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Biology and Management of Invasive Hawkweeds (Hieracium spp.)
in the Pacific Northwest.

Linda M. Wilson and Timothy S. Prather 
P.O. Box 442339, Department Plant, Soil and Entomological Sciences, University of Idaho, Moscow 83844-2339, lwilson@uidaho.edu

In the western United States and Canada, there are about fourteen species of non-native hawkweeds (Hieracium spp.) belonging to two subgenera.  Hawkweeds are fibrous-rooted, perennial herbs growing from a stout rhizome.  Collectively, they possess many characteristics that enable a species to be invasive and dominate susceptible habitats: perennial, apomictic, high seed production and germinability, long distance seed dispersal, spread/regenerate from rhizome and stolon fragments, adventitious root buds, rapid generation time (ca. 63 days), and broad latitudinal range. In addition, several hawkweed species, particularly H. aurantiacum, orange hawkweed, are popular ornamental species, further increasing spread.  Invasive hawkweeds infest a range of habitats in the Pacific Northwest, predominantly occurring in open fields, mountain meadows, clearings in forest zones, permanent pastures, and other modified habitats where the soil is well drained, coarse-textured, and moderately low in organic matter.  Management of hawkweed-invaded sites has relied mostly on selective herbicides, which are effective in suppressing hawkweeds but reinvasion occurs unless other plant species fill the gaps left by hawkweed removal.  Hawkweeds are thought to persist in these sites because they capture nitrogen in nutrient-poor soils, thus limiting nutrients available to competing plants.  Fertilizers and soil fertility management have been used to effectively control hawkweeds in some areas, especially in new hawkweed infestations or where hawkweed density is relatively low.  Long-term management of hawkweed needs to emphasize altering conditions in the plant community to favor grasses and desirable forbs following initial hawkweed control efforts.  Alternative control solutions, including biological control, are being investigated.