Hieracium, commonly known as the hawkweeds, is a large genus of flowering plants in the daisy family Asteraceae. Its members are perennial herbs ranging from low rosettes only a few centimetres tall to robust stems well over a metre in height, growing from taproots or woody rootstocks. Plants typically produce a single erect stem with a mixture of basal and stem (cauline) leaves whose margins are dentate to deeply cut, and like other members of the chicory tribe they exude milky latex when broken.
The flower heads are the most recognisable feature of the genus. Each head is composed entirely of strap-shaped ligulate florets — there are no disc florets — and is usually a clear yellow, though some species bear white or occasionally orange flowers. The ray corollas are tipped with 3–5 small teeth, and the heads are often clustered tightly at the top of the stem. After flowering, the achenes are dispersed on a pappus of slender bristles, helping the lightweight seeds drift on the wind.
Hieracium occupies temperate and mountainous tropical regions on every continent except Antarctica, with strongholds in the European Alps and other northern temperate uplands, and a substantial native presence in North America. The genus is notorious for its taxonomic complexity: most plants in the genus reproduce by apomixis, setting seed asexually so that each lineage breeds true and even tiny morphological variants persist as distinct clones. As a result more than 9,000–10,000 species and subspecific names have been published, although modern botanists generally recognise roughly 800 species, with many regional treatments choosing to work at the level of broader species aggregates.
The genus has a notable place in the history of biology. After his celebrated work on peas, Gregor Mendel turned to Hieracium to test his ideas about inheritance, but the apomictic seed production of his subjects produced anomalous results that obscured the inheritance patterns he had previously found. Today the closely related mouse-ear hawkweeds — including the familiar Hieracium pilosella and orange hawkweed — are usually placed in the segregate genus Pilosella, leaving Hieracium in the strict sense to the "true" hawkweeds typified by Hieracium murorum, the wall hawkweed.
Although several Hieracium species are pretty enough to occur in gardens as low-maintenance perennials, the genus is far better known for its weedy potential. Many polyploid apomictic species spread aggressively outside their native range, and all Hieracium species are banned under New Zealand's National Pest Plant Accord. Several are listed as noxious weeds in parts of the United States and are declared invasive in Australian alpine regions, where they displace native vegetation and reduce livestock forage in pastures.
Etymology
The name Hieracium comes from the ancient Greek hierax, meaning "hawk." Classical writers used the related term hierakion for the plants, and the association with hawks reflects an old European folk belief that birds of prey ate the milky sap to sharpen their eyesight. The English common name hawkweed preserves the same connection.
Distribution
Hieracium is native across Africa, Asia, Europe, North America, Central America and South America, and is essentially cosmopolitan in temperate and mountainous tropical regions. The European Alps and other northern uplands hold particularly high diversity — Switzerland alone treats 47 species and aggregate taxa in its national flora — and North America has its own native, mainly diploid sexual lineages alongside introduced polyploid apomicts. The genus is conspicuously absent from Antarctica and most lowland tropical environments.
Ecology
Most Hieracium species reproduce by apomixis, producing genetically identical seeds without fertilisation. This breeding system locks small morphological variants into persistent clonal lineages and is the reason so many narrowly defined microspecies have been described. Native North American hawkweeds are by contrast usually diploid and sexual, and occasionally form hybrids with neighbouring species; introduced European species, often polyploid apomictic complexes, lack clear species boundaries and can spread rapidly. The achenes are wind-dispersed on a slender pappus, allowing the plants to colonise open habitats such as alpine meadows, pastures, roadsides, rocky slopes and woodland clearings.
Taxonomy
Hieracium L. was published by Linnaeus in Species Plantarum (1753, p. 799) and sits in family Asteraceae, order Asterales. GBIF currently indexes 12,826 descendant taxa under the genus, and more than 9,000–10,000 species and subspecific names have been published in total — a reflection of historical splitting driven by apomictic microspecies rather than of genuine species diversity. Modern treatments usually recognise about 800 accepted species, and many regional floras work at the level of species aggregates ("aggr.") rather than describing every clonal microspecies. The closely related mouse-ear hawkweeds, including the very common Hieracium pilosella and the orange hawkweed, are now generally placed in the segregate genus Pilosella, leaving Hieracium in the strict sense for the "true" hawkweeds typified by H. murorum.
Conservation
At the genus level Hieracium is more often a conservation problem than a conservation concern. All Hieracium species are banned under New Zealand's National Pest Plant Accord, which prohibits their sale, propagation and distribution. Several are listed as noxious weeds in parts of the United States, and members of the genus are declared invasive in Australian alpine regions, where they outcompete native vegetation and reduce livestock forage in mountain pastures.
History
Hieracium has a small but memorable role in the history of biology. After publishing his now-famous results on inheritance in peas, Gregor Mendel turned to Hieracium to test whether the same patterns held in other plants. Because most hawkweeds reproduce by apomixis — setting seed asexually — Mendel's crosses produced anomalous offspring that did not follow the segregation ratios he had observed in Pisum. The discrepancy was not understood until decades later, and the Hieracium experiments are sometimes cited as one reason Mendel's work on peas was not immediately accepted.
