Hieracium, known by the common name hawkweed and classically as hierakion, is a genus of the sunflower, and closely related to dandelion, chicory, prickly lettuce and sow thistle, which are part of the tribeCichorieae. Hawkweeds, with their 10,000+ recorded species and subspecies, do their part to make Asteraceae the second largest family of flowers. Some botanists group all these species or subspecies into approximately 800 accepted species, while others prefer to accept several thousand species. Since most hawkweeds reproduce exclusively asexually by means of seeds that are genetically identical to their mother plant, clones or populations that consist of genetically identical plants are formed and some botanists prefer to accept these clones as good species whereas others try to group them into a few hundred more broadly defined species. What is here treated as the single genus Hieracium is now treated by most European experts as two different genera, Hieracium and Pilosella, with species such as Hieracium pilosella, Hieracium floribundum and Hieracium aurantiacum referred to the latter genus. Many members of the genus Pilosella reproduce both by stolons and by seeds, whereas true Hieracium species reproduce only by seeds. In Pilosella, many individual plants are capable of forming both normal sexual and asexual seeds, whereas individual plants of Hieracium only produce one kind of seeds. Another difference is that all species of Pilosella have leaves with smooth margins whereas most species of Hieracium have distinctly dentate to deeply cut or divided leaves.
Hieracium or hawkweeds, like others in the family Asteraceae, have mostly yellow, tightly packed flower-heads of numerous small flowers but, unlike daisies and sunflowers in the same family, they have not two kinds of florets but only strap-shaped florets, each one of which is a complete flower in itself, not lacking stamens, and joined to the stem by leafy bracts. As in other members of the tribe :wikispecies:Cichorieae|Cichorieae, each ray :wiktionary:corolla#Noun|corolla is tipped by 3 to 5 teeth.
Erect single, :wiktionary:glabrous|glabrous or hairy stems, sometimes branched away from the point of attachment, sometimes branched throughout. The hairiness of hawkweeds can be very complex: from surfaces with scattered to crowded, tapered, whiplike, straight or curly, smooth to :wiktionary:seta#Noun|setae; "stellate-pubescent" or surfaces with scattered to crowded, branched hairs; and ":wiktionary:stipes|stipitate-:wiktionary:gland#Noun|glandular" or surfaces with scattered to crowded gland-tipped hairs mostly. Surfaces of stems, leaves, :wiktionary:peduncle#Noun|peduncles, and phyllaries may be glabrous or may bear one, two, or all three of the types of hairs mentioned above. Like the other members of the Chicory tribe, hawkweeds contain a milky latex.
The classification of Hieracium into species is notoriously difficult. One reason is the apomictic reproduction, which tends to produce a lot of minor geographical variation. Over 9000 species names have been published in Hieracium but some botanists regard many of those as synonyms of larger species.
The list below is a selection of species that have been accepted by the USDA Natural Resources Conservation Service and Canada. A more complete list is given in the list of Hieracium species.
Hieracium triste Willd. ex Spreng. – woolly hawkweed
Hieracium umbellatum L. – narrowleaf hawkweed
Hieracium venosum L. – rattlesnakeweed
Plant pest
All species of the genus Hieracium are classed as invasive species throughout New Zealand. They are banned from sale, propagation and distribution under the National Pest Plant Accord. Hieracium is a pasture weed that reduces available feed for livestock and displaces the indigenous plants. It is a particular threat in alpine ecosystems previously dominated by native tussocks, though it will colonise habitats from bare ground, to exotic pine forest, to native Southern Beech forest. In the United States, many species of Hieracium have been introduced and all species present are considered noxious weeds in one or more states. In Australia, hawkweeds are invasive pests in alpine regions, all species of Hieracium are listed or declared under various State Acts.
