagated either by seed (autumn-sown nuts, stratified through winter) or vegetatively from suckers and layered shoots arising from the multi-stemmed base — the same regenerative habit that makes the genus so amenable to coppicing on roughly seven-year cycles. Named cultivars of C. avellana and C. maxima are usually maintained by layering or grafting onto seedling rootstock so that ornamental traits such as contorted stems or purple foliage breed true.
Conservation
The genus as a whole carries no global conservation listing: no Corylus species appear in the IUCN Global Invasive Species Database, and hazel populations in Britain face no significant pest or pathogen threats, though browsing damage from deer can suppress unprotected coppiced stools. In North American orchards, eastern filbert blight (Anisogramma anomala) is an industry-defining disease pressure on cultivated C. avellana, with breeding programmes drawing on resistant C. americana germplasm — context that frames hazel conservation as more about disease and habitat management than about wild rarity.
Cultural uses
Few European trees carry as dense a layer of cultural meaning as hazel. Its pliable, straight-grained wood has been worked since prehistory into wattle hurdles, withy fences, basketry, thatching spars, net stakes, and the divining rods used by water-finders, while the nuts themselves have been a staple wild food and a commercial crop. Folklore amplifies the practical: Celtic tradition held that nine hazel trees surrounded a sacred pool whose nuts fed wisdom-bearing salmon, in Ireland the species was known as the "Tree of Knowledge", and across Europe hazel rods served as protection against evil spirits and snakes while nuts were carried as charms against rheumatism. Grimms' fairy tales preserve the same belief system, with a hazel branch warding off snakes and the hazel tree at Cinderella's mother's grave granting wishes. American hazel bark also figured in Indigenous and settler-era medicine for hives, fevers, and wounds.
History
The hazels are an ancient lineage: fossil records of Corylus extend back to the Cretaceous, and the genus may have featured in dinosaur diets, with Corylus johnsonii from Ypresian (early Eocene) deposits in Ferry County, Washington, standing as the oldest unambiguously confirmed species. Linnaeus formalised the modern genus in Species Plantarum in 1753, anchoring it to the European common hazel (C. avellana L.) and the American hazel (C. americana Marshall, 1785) was added soon after as Euro-American botany matured. Commercial hazelnut cultivation dominated rural economies in regions such as Britain into the early twentieth century before global trade shifted production toward Türkiye, Italy, and the US Pacific Northwest.
Taxonomy notes
Corylus L. sits in the birch family Betulaceae, order Fagales, with Corylus avellana L. as type species. POWO and Kew accept sixteen species in the genus, while Wikipedia notes a working range of roughly 14–18 depending on how the contested eastern Asian segregates are split, and the GBIF backbone lists 60 descendants once infraspecific names and synonyms are included. The IPNI record (id 13489-1) anchors the name to Linnaeus's Species Plantarum, p. 998 (1753). Together these databases agree on placement and authorship but reflect ongoing disagreement about species-level circumscription, particularly within Asian hazels.
