Hieracium aurantiacum Risk – California Invasive Plant Council

Created by: Nicole Valentine
Created on: Monday, Sep 27th, 2021

Evaluation Summary

Hieracium aurantiacum (Orange hawkweed) is an herbaceous perennial forb that reproduces by seed, rhizome, and stolon. Because of the various reproduction options, this plant also has various dispersal routes. Once established, Hieracium aurantiacum can colonize areas quickly, forming dense mats and excluding forage and native species. Hieracium aurantiacum was introduced in 1875 to the US in Vermont as a garden ornamental, was marketed as a garden ornamental for a long period, and has since repeatedly escaped cultivation. This plant is listed as a noxious weed in the western US in Colorado, Idaho, Oregon, Montana, and Washington. It is also rated by the California Department of Food and Agriculture as a B-rated weed.

General Evaluation Information

Date of Evaluation:

September 27, 2021

Evaluation Time (hrs):

5 Hours

Evaluation Status:

Completed

Plant Information

Plant:

Hieracium aurantiacum

Plant Material:

If the plant is a cultivar, and if the cultivar's behavior differs from its parent's (behavior), explain how:

Regional Information

Region Name:

California

Climate Matching Map

These maps were built using a toolkit created in collaboration between GreenInfo Network, PlantRight, Cal-IPC, and Agricultural Sustainability Institute at UC Davis.

PRE Climate Map (external link)

Climate Matching Maps PDF:

Attachment	Size
CA_ClimateMatch.pdf	503.38 KB

Invasive History and Climate Matching

1. Has the species (or cultivar or variety, if applicable; applies to subsequent "species" questions) become naturalized where it is not native?

Yes or No:

Yes

Points:

Confidence Level:

Very High

Answer / Justification:

Hieracium aurantiacum is native to alpine and hillside meadows in northern and central Europe. Hieracium aurantiacum has naturalized across Japan, the South Island of New Zealand, southeastern Australia (Kosciuszko National Park in New South Wales), western Iceland, the Falkland Islands, along the southern Andes in Chile, and in Canada (southwestern British Colombia, New Foundland, and New Brunswick). In the US Hieracium aurantiacum has naturalized in Colorado, Washington, Oregon, Idaho, Montana, southern Alaska (Kodiak Island and Kenai Peninsula), surrounding the Great Lakes, and New England with dense occurrences from Pennsylvania to Maine.

Reference(s):

GBIF (2016). GBIF Backbone Taxonomy.

iNaturalist Network (0). iNaturalist.

2. Is the species (or cultivar or variety) noted as being naturalized in the US or world in a similar climate?

Yes or No:

Yes

Points:

Confidence Level:

Very High

Answer / Justification:

Hieracium aurantiacum has been naturalized in areas with a similar climate: across Japan, the South Island of New Zealand, southeastern Australia (Kosciuszko National Park in New South Wales), along the southern Andes in Chile, and in Canada (southwestern British Colombia, New Brunswick). In the US Hieracium aurantiacum has naturalized in Colorado, Washington, Oregon, Idaho, Montana, southern Alaska (Kodiak Island and Kenai Peninsula). There are also occurrences in southern New England that match the climate.

Reference(s):

GBIF (2016). GBIF Backbone Taxonomy.

iNaturalist Network (0). iNaturalist.

3. Is the species (or cultivar or variety) noted as being invasive in the U.S. or world?

Yes or No:

Yes

Points:

Confidence Level:

Very High

Answer / Justification:

Hieracium aurantiacum displaces desired native and forage species in Japan, southeastern Australia, southern Alaska, New Zealand, and British Colombia. Hieracium aurantiacum in southern Alaska forms monocultures, reduces plant diversity, and decreases pasture productivity (Seefeldt and Conn 2011). Australian agriculture estimates potential production losses from H. aurantiacum to cost $AUD 48 million (Brinkley and Bomford 2002). Hieracium aurantiacum is also listed as a noxious weed in Colorado, Idaho, Minnesota, Montana, Oregon, and Washington (CABI). It is also rated by the California Department of Food and Agriculture as a B-rated weed (CDFA).

Reference(s):

[Anonymous] (2007). CABI Invasive Species Compendium.

Seefeldt, S. S., & Conn J. S. (2011). Control of Orange Hawkweed (Hieracium aurantiacum) in Southern Alaska. Invasive Plant Science and Management. 4(1), 87-94.

Weeds of Australia (2016). Hieracium aurantiacum.

Brinkley, T., & Bomford M. (2002). Agricultural Sleeper Weeds in Australia What is the potential threat?.

CDFA (0). CDFA's Division of Plant Health's Pest Ratings and Proposals.

4. Is the species (or cultivar or variety) noted as being invasive in the US or world in a similar climate?

Yes or No:

Yes

Points:

Confidence Level:

Very High

Answer / Justification:

Hieracium aurantiacum is invasive in areas with a similar climate to California such as Japan, southeastern Australia, New Zealand, southern Alaska, British Colombia, Colorado, Idaho, and Washington (CABI, Weeds of Australia). Hieracium aurantiacum in southern Alaska forms monocultures, reduces plant diversity, and decreases pasture productivity (Seefeldt and Conn 2011). Australian agriculture estimates potential production losses from H. aurantiacum to cost $AUD 48 million (Brinkley and Bomford 2002). Hieracium aurantiacum is also listed as a noxious weed in Colorado, Idaho, Minnesota, and Washington (CABI). It is also rated by the California Department of Food and Agriculture as a B-rated weed (CDFA).

Reference(s):

[Anonymous] (2007). CABI Invasive Species Compendium.

Weeds of Australia (2016). Hieracium aurantiacum.

Seefeldt, S. S., & Conn J. S. (2011). Control of Orange Hawkweed (Hieracium aurantiacum) in Southern Alaska. Invasive Plant Science and Management. 4(1), 87-94.

Brinkley, T., & Bomford M. (2002). Agricultural Sleeper Weeds in Australia What is the potential threat?.

5. Are other species of the same genus (or closely related genera) invasive in a similar climate?

Yes or No:

Yes

Points:

Confidence Level:

Very High

Answer / Justification:

There are numerous Hieracium species that are listed or implicated as invasive in areas with a similar climate. All Hieracium species are prohibited in Australia and New Zealand (CABI). In New Zealand, present invasive species include Pilosella officinarum, Pilosella piloselloides subspecies praealta, Hieracium lepidulum, and Pilosella caespitosa (AgPest). Hieracium caespitosum displaces native and forage species and is listed as a noxious weed in Washington, Oregon, Montana, and Idaho (Yellow hawkweed identification and control, Invasive Plant Atlas).

Reference(s):

[Anonymous] (2007). CABI Invasive Species Compendium.

Invasive Plant Atlas of the United States (0). Herbs/Forbs: Invasive Plant Atlas of the United States.

AgPest (2021). Hieracium, hawkweed.

[Anonymous] (2018). Yellow hawkweed identification and control.

6. Is the species (or cultivar or variety) found predominately in a climate matching the region of concern?

Yes or No:

Yes

Points:

Confidence Level:

Very High

Answer / Justification:

Although Hieracium aurantiacum occurs in areas of cooler climates, it predominately occurs in a climate matching the region of concern. The areas where Hieracium aurantiacum occurs in a similar climate to California include: throughout Central Europe (Germany, Czechia, Switzerland, Austria, Netherlands, Romania, and Belgium), across Japan, the South Island of New Zealand, southeastern Australia (Kosciuszko National Park in New South Wales), along the southern Andes in Chile, and in Canada (southwestern British Colombia, New Brunswick). In the US Hieracium aurantiacum has naturalized in areas with a similar climate such as Colorado, Washington, Oregon, Idaho, Montana, southern Alaska (Kodiak Island and Kenai Peninsula). There are also occurrences in southern New England that match the climate. Areas where Hieraceum occurs that do not match the climate include: Northern Europe (Sweden, Norway, Finland, Denmark), New Foundland, western Iceland, the Falkand Islands, areas in the US (surrounding the Great Lakes, northern New England).

Reference(s):

GBIF (2016). GBIF Backbone Taxonomy.

iNaturalist Network (0). iNaturalist.

Impact on Native Plants and Animals

7. Does this plant displace native plants and dominate (overtop or smother) the plant community in areas where it has established?

Yes or No:

Yes

Points:

Confidence Level:

Very High

Answer / Justification:

Hieracium aurantiacum has been documented in southeast Alaska, Montana, and Minnesota to quickly colonize areas, forming dense mats, leading to a loss of native plant diversity (Heutte et al 2003, Rinella et al 2009, MN DNR). In Hokkaido, Japan some populations reached a density of >1000 plants/m2 (Suzuki and Narayama 1977). Hieracium aurantiacum is also pollen-alleopathic, reducing pollen growth and germination in other plants (Murphy 2001).

Reference(s):

Rinella, M. J., Sheley R. L., Mangold J., & Kittle R. (2009). Orange Hawkweed and Meadow Hawkweed Complex.

Heutte, T., Bella E., Snyder J., & Shephard M. (2003). Invasive Plants and Exotic Weeds of Southeast Alaska.

MNDNR (2021). Orange hawkweed (Hieracium aurantiacum).

Suzuki, S., & Narayama T. (1977). Orange hawkweed (Hieracium aurantiacum L.) as an alien pasture weed in Hokkaido.. Research Bulletin, 45-56.

Murphy, S. D. (2001). The Role of Pollen Allelopathy in Weed Ecology . Weed Technology. 15(4), 867-872.

8. Is the plant noted as promoting fire and/or changing fire regimes?

Yes or No:

Points:

Confidence Level:

Medium

Answer / Justification:

Because Hieracium aurantiacum establishment is favored by disturbance and high-light conditions, fire may benefit Hieracium aurantiacum; however, there is no evidence that Hieracium aurantiacum promotes fire. Hieracium aurantiacum is not a major fuel source as it is a perennial herbaceous forb 10 to 36 inches tall with milky sap and leafy stolons (FEIS). Hieracium aurantiacum sprouts from stolons and rhizomes after mechanical disturbance and belowground rhizomes may survive fire (FEIS). Hieracium aurantiacum in Montana is most abundant in sites that have been burned and salvage logged (Jones and Crone 2007).

Reference(s):

Jones, A., & Crone E. (2009). The Effect of Disturbance History on Hawkweed Invasion (Montana). Ecological Restoration. 27(2), 139-140.

U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer) (1992). Fire Effects Information System (FEIS).

9. Is the plant a health risk to humans or animals/fish? Has the species been noted as impacting grazing systems?

Yes or No:

Yes

Points:

Confidence Level:

Low

Answer / Justification:

Despite differing reports on whether Hieracium aurantiacum is palatable to animals, Hieracium aurantiacum displaces desirable forage species ultimately resulting in less grazing productivity. WSU reports that Hieracium aurantiacum is unpalatable to animals whereas Boswell and Espie and Rinella and Sheley report that Hieracium species have moderate to high nutritive values (WSU 2013, Boswell and Espie 2002, Rinella and Sheley 2002). Overall Hieracium aurantiacum still has lower nutritional quality than pasture species, and as an aggressive competitor of pasture and range species, Hieracium aurantiacum reduces grazing productivity (Williams et al 2007). There is no evidence that Hieracium aurantiacum is a health risk to humans or animals/fish.

Reference(s):

Boswell, C., & Espie P. (2002). Sheep fancy Hieracium flowers: monitoring grazing of Hieracium on mid-altitude blocks at Tara Hills research station during 1996/7..

Rinella, M., & Sheley R. (2002). Orange and Meadow Hawkweed. MontGuide Fact Sheet.

Williams, N., Hahs A., Morgan J., & Holland K. (2007). A dispersal constrained habitat suitability model for Orange Hawkweed (Hieracium aurantiacum) on the Bogong High Plains, Victoria.

Washington State University (WSU) (2013). Orange hawkweed.

10. Does the plant produce impenetrable thickets, blocking or slowing movement of animals, livestock, or humans?

Yes or No:

Points:

Confidence Level:

Medium

Answer / Justification:

There are multiple reports of Hieracium aurantiacum forming dense mats and monocultures, but it is inferred that it does not obstruct passage due to its small stature (Heutte et al 2003, Rinella et al 2009, and MN DNR). Hieracium aurantiacum is a perennial herbaceous forb 10 to 36 inches tall (FEIS).

Reference(s):

U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer) (1992). Fire Effects Information System (FEIS).

Heutte, T., Bella E., Snyder J., & Shephard M. (2003). Invasive Plants and Exotic Weeds of Southeast Alaska.

Rinella, M. J., Sheley R. L., Mangold J., & Kittle R. (2009). Orange Hawkweed and Meadow Hawkweed Complex.

Minnesota Department of Natural Resources (2016). Minnesota Department of Natural Resources (MNDNR).

Reproductive Strategies

11. Does this species (or cultivar or variety) reproduce and spread vegetatively?

Yes or No:

Yes

Points:

Confidence Level:

Very High

Answer / Justification:

Hieracium aurantiacum is rhizomatous and stoloniferous (FEIS, Stergios 1976). Most established populations of hawkweed expand vegetatively (Callihan et al 1997). Hieracium aurantiacum can form a 0.5 m2 mat in its first year from asexual growth (Williams et al 2007). Hieracium aurantiacum sprouts from stolons and rhizomes after mechanical disturbance (FEIS).

Reference(s):

U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer) (1992). Fire Effects Information System (FEIS).

Williams, N., Hahs A., Morgan J., & Holland K. (2007). A dispersal constrained habitat suitability model for Orange Hawkweed (Hieracium aurantiacum) on the Bogong High Plains, Victoria.

Stergios, B. (1976). Achene production, dispersal, seed germination, and seedling establishment of Hieracium aurantiacum in an abandoned field community. Canada Journal of Botany. 54, 1189-1197.

Callihan, R. H., Wilson L. M., McCaffrey J. P., & Miller T. (1997). Hawkweeds. PNW 499.

12. If naturally detached fragments from this plant are capable of producing new plants, is this a common method of reproduction for the plant?

Yes or No:

Yes

Points:

Confidence Level:

High

Answer / Justification:

Hieracium aurantiacum is capable of producing new plants from detached fragments. Any activities that disturb plants, such as digging or grazing, can stimulate the growth of new plants from fragmented roots, stolons, and rhizomes (Rinella and Sheley 2002). Dispersal of plant fragments by water downslope has also been demonstrated for populations in Hobart (Williams et al 2007). Since water is capable of detaching fragments, this is likely not an uncommon event.

Reference(s):

Rinella, M., & Sheley R. (2002). Orange and Meadow Hawkweed. MontGuide Fact Sheet.

Williams, N., Hahs A., Morgan J., & Holland K. (2007). A dispersal constrained habitat suitability model for Orange Hawkweed (Hieracium aurantiacum) on the Bogong High Plains, Victoria.

13. Does the species (or cultivar or variety) commonly produce viable seed?

Yes or No:

Yes

Points:

Confidence Level:

High

Answer / Justification:

Most new Hieracium infestations are likely started by seeds, then expand vegetatively as they are established (Callihan et al 1996). Seed production is density-dependent, with production decreasing as population density increases (Rinella et al 2009). H. aurantiacum is an apomict, producing seeds without pollination (Bicknell and Borst 1994, Bicknell et al. 2003).

Reference(s):

Callihan, R. H., Wilson L. M., McCaffrey J. P., & Miller T. (1997). Hawkweeds. PNW 499.

Rinella, M. J., Sheley R. L., Mangold J., & Kittle R. (2009). Orange Hawkweed and Meadow Hawkweed Complex.

Bicknell, R., & Borst N. (1994). Agrobacterium-mediated transformation of Hieracium aurantiacum. International Journal of Plant Sciences. 467-470.

Bicknell, R., Lambie S., & Butler R. (2003). Quantification of progeny classes in two facultatively apomictic accessions of Hieracium.. Hereditas. 11-20.

14. Does this plant produce copious viable seeds each year (> 1000)?

Yes or No:

Yes

Points:

Confidence Level:

High

Answer / Justification:

Hieracium aurantiacum is capable of producing copious viable seeds when analyzing single plant production as well as an area of plants. Because Hieracium aurantiacum seed production is density-dependent, seed production is greater on the periphery of Hieracium aurantiacum populations, where plant density is typically lower than in population interiors (Rinella et al 2009). Each Hieracium aurantiacum rosette generally produces 1 flowering stem but can produce up to 30 stems (Rinella et al 2009). A single-flowered stem has a range of producing 60 to 1500 seeds (Rinella et al 2009). When analyzing a square-meter mat of Hieracium aurantiacum up to 40,000 seeds are produced per year (National Heritage Trust 2003). Hieracium aurantiacum plants in New Zealand had 91% seed set (Koltunow et al 1998).

Reference(s):

Rinella, M. J., Sheley R. L., Mangold J., & Kittle R. (2009). Orange Hawkweed and Meadow Hawkweed Complex.

Trust, N. Heritage (2003). Orange hawkweed (Hieracium aurantiacum).

Koltunow, A. M., Johnson S. D., & Bicknell R. A. (1998). Sexual and apomictic development in Hieracium. Sexual Plant Reproduction. 11, 213–230.

15. Is there significant germination (>25%) of seeds the next growing season, with no requirement of an infrequent environmental condition for seeds to germinate (i.e. fire) or long dormancy period?

Yes or No:

Yes

Points:

Confidence Level:

Very High

Answer / Justification:

Hieracium aurantiacum seeds do not have an after-ripening period and can germinate immediately after dropping from the plant (FEIS). In New Zealand 71% of Hieracium aurantiacum seeds germinated on nutrient agar within 4 days (Koltunow et al 1998). In Michigan germination rates decline through the season; for seeds gathered in June, July, August, and September, total germination was 66%, 28%, 18.5%, and 13% respectively (Stergios 1976). Seeds may survive up to 7 years in the soil (FEIS).

Reference(s):

Stergios, B. (1976). Achene production, dispersal, seed germination, and seedling establishment of Hieracium aurantiacum in an abandoned field community. Canada Journal of Botany. 54, 1189-1197.

U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer) (1992). Fire Effects Information System (FEIS).

Koltunow, A. M., Johnson S. D., & Bicknell R. A. (1998). Sexual and apomictic development in Hieracium. Sexual Plant Reproduction. 11, 213–230.

16. Does this plant produce viable seed within the first three years (for an herbaceous species) to five years (for a woody species) after germination?

Yes or No:

Yes

Points:

Confidence Level:

Very High

Answer / Justification:

Hieracium aurantiacum in Michigan did not produce seed in its first year but flowered the following spring (Stergios 1976). In greenhouse experiments 35% of Hieracium aurantiacum seedlings flowered within three months (Howe 2002).

Reference(s):

Stergios, B. (1976). Achene production, dispersal, seed germination, and seedling establishment of Hieracium aurantiacum in an abandoned field community. Canada Journal of Botany. 54, 1189-1197.

Howe, K. (2002). The ecology of invasions in a Minnesota grassland: characteristics of invasive species and invaded communities and the effects of global change. Zoology,

17. Does this plant continuously produce seed for >3 months each year or does seed production occur more than once a year?

Yes or No:

Points:

Confidence Level:

High

Answer / Justification:

Hieracium aurantiacum reportedly flowers for less than three months in the majority of locations and for only a month in the area of interest. Hieracium aurantiacum flowers from January to March in Australia (National Heritage Trust 2003). In North America, out of 11 locations, 5 locations flowered for 3 months or longer (FEIS). In the Pacific Northwest Hieracium aurantiacum flowers from mid-June to early August (Callihan et al 1997). Calflora reports the flowering period for Hieracium aurantiacum in California is August.

Reference(s):

Trust, N. Heritage (2003). Orange hawkweed (Hieracium aurantiacum).

Calflora (0). Calflora: Information on California plants for education, research and conservation, with data contributed by public and private institutions and individuals, including the Consortium of California Herbaria.

Callihan, R. H., Wilson L. M., McCaffrey J. P., & Miller T. (1997). Hawkweeds. PNW 499.

U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer) (1992). Fire Effects Information System (FEIS).

Dispersal

18. Are the plant’s propagules frequently dispersed long distance (>100 m) by mammals or birds or via domestic animals?

Yes or No:

Yes

Points:

Confidence Level:

Medium

Answer / Justification:

It is likely Hieracium aurantiacum is spread long distances through their seeds and detached fragments. Hieracium aurantiacum seeds have minute barbs along their ribs that enable them to stick to hair, feathers, and fur (Williams et al 2007). Hieracium aurantiacum can be dispersed through grazing as grazing can stimulate the growth of new plants from fragmented roots, stolons, and rhizomes (Rinella and Sheley 2002).

Reference(s):

Williams, N., Hahs A., Morgan J., & Holland K. (2007). A dispersal constrained habitat suitability model for Orange Hawkweed (Hieracium aurantiacum) on the Bogong High Plains, Victoria.

Rinella, M., & Sheley R. (2002). Orange and Meadow Hawkweed. MontGuide Fact Sheet.

19. Are the plant’s propagules frequently dispersed long distance (>100 m) by wind or water?

Yes or No:

Yes

Points:

Confidence Level:

Very High

Answer / Justification:

Hieracium aurantiacum seeds can disperse by wind and water and H. aurantiacum fragments can also disperse by water. Hieracium aurantiacum has small seeds with a bristly tuft that enables long-distance wind-dispersal (FEIS). Stergios found that the majority of H. aurantiacum seeds in Michigan were deposited within 2 m of the source patch with occasional long-distance dispersement of seeds from the source population (1976). In lower-quality environments, H. aurantiacum invests in seed production more than vegetative spread (Jones and Crone 2009). Cousens and Williams found that most of the new populations of H. aurantiacum were likely spread by prevailing winds (2011). Dispersal of seeds and plant fragments by water downslope has also been demonstrated for populations in Hobart (Williams et al 2007).

Reference(s):

U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer) (1992). Fire Effects Information System (FEIS).

Williams, N., Hahs A., Morgan J., & Holland K. (2007). A dispersal constrained habitat suitability model for Orange Hawkweed (Hieracium aurantiacum) on the Bogong High Plains, Victoria.

Jones, A., & Crone E. (2009). The Effect of Disturbance History on Hawkweed Invasion (Montana). Ecological Restoration. 27(2), 139-140.

Stergios, B. (1976). Achene production, dispersal, seed germination, and seedling establishment of Hieracium aurantiacum in an abandoned field community. Canada Journal of Botany. 54, 1189-1197.

Cousens, R., & Williams N. (2011). Improved Detection and Eradication of Hawkweed (Hieracium) Experiments and second-generation dispersal models.

20. Are the plant’s propagules frequently dispersed via contaminated seed (agriculture or wildflower packets), equipment, vehicles, boats or clothing/shoes?

Yes or No:

Yes

Points:

Confidence Level:

High

Answer / Justification:

Hieracium aurantiacum seeds have minute barbs along their ribs that enable them to stick to clothing and vehicles (Williams et al 2007). Hikers, road maintenance, and ski equipment have been implicated in the species dispersal (National Heritage Trust 2003). Any activities that disturb plants, such as digging, can stimulate the growth of new plants from fragmented roots, stolons, and rhizomes (Rinella and Sheley 2002). Hieracium aurantiacum has also been found as part of wildflower seed mixes (National Heritage Trust 2003). Siskiyou County had a population of Hieracium aurantiacum resulting from escaped nursery propagules (personal comm. Marla Knight 2021).

Reference(s):

Williams, N., Hahs A., Morgan J., & Holland K. (2007). A dispersal constrained habitat suitability model for Orange Hawkweed (Hieracium aurantiacum) on the Bogong High Plains, Victoria.

Trust, N. Heritage (2003). Orange hawkweed (Hieracium aurantiacum).

Rinella, M., & Sheley R. (2002). Orange and Meadow Hawkweed. MontGuide Fact Sheet.

Evaluation Notes

CABI:
https://www.cabi.org/isc/datasheet/27160
Accessed 10/01/2021

Calflora:
https://www.calflora.org/app/taxon?crn=4186
Accessed 10/01/2021

CDFA:
https://blogs.cdfa.ca.gov/Section3162/?p=3825
Accessed 10/01/2021

FEIS:
https://www.fs.fed.us/database/feis/plants/forb/hieaur/all.html
Accessed 10/01/2021

GBIF:
https://www.gbif.org/species/3136120
Accessed 10/01/2021

iNaturalist:
https://www.inaturalist.org/taxa/711103-Pilosella-aurantiaca
Accessed 10/01/2021

Invasive Plant Atlas (Hieracium caespitosum):
https://www.invasiveplantatlas.org/subject.html?sub=4424
Accessed 10/01/2021