Chlorocrepis

''Hieracium'' (),
known by the common name hawkweed and classically as (from ancient Greek ἱέραξ, 'hawk'),
is a genus of flowering plant in the family Asteraceae, and closely related to dandelion (''Taraxacum''), chicory (''Cichorium'') and sow thistle ('' Sonchus''),

which are part of the tribe Cichorieae. Hawkweeds, with their 10,000+ recorded species and subspecies, do their part to make Asteraceae the second largest family of flowering plants.

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 (apomixis or agamospermy), clones or populations that consist of genetically identical plants are formed and some botanists (especially in UK, Scandinavia and Russia) prefer to accept these clones as good species (arguing that it is impossible to know how these clones are interrelated) whereas others (mainly in Central Europe and USA) 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 (runners like those of strawberries) 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 (apomictic) seeds, whereas individual plants of ''Hieracium'' only produce one kind of seeds. Another difference is that all species of ''Pilosella'' have leaves with smooth (entire) margins whereas most species of ''Hieracium'' have distinctly dentate to deeply cut or divided leaves.

Description

Flowers and flower-heads

''Hieracium'' or hawkweeds, like others in the family Asteraceae, mostly have 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 (spatulate) 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 Cichorieae, each ray corolla is tipped by 3 to 5 teeth.

Bracts, stems and leaves

Erect single, 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 setae; " stellate-pubescent" or surfaces with scattered to crowded, dendritically branched (often called, but seldom truly, "stellate") hairs; and " stipitate-glandular" or surfaces with scattered to crowded gland-tipped hairs mostly. Surfaces of stems, leaves, 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.

Ecology

The large yellow underwing moth (''Noctua pronuba'') feeds on ''Hieracium'' species.

Distribution

''Hieracium'' species are native to Africa, Asia, Europe, North America, Central America and South America.

Species

The classification of ''Hieracium'' into species is notoriously difficult. One reason is the apomictic reproduction (in which plants asexually produce seeds), 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.

Europe

*''Hieracium attenboroughianum'' – Attenborough’s hawkweed
*'' Hieracium bakerianum''
*'' Hieracium hethlandiae'' – Cliva Hill hawkweed
*'' Hieracium lepidulum'' Stenstr. ex Dahlst. – tussock hawkweed
*'' Hieracium lucidum'' Guss. – Sicilian sparviere
*'' Hieracium insolitum'' (Zahn) Üksip
*''Hieracium villosum'' Jacq.

North America

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 albertinum'' – houndstongue hawkweed
*'' Hieracium albiflorum'' Hook. – white hawkweed
*''Hieracium × alleghaniense'' Britt. (pro sp.)
*''Hieracium argutum'' Nutt. – southern hawkweed
*'' Hieracium × atramentarium'' (Naegeli & Peter) Zahn ex Engl. (pro sp.)
*'' Hieracium atratum'' Fries – polar hawkweed
*'' Hieracium bolanderi'' Gray – Bolander's hawkweed
*''Hieracium × brachiatum'' Berth. ex DC.
*'' Hieracium canadense'' Michx. – Canada hawkweed
*'' Hieracium carneum'' Greene – Huachuca hawkweed
*'' Hieracium × fassettii'' Lepage
*'' Hieracium fendleri'' Schultz-Bip. – yellow hawkweed
*''Hieracium × fernaldii'' Lepage
*''Hieracium × fuscatrum'' Naegeli & Peter (pro sp.)
*'' Hieracium glomeratum'' Froel. – queen-devil hawkweed
*'' Hieracium gracile'' Hook. – slender hawkweed
*'' Hieracium greenei'' Gray – Greene's hawkweed
*'' Hieracium greenii'' Porter & Britt. – Maryland hawkweed
*'' Hieracium × grohii'' Lepage
*'' Hieracium gronovii'' L. – queendevil
*'' Hieracium horridum'' Fries – prickly hawkweed
*'' Hieracium kalmii'' L. – Kalm's hawkweed
*'' Hieracium lachenalii'' K.C. Gmel. – common hawkweed
*'' Hieracium laevigatum'' Willd. – smooth hawkweed
*'' Hieracium lemmonii'' Gray – Lemmon's hawkweed
*'' Hieracium longiberbe'' T.J. Howell – longbeard hawkweed
*'' Hieracium longipilum'' Torr. – hairy hawkweed
*'' Hieracium maculatum'' Sm. – spotted hawkweed
*'' Hieracium marianum'' Willd. – Maryland hawkweed
*'' Hieracium megacephalum'' Nash – coastal plain hawkweed
*''Hieracium murorum'' L. – wall hawkweed
*''Hieracium paniculatum'' L. – Allegheny hawkweed
*'' Hieracium parryi'' Zahn in H.G.A. Engler – Parry hawkweed
*'' Hieracium piloselloides'' Vill. – tall hawkweed
*'' Hieracium praealtum'' Vill. ex Gochnat – kingdevil
*'' Hieracium pringlei'' Gray – Pringle's hawkweed
*'' Hieracium robinsonii'' (Zahn) Fern. – Robinson's hawkweed
*'' Hieracium rusbyi'' Greene – Rusby's hawkweed
*''Hieracium sabaudum'' L. – New England hawkweed
*''Hieracium scabrum'' Michx. – rough hawkweed
*'' Hieracium schultzii'' Fries – roughstem hawkweed
*'' Hieracium scouleri'' Hook. – Scouler's woollyweed
*'' Hieracium scribneri'' Small – Scribner's hawkweed
*'' Hieracium traillii'' – Maryland hawkweed
*'' Hieracium triste'' Willd. ex Spreng. – woolly hawkweed
*''Hieracium umbellatum'' L. – narrowleaf hawkweed
*'' Hieracium venosum'' L. – rattlesnakeweed

Some species are now placed in the genus ''Pilosella'':
*''Hieracium aurantiacum'' L., syn. of ''Pilosella aurantiaca'' – orange hawkweed
*''Hieracium caespitosum'' Dumort., syn. of '' Pilosella caespitosa'' – meadow hawkweed
*''Hieracium flagellare'' Willd., syn. of '' Pilosella flagellaris'' – whiplash hawkweed
*''Hieracium floribundum'' Wimmer & Grab., syn. of '' Pilosella floribunda'' – kingdevil hawkweed
*''Hieracium lactucella'' Wallr., syn. of '' Pilosella lactucella'' – European hawkweed
*''Hieracium pilosella'' L., syn. of '' Pilosella officinarum'' – mouse-ear hawkweed

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.

References

Further reading

*
* McCosh, D. and Rich, T.C.G. 209. ''Hieracium proximum'' (Caithness Hawkweed) in Ireland. ''Ir. Nat J.'' 30: 54.
* Rich, T.C.G., Cotton, D.C.F., Hood, R.L.I.B., Houston, L., McCosh, J. and Jackson, M.B.W. 2009. Conservation of Ireland's biodiversity: status of the Irish endemic ''Hieracium basalticola'' Pugsley (Basalt Hawkweed) (''Asteraceae''). ''Ir. Nat J.'' 30: 79–89.

External links

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{{Taxonbar, from=Q16526

Asteraceae genera
Taxa named by Carl Linnaeus