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Understanding Hawkweeds
Hieracium and Pilosella (Asteraceae, Compositae)

By Arieh Tal, 2016

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Hawkweeds (Hieracium and Pilosella) comprise a large group of related species that occur mostly in temperate or mountainous regions of the northern hemisphere.  The majority are found in Europe and adjacent territories.  Hundreds of species of hawkweeds and thousands of varieties and hybrids have been described by botanists.  A much smaller number of species occur in North America, some of which were introduced from Europe, beginning in the late 19th century.  Many are considered weedy and invasive, especially the non-native species.

General Description

Hawkweeds can be described as herbaceous perennials with milky sap, ligulate flower heads, typically yellow florets, heads arranged either in small, congested, terminal clusters or in larger, longer-branched, "open" inflorescences.  (A few species have red-orange or white florets.)

Hawkweed fruits (cypselae) are more/less round in cross-section, longitudinally 10+ ribbed and either cylinder-shaped (columnar), with a truncated summit (like a bullet), or spindle-shaped and narrowed toward the summit.  The pappus of hair-like, brittle, minutely-barbed bristles is in 1 or 2+ series.

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Fig. 1.  Fruits (cypselae) of Hieracium umbellatum (left) and Pilosella piloselloides (right).  Scale in millimeters.
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Primary leaves are either alternate on stems or mostly basal, unlobed or with shallow lobes, and not usually with many prominent teeth.

Stems, branches, peduncles and involucral bracts are covered with a more or less dense variety of hairs, including minute, branched (i.e., stellate) hairs; as well as unbranched hairs, which can be either short and gland-tipped, or  broad-based, long, spreading and somewhat stiff, or just long and spreading but softer.

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Fig. 2.  Section of stem of Pilosella officinarum (Asteraceae) showing covering of hairs:  branched (stellate), gland-tipped (stalked glands), and broad-based, long, spreading, stiff hairs.

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Some hawkweed species are colony-forming by stolonsApomixis and polyploidy are common, and hybridization is frequent in some species.

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Figure 3.  (A) many-branched inflorescence and leafy-stemmed habit of H. scabrum; (B) open inflorescence and leafy-stemmed habit of H. paniculatum; (C) congested-terminal inflorescence and scape-like stems of Pilosella aurantiaca; (D) congested to open, branched, terminal inflorescence and scape-like stems of Pilosella piloselloides.

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Similar Genera

Related Composite family genera with milky sap and yellow-ligulate flower heads of approximately the same size include:  Taraxacum/Leontodon/Scorzoneroides (dandelions), Crepis (hawksbeard), Sonchus (sow thistle), Hypochaeris (cat's ear), and Picris (oxtongue).

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Approximately 36 species of hawkweeds occur in North America Strother (2006).  Roughly half of the species are found east of the Mississippi River, and most of the rest are distributed across mountain and pacific coastal states, as well as in western Canadian provinces.

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Hawkweed Species of New York and New England

* Scientific Name. English Name Synonyms
Hieracium gronovii beaked hawkweed
* Hieracium lachenalii common hawkweed Hieracium vulgatum
* Hieracium murorum wall hawkweed
Hieracium paniculatum panicled hawkweed
Hieracium robinsonii Robinson's hawkweed Hieracium smolandicum
* Hieracium sabaudum Savoy hawkweed
Hieracium scabrum rough hawkweed
Hieracium umbellatum northern hawkweed Hieracium canadense
Hieracium venosum rattlesnake weed
* Pilosella aurantiaca orange h., devil's paintbrush Hieracium aurantiacum
* Pilosella caespitosa yellow king devil Hieracium pratense
* Pilosella flagellaris large mouse-ear or whiplash h. Hieracium flagellare
* Pilosella officinarum mouse-ear hawkweed Hieracium pilosella
* Pilosella piloselloides glaucous hawkweed Hieracium piloselloides
Pilosella ◊floribundum hybrid hawkweed Hieracium ◊floribundum
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* Not native

[ Notes:  There is an 1893 collection of Pilosella lactucella from Oswego County, NY at BH-Cornell.  It is uncertain whether the plants are still present there.  Hieracium kalmii is considered by some to be a species (rather than a synonym of H. umbellatum).  It is not clear whether this taxon is found in NY.  ]

Hieracium or Pilosella?
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Botanists have not been in agreement regarding the taxonomic placement of the species referred to here as Pilosella.  Following Fernald (1950), North American botanists have tended to treat Pilosella species as a subgenus of a more broadly defined Hieracium (i.e., subgenus pilosella).  Some European botanists have argued for a broad circumscription of Hieracium, which would include the Pilosella species, whereas others have tended to place these taxa into two separate genera.

Based mainly on morphology, Sell (1987), Bremer (1994), Lack (2007), and others, have made a strong case for the two genera concept.  Bršutigam and Greuter (2007) also argued for the two genera concept, based on both morphology and on a number of cited molecular studies.

In a molecular phylogenetic study of the relationships among North American species, Gaskin and Wilson (2007) found support for retaining Pilosella species at subgeneric rank.  However, their results are based solely on North American taxa, which represent but a small subset of all Hieracium species known world wide, and hence their findings are limited.

Most empirical research on hawkweeds during the past 15 years has been conducted in Europe.  Significantly, Kilian, et al. (2009) summarized the results of a recent molecular phylogenetic analysis of a large dataset (428 taxa of 83 genera of the tribe Cichorieae), including North American taxa.  The authors found strong support for recognition of Pilosella as a separate genus.  They also found good support for a subclade consisting of North American Hieracium taxa.

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Hieracium and Pilosella can be differentiated as follows:

>> A majority of Pilosella species regularly produce stolons, whereas Hieracium species (i.e., excluding Pilosella) do not produce stolons.

>> Pilosella species retain a basal rosette of leaves during the flowering period, while most Hieracium species lose basal leaves by flowering.  (H. murorum, H. lachenalii, H. robinsonii and H. venosum retain basal rosettes.)

>> Stems of Pilosella species are soft and scapose, but Hieracium species tend to have stems that are firm, and bear from several to many, well-developed stem leaves.  (H. murorum often has only 1 or 2 well-developed lower stem leaves.)

>> The fruits (cypselae) of Pilosella species vary from 1.0-2.5 mm in length, compared with the fruits of Hieracium species, which vary from 2.5-5.0+ mm in length.  Though fruits of both genera have 10 ribs, the ribs of Pilosella species extend very slightly above the flat summit of the cypsela, while the ribs of Hieracium species do not extend above the flat summit of the cypsela, and in some species also connect together to form an obscure ring at the summit of the cypsela.

>> The cypselae of Pilosella species bear 1 series of pappus bristles, but the cypselae of Hieracium species typically bear 2 or more series of pappus bristles.

>> Pilosella species tend to reproduce both sexually and by apomixis, forming hybrids readily, while Hieracium species usually reproduce only by apomixis, forming hybrids less often.  (Hand et al., 2015; Kilian et al., 2009)

>> Pilosella species tend to flower in May and June (sometimes with a second flowering period in autumn), while Hieracium species tend to bloom in July and August.

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Many attempts have been made to distinguish, based on morphology, between Eurasian hawkweeds and indigenous North American species (i.e., Hieracium sensu stricto).  Other than the division of Hieracium into subgenera by Fernald (1950), no other convincing taxonomic treatment based on morphology has come to my attention.  Fernald's division of the genus Hieracium into 3 subgenera (Pilosella, Archieracium & Stenotheca) does not appear to be recognized by most researchers today, however.  Haines (2011) briefly noted that "The morphological distinction between Hieracium and Stenotheca is relatively weak and does not seem to justify generic separation."  My morphological examination (unpublished) of North American Hieracium species using Fernald's subgeneric definitions and Strother's (2006) measurements of various relevant morphological characters did not reveal significant differences between Archieracium (i.e., Eurasian species) and Stenotheca (New World species).

Though the distinctions between Pilosella and Hieracium outlined above seem fairly strong, they are not absolute.  Some character state gradations across the entire set of hawkweed species are nevertheless observable, especially with regard to the presence (or absence) of basal leaves at flowering, or the number and size of stem leaves.  For example, Hieracium venosum and H. murorum tend to retain basal leaves at flowering and have scapose stems with very few leaves, character states shared with Pilosella species.  Stem leaves that do occur for those two species tend to be much smaller than their basal leaves, or are scale-like.  Perhaps the two genera should best be differentiated by their fruits and their differing reproductive modes.  Hybrids between Pilosella and Hieracium are very rare.

For now, we will need to wait for additional molecular phylogenetic research to unravel the mysteries of these complex species.

nb:  The type of the genus Hieracium is H. murorum L.

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Selected References

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Bršutigam, S. and W. Greuter.  2007.  A new treatment of Pilosella for the Euro-Mediterranean flora.   Willdenowia 37: 123-137.
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Bremer, K. 1994.  Asteraceae: Cladistics & Classification. Timber Press, Portland.
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Fehrer, J. and J. Chrtek.  2011.  Evolution of the American Hieracium subgenus Chionoracium.  12th International Hieracium Workshop, June 19-23, SpŚnhult, Sweden.
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Fernald, M. L.  1950.  Gray's Manual of Botany, 8th (Centennial) Edition.  American Book Company, New York.
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Garland, M. A.  1990.  Infrageneric Names Applicable to Hieracium subgenus Chionoracium (Compositae:Lactuceae).  Taxon, 39: 119-124.
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Gaskin, J. F. and L. M. Wilson.  2007.  Phylogenetic Relationships among Native and Naturalized Hieracium (Asteraceae) in Canada and the United States Based on Plastid DNA Sequences.  Systematic Botany 32:278-485.
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Gleason, H. A. and A. Cronquist.  1991.  Manual of Vascular Plants of Northeastern United States and Adjacent Canada, 2nd ed.  The New York Botanical Garden, Bronx, NY.
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Haines, A.  2011.  The New England Wild Flower Society's Flora Novae-Angliae: A Manual for the Identification of Native and Naturalized Higher Vascular Plants of New England.  Yale University Press, New Haven, CT.
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Hand, M. L., P. VŪt, A. KrahulcovŠ, S. D. Johnson, K. Oelkers, H. Siddons, J. Chrtek, J. Fehrer and A. M. Koltunow.  2015.  Evolution of apomixis loci in Pilosella and Hieracium (Asteraceae) inferred from the conservation of apomixis-linked markers in natural and experimental populations.  Heredity 114:17-26.
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Kilian, N., B. Gemeinholzer and H. W. Lack.  2009.  Cichorieae.  pp. 343-383  in:  Funk, V. A., A. Susanna, T. F. Stuessy and R. J. Bayer (eds.),  Systematics, Evolution, and Biogeography of Compositae.  International Association for Plant Taxonomy.  Vienna, Austria.
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Lack, H. W.  2007.  Cichorieae.  pp. 180-199  in:  Kadereit, J. W. and C. Jeffrey (eds.), The Families and Genera of Vascular Plants, vol. 8,  Flowering Plants.  Eudicots.  Asterales.  Springer, Berlin.
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Sell, P. D.  1987.  An introduction to the study of the British Hieracia, 1. History and classification.  Watsonia, 16: 365-371.
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Sell, P. D. and C. West.  1974.  Hieracium.  Notes from the Royal Botanic Garden Edinburgh, 33: 241-248.
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Strother, J. L.  2006.  Hieracium.  pp. 278-294  in:  Flora of North America Editorial Committee (eds.)  Flora of North America, volume 19.  Oxford University Press, New York.

 

 

 

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