About Hawkweeds in the Western US and Canada

Invasion Success

Habitat Preference

The name Hieracium comes from the Greek ‘hierax’, meaning hawk; allegedly keen-sighted hawks of yore ate the sap of the brightly colored plants to sharpen their eyesight.  In North America, invasive hawkweeds are an eyesore – and they are among the most troublesome weeds in the Pacific Northwest (PNW).  The first species arrived in the region as recently as seventy years ago, probably from the western expansion of infestations from eastern Canada and the US during or shortly after the Second World War.  There are now about 14 species of invasive hawkweeds in the PNW.  Rapid spread of hawkweed has been possible because much of the coastal and inland regions encompassing Washington, Idaho, Oregon, Montana and British Columbia is considered susceptible to invasion by these aggressive weeds.    

Invasive hawkweed commonly occur as populations of intermediate types throughout the PNW, making identification of invasive hawkweed species difficult.  Abundant variation in plant characteristics due to apomixis and perhaps occasional hybridization, environmental and site influences, and natural variation (polymorphism) has resulted in the description of thousands of species, subspecies and types worldwide. 

INVASION SUCCESS

 Hawkweeds possess many characteristics that allow a species to become invasive: perennial, apomictic, high seed production and germinability, long distance seed dispersal, spread/regenerate from root fragments, root buds, rhizomes and stolons, rapid generation time (ca. 63 days), and broad latitudinal range.  In addition, several hawkweed species, particularly orange hawkweed, are popular ornamentals.  They have been and continue to be spread by intentional and accidental human activities. 

HABITAT PREFERENCES 

Invasive hawkweeds infest similar habitats in Washington, Idaho, Oregon, Montana and British Columbia.  Found predominantly in open fields, mountain meadows and clearings in forest zones, hawkweeds also infest permanent pastures, cleared timber units, abandoned farmland and other modified habitats where the soil is well drained, coarse-textured, and moderately low in organic matter.  

Hawkweeds are preadapted to many habitats in the Pacific and Inland Northwest, and mesic habitats in the Intermountain West.  Throughout most of the PNW, hawkweeds are generally found at elevations ranging from 725 m (2400 feet) to over 1700 m (5500 feet).  They occur more commonly at lower elevations above 51°N latitude.  None of the invasive hawkweeds are found in the natural grasslands or shrub-steppe of the PNW, and they have not proven to be invasive in the dry habitats of southcentral British Columbia and central Washington.  

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