Pilosella aurantiaca (orange hawkweed)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Plant Type
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Hosts/Species Affected
- Biology and Ecology
- Latitude/Altitude Ranges
- Air Temperature
- Rainfall Regime
- Soil Tolerances
- Natural enemies
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Pathway Vectors
- Plant Trade
- Impact Summary
- Economic Impact
- Environmental Impact
- Risk and Impact Factors
- Prevention and Control
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Pilosella aurantiaca (L.) F.W. & C.H. Schultz
Preferred Common Name
- orange hawkweed
Other Scientific Names
- Hieracium aurantiacum L.
International Common Names
- English: devil's paint brush; fox-and-cubs; orange red king devil
- French: epervière orangée
Local Common Names
- Austria: orangerotes habichskraut
- Canada: bouquets rouges; marguerite rouge
- France: epervière de Hongrie
- Germany: branntweinblume; orangerotes habichskraut; pomeranzen-habichskraut
- Japan: korintanpopo
- Netherlands: havikskruid, oranje
- Switzerland: orangerotes habichskraut
- HIEAU (Hieracium aurantiacum)
Summary of InvasivenessTop of page
P. aurantiaca is a perennial herb which has spread rapidly in North America after its introduction as an ornamental and/or the contamination of pasture seeds from its native range in Europe. Although it is known to be a noxious weed elsewhere, it continues to be available as a garden ornamental and is therefore likely to spread further. It is an undesirable invader on account of its competitiveness, prolific seed production and vigorous vegetative growth leading to a drastic change in vegetation, loss in forage for stock, and loss of biodiversity. All Hieracium/Pilosella species are prohibited entry to Australia and New Zealand, and in the USA, P. aurantiaca is a declared weed in Colorado, Idaho, Minnesota and Washington.
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Dicotyledonae
- Order: Asterales
- Family: Asteraceae
- Genus: Pilosella
- Species: Pilosella aurantiaca
Notes on Taxonomy and NomenclatureTop of page
P. aurantiaca is often referred to by the synonym Hieracium aurantiacum L. The scientific name Hieracium is of Greek origin and means 'hawk'. Hawkweed refers to the fact that many species of this genus grow at higher altitudes which are only accessible by hawks. According to other sources hawks sharpened their faces with the latex sap of hawkweeds (Zahn, 1987). During the sixteenth and seventeenth centuries, botanists used to include various other yellow-flowering Asteraceae under the name Hieracium, e.g. Sonchus spp., Tragopogon spp., Crepis spp., Hypochoeris spp., or Leontodon spp. (Marzell, 1972). Hieracium species are perennial rhizomatous herbs comprising 850-1000 species worldwide with most species occurring in western Europe, where especially mountainous regions are species-rich (Gottschlich, 1996). The genus Hieracium consists of the three subgenera Hieracium, Pilosella and Stenotheca and H. officinarum was thought to belong to the subgenus Pilosella, section Pratensina. Most species in this subgenus produce stolons for vegetative reproduction in contrast to species in the subgenera Hieracium and Stenotheca which do not produce stolons.
Sell and West (1976) and other authors prefer to consider Hieracium and Pilosella as separate genera and hence the name Pilosella aurantiaca. The recognition of a distinct genus is thought to be justified both morphologically (Pilosella is distinct from Hieracium by cypsela features) and phylogenetically (Bräutigam and Greuter, 2007). Detailed justification for this name change can be found in Bräutigam and Greuter (2007). For the purpose of this datasheet and in keeping with The Plant List (2013) the name Pilosella aurantiaca will be applied. Pilosella refers to the Latin word 'pilosus', which means 'hairy' (Zahn, 1987).
DescriptionTop of page
P. aurantiaca is a red-flowering, perennial herb with usually 2-3 (up to 6) bright-green, soft rosette leaves which are covered with numerous, pale simple eglandular hairs on both the upper and lower leaf surface and the leaf margin. Rosette leaves are 8-15 cm (up to 22 cm) long and 1-4 cm wide with sparse stellate hairs restricted to the lower leaf surface. Flowering stems are 20-40 cm (up to 65 cm) long with sparse stellate, numerous, dark simple eglandular hairs (1-6 mm long) and fewer, shorter, dark glandular hairs above, with 1-4 leaves similar to those of the rosette. The flowering stem produces 2-12 (-25) flower heads with orange-red florets. The fruit is an achene up to 3 mm long and 1 mm wide. The pappus is brown and brittle.
Plant TypeTop of page
DistributionTop of page
P. aurantiaca is native to Europe with an original subalpine distribution resulting in disjunctive areas (Zahn, 1987; Meusel and Jäger, 1992). P. aurantiaca reaches its most eastern distribution in southern Russia, France in the west and Bulgaria in the south (Sell and West, 1976). However, records in Belgium, Britain, Denmark, the Netherlands and Iceland are uncertain and may represent plants that have escaped from gardens or parks (Sell and West, 1976). Due to its use as a garden ornamental and rock garden plant, P. aurantiaca has been repeatedly introduced into exotic locations including New Zealand (Webb et al., 1988), Japan (Suzuki and Narayama, 1977) and North America (Fernald, 1950; Gleason and Cronquist, 1991; Birdsall and Quimby, 1996; Wilson and Callihan, 1999).
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.Last updated: 15 Dec 2020
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Japan||Present||Present based on regional distribution.|
|-Hokkaido||Present||Introduced||Invasive||First reported: 1950s|
|Russia||Present||Present based on regional distribution.|
|-Russian Far East||Present||Introduced|
|Serbia and Montenegro||Present||Native|
|Canada||Present||Present based on regional distribution.|
|-British Columbia||Present, Widespread||Introduced||Invasive|
|-Newfoundland and Labrador||Present||Introduced|
|-Prince Edward Island||Present||Introduced|
|United States||Present, Localized||Introduced||Invasive||First reported: before 1818|
|Australia||Present||Present based on regional distribution.|
|-New South Wales||Present||Introduced|
History of Introduction and SpreadTop of page
P. aurantiaca has been repeatedly introduced to other countries as a garden ornamental. The original dates are rarely known, though the first records of naturalization give some indication, e.g. before 1818 in the USA (Birdsall and Quimby, 1996), 1911 in New Zealand (Webb et al., 1988) and the 1950s in Japan (Suzuki and Narayama, 1977). In addition, P. aurantiaca is naturalized in the Altai and Sakhalin, Russia and on the Falkland Islands (Meusel and Jäger, 1992). It is also likely that P. aurantiaca, like P. officinarum, was introduced through contamination of pasture seeds or seeds for lawns (Meusel and Jäger, 1992), since the seeds of both these species are of comparable size.
In New Zealand, P. aurantiaca occurs on waste land, grassland, scrub, tussock grassland, roadsides, lawns, gardens and pastures (Webb et al., 1988). However, according to Hunter (1992), P. aurantiaca is not a major weed problem. In Japan, P. aurantiaca is abundant in Hokkaido, Japan and is spreading along coasts and inland along roadsides and into pastures (Suzuki and Narayama, 1977). In North America, P. aurantiaca infests forests, pastures, meadows, and wetlands (Birdsall and Quimby, 1996). The most heavily infested states in the USA are Minnesota to Iowa, Ohio, Oregon, Washington (Birdsall and Quimby, 1996) and Montana (Wilson and Callihan, 1999).
P. aurantiaca is still available in nurseries in North America and Australia. As of 2003, P. aurantiaca was being sold as a garden ornamental in online stores in the USA and promoted as a plant which is easy to cultivate; "This is one of those plants that is best planted in one of those locations where everything else you have tried has not performed well". P. aurantiaca was also seen for sale in markets in Oregon in 2002 but the plants were destroyed (Oregon Invasive Species Council, 2002). Although P. aurantiaca is prohibited by the Australian authorities, it escaped regulations by being sold under its synonym name Pilosella aurantiaca (Lloyd, 2003).
Risk of IntroductionTop of page
Further spread is highly probable, owing to the risks of both accidental movement (as a seed contaminant or with machinery or soil) and deliberate introduction as an ornamental. This is encouraged by the availability from commercial nurseries via mail-order catalogues and websites. However, entry of all Hieracium/Pilosella species is already prohibited in Australia and New Zealand. In the USA, P. aurantiaca is a declared weed in Colorado, Idaho, Montana, Oregon, and Washington (USDA-NRCS, 2016).
HabitatTop of page
Within its native range, the presence of P. aurantiaca varies from sparse to common and it occurs on meadows and pastures especially in subalpine regions. The original distribution of P. aurantiaca outside the Alps, low mountain ranges and Sudeten Mountains is often difficult to determine since it is frequently used as an ornamental in parks, gardens and cemeteries from where it escapes easily. In North America, species of Hieracium and Pilosella are primarily weeds of moist pastures, forest meadows, forest clearings, abandoned fields and roadsides (Wilson and Callihan, 1999).
Habitat ListTop of page
|Terrestrial||Managed||Managed forests, plantations and orchards||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Managed grasslands (grazing systems)||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Disturbed areas||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Rail / roadsides||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Urban / peri-urban areas||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Natural / Semi-natural||Natural forests||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Natural / Semi-natural||Natural grasslands||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Natural / Semi-natural||Riverbanks||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Natural / Semi-natural||Wetlands||Present, no further details|
Hosts/Species AffectedTop of page
P. aurantiaca is not a weed of crops but is a serious weed of natural and managed pastures, also in forest clearings and margins.
Biology and EcologyTop of page
P. aurantiaca is a facultative apomictic species. The chromosome numbers recorded are 2n=18, 27, 36, 45, 54, 63 and 72. Before the discovery of a diploid apomictic biotype of P. aurantiaca, only polyploid apomictic biotypes were known in the subgenus Pilosella (Bicknell, 1997). The process of aposporie where the gametophytes develop from somatic cells is typical for apomicts in the subgenus Pilosella (Gottschlich, 1987). In the field, tetraploid and pentaploid populations of P. aurantiaca do not usually coexist, but where they do, hexaploid and heptaploid plants are also found, although to a very low extent. The following events lead to the richness in ploidy levels of P. aurantiaca: (a) aposporie, (b) fertilization of reduced egg cells, (c) fertilization of unreduced egg cells, or (d) fertilization with unreduced pollen (Skalinska, 1971). P. aurantiaca can produce hybrids with numerous other Hieracium species including P. caespitosa, H. cymosum, H. hoppeanum, P. officinarum and P. lactucella (Meusel and Jäger, 1992), also H. fuscoatrum, H. guthnickianum and P. stoloniflora. The most closely related main species is P. caespitosa (Meusel and Jäger, 1992).
Propagation of P. aurantiaca is by seeds and stolons. Mature seeds can germinate as soon as they are released from the plant (Stergios, 1976). Panebianco and Willemsen (1976) recorded a high seed and seedling mortality for P. caespitosa, though concluding that establishment of only a small number of rosettes is needed to ensure the success of P. caespitosa since it is a perennial and multiplies vegetatively. When P. aurantiaca initiates flowering, stolons and rhizomes initiated from axillary buds at the base of rosette leaves begin to grow (Wilson and Callihan, 1999). It is likely that vegetative reproduction within an existing P. aurantiaca population is more important for propagation than rosettes derived from seeds, as recorded for P. officinarum and the closely related P. floribunda. Makepeace (1985) recorded a large difference in turnover of P. officinarum plants between field sites, 5-173 new rosettes per 100 existing rosettes and that within existing populations, spread occurs mainly by vegetative means (i.e. stolon production). Similar results were obtained for P. floribunda where only 1% of new plants in a population were derived from seedlings (Thomas and Dale, 1975). Nonetheless, seeds remain important for long-distance spread and a mixed strategy of clonal growth and reproduction by seeds in P. officinarum may be necessary to maintain populations of this species in the presence of high interspecific competition and a shortage of open space (Winkler and Stöcklin, 2002).
Physiology and Phenology
Depending on the altitude of the plant, flowering occurs between June and August. No references were found regarding allelopathic substances in P. aurantiaca (e.g. umbelliferone) as recorded for P. officinarum (Makepeace et al., 1985) although this does not necessarily indicate that they are absent in P. aurantiaca.
In New Zealand, species of Hieracium and Pilosella are particularly abundant in sub-humid to humid mountain to lower sub-alpine bioclimates. The optimal rainfall range for vigorous communities appears to be 600-1200 mm, but occur locally where annual rainfall exceeds 3000 mm (Hunter, 1992). In southern Norway it can be found up to 1250 m altitude, up to 2200 m in the southern Carpathian Mountains and up to 2600 m in the Alps (Meusel and Jäger, 1992).
Latitude/Altitude RangesTop of page
|Latitude North (°N)||Latitude South (°S)||Altitude Lower (m)||Altitude Upper (m)|
Air TemperatureTop of page
|Parameter||Lower limit||Upper limit|
|Absolute minimum temperature (ºC)||-33|
|Mean annual temperature (ºC)||5||10|
|Mean maximum temperature of hottest month (ºC)||15||20|
|Mean minimum temperature of coldest month (ºC)||-6||-5|
RainfallTop of page
|Parameter||Lower limit||Upper limit||Description|
|Dry season duration||0||8||number of consecutive months with <40 mm rainfall|
|Mean annual rainfall||600||3000||mm; lower/upper limits|
Rainfall RegimeTop of page
Soil TolerancesTop of page
- seasonally waterlogged
Special soil tolerances
Natural enemiesTop of page
|Natural enemy||Type||Life stages||Specificity||References||Biological control in||Biological control on|
|Aulacidea pilosellae||Herbivore||Plants|Leaves; Plants|Seeds|
|Aulacidea subterminalis||Herbivore||Plants|Stems||to species||Canada, USA and New Zealand||P. officinarum, H. flagellare and P. aurantiaca|
|Cheilosia psilophthalma||Herbivore||Plants|Growing point; Plants|Stems||to genus||New Zealand||P. aurantiaca, P. caespitosa, H. lepidulum, P. officinarum and H. praealtum|
|Cheilosia urbana||Herbivore||Plants|Roots||to species||Canada, USA and New Zealand||P. officinarum; P. aurantiaca, P. caespitosa, H. lepidulum and H. praealtum|
|Noeeta pupillata||Herbivore||Plants|Inflorescence; Plants|Seeds|
|Oxyptilus pilosellae||Herbivore||Plants|Growing point; Plants|Stems||to genus||New Zealand||P. aurantiaca, H. lepidulum, P. officinarum and H. praealtum|
Notes on Natural EnemiesTop of page
P. aurantiaca is not attacked to any noticeable degree by phytophagous insects in New Zealand (Syrett and Smith, 1998) therefore they may have a competitive advantage over native rangeland species. In its native range, P. aurantiaca is associated with a range of specialized phytophagous insects (Grosskopf et al., 2001; 2003). The main insect groups associated with species of Hieracium and Pilosella are Aulacidea spp., Cheilosia spp., Oxyptilus spp., gall midges and tephritids developing in the flower heads. In contrast to other weeds no host-specific Coleoptera have been found.
Means of Movement and DispersalTop of page
Propagation ofP. aurantiaca occurs by seeds, which are produced in large numbers and are wind-dispersed. P. aurantiaca also reproduces vegetatively by stolons and rhizomes which could be dispersed by water.
Contamination of agricultural seed is probably a main pathway for introduction of P. aurantiaca, as in the case with P. officinarum or P. caespitosa. Accidental spread is also likely through soil contaminated with seeds or stolon fragments, or by the dumping of garden waste and the movement of soil or machinery during earthworks.
Deliberate introduction of P. aurantiaca is quite likely, as it is used as an ornamental plant. Such introduction is encouraged by the availability of seed and rosettes, either from commercial nurseries, at farmers markets or via mail-order catalogues, from websites in Canada, Australia or the USA.
Pathway VectorsTop of page
Plant TradeTop of page
|Plant parts liable to carry the pest in trade/transport||Pest stages||Borne internally||Borne externally||Visibility of pest or symptoms|
|Fruits (inc. pods)||seeds|
|Growing medium accompanying plants||roots; seeds; stems|
Impact SummaryTop of page
|Fisheries / aquaculture||None|
Economic ImpactTop of page
P. aurantiaca can from dense stands which may replace forage plants decreasing productivity and land value (Birdsall and Quimby, 1996).
Environmental ImpactTop of page
Risk and Impact FactorsTop of page
- Proved invasive outside its native range
- Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
- Highly mobile locally
- Has high reproductive potential
- Negatively impacts agriculture
- Negatively impacts tourism
- Reduced amenity values
- Reduced native biodiversity
- Competition - monopolizing resources
- Highly likely to be transported internationally accidentally
- Highly likely to be transported internationally deliberately
- Difficult/costly to control
UsesTop of page
Potential benefits of Hieracium and Pilosella species in New Zealand include being a source of umbelliferone (used in the manufacture of sun-screens and sun-tan lotions), a food source for stock, soil conservation, horticultural use, a pollen source for bees, suppression of other weeds and seed for herbal purposes, although these benefits are negligible in comparison to the negative impacts of this group of plants (Grundy, 1989). Due to its attractive flowers, P. aurantiaca has also been widely grown as an ornamental in parks and gardens.
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
The application of fertilizers can help to control P. aurantiaca by increasing the competitive ability of more desirable species such as grasses, legumes and other beneficial forbs. The main method of control at present is to improve land with application of fertilizer, oversowing with pasture species and a good grazing management (Wilson and Callihan, 1999).
Mechanical control of P. aurantiaca has had limited success since disturbing the stolons and rhizomes may only help the plant to spread. In addition, due to the mat-forming growth P. aurantiaca can successfully escape mowing. Although mowing prevents seed production by removing flowering stems, repeated mowing encourages faster vegetative spread (Wilson and Callihan, 1999).
Chemical control is especially promising on small infestations to prevent further spread. P. aurantiaca is effectively controlled by dicamba, clopyralid, picloram, and picloram + 2,4-D, (Whitson et al., 2000). However, total elimination is unlikely and a follow-up with fertilizer and top dressing is considered essential (Grundy, 1989). A problem with chemical control is that the chemicals have as much, or greater, impact on many desirable pasture species (Grundy, 1989). In many areas in New Zealand chemical control is uneconomic and thus not considered on a large scale (Grundy, 1989).
Since chemical and mechanical control are ineffective and/or not economical a programme to develop biological control was initiated in 1992 (Syrett and Smith, 1998). P. aurantiaca is not attacked to any noticeable degree by specialized phytophagous insects in New Zealand (Syrett and Smith, 1998), and may therefore have a competitive advantage over native rangeland species. The plume moth, Oxyptilus pilosellae, the cynipid, Aulacidea subterminalis, the syrphid, Cheilosia urbana and Cheilosia psilophthalma are native to Europe and feed on different parts of P. aurantiaca and after testing were approved for release in New Zealand (Syrett et al., 1999; Grosskopf et al., 2002; Klöppel et al., 2003).
Since P. aurantiaca is a noxious weed in North America a programme was also initiated. Major differences between the programmes in New Zealand and North America include the presence of native Hieracium and Pilosella species. Potential biological control agents are presently being tested at CABI Switzerland for use in North America. However after assessment, the four agents released in New Zealand were dismissed due to difficulties in rearing some agents, feeding on non-target species and limited damage caused by the agent. However, Aulacidea pilosellae causes small galls on the midrib of leaves, stolons and flower stalks of several target species in the genus Pilosella and appears to have a restricted host range. The wasp is being studied in collaboration with Agriculture and Agri-Food Canada (AAFC) and Montana State University (MSU).
A joint petition (Littlefield et al., 2009) for field release of the gall wasp Aulacidea subterminalis in the USA and Canada was submitted and approved in 2010. Since summer 2011, several releases have taken place in British Columbia, Canada and Montana, USA on P. aurantiaca and H.flagellare, and sites are being monitored. So far, establishment has only been confirmed on H. flagellare.
A joint petition for the field release of the root-feeding hoverfly Cheilosia urbana in North America was submitted to the United States Department of Agriculture – Animal and Plant Health Inspection Service (USDA-APHIS) Technical Advisory Group (TAG) and the Canadian Biological Control Review Committee in December 2014. The agent was approved for release in Canada by the Canadian Food Inspection Agency (CFIA) in April 2016 and recommended for release by TAG for the USA in May 2016. Surveys in Switzerland to collect the hoverfly in view of future releases in North America started in 2016.
Screening of the rust Puccinia hieracii var. piloselloidarum started in 2012 at Agriculture and Agri-Food Canada (AAFC), using samples field collected from various Pilosella species in Europe by CABI. Due to the very limited success with the inoculations in the laboratory, work with the rust was discontinued.
Strategies for sustainable, long-term management include prevention, early detection, minimizing disturbance, maintaining soil health, controlling grazing, judicious use of herbicides, periodic applications of fertilizer, and cooperation between private and public land managers (Wilson and Callihan, 1998). Biological control incorporated in an integrated control strategy may strengthen the competitiveness of desirable pasture species and help control P. aurantiaca (Grundy, 1989; Wilson and Callihan, 1999).
ReferencesTop of page
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Stergios BG, 1976. Achene production, dispersal, seed germination and seedling establishment of Hieracium aurantiacum in an abandoned field community. Canadian Journal of Botany, 54(11):1189-1197
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