Control of Orange Hawkweed (Hieracium aurantiacum) in Southern Alaska

Authors: Seefeldt, Steven; Conn, Jeffery

Submitted to: Invasive Plant Science and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/24/2010
Publication Date: 4/20/2011
Citation: Seefeldt, S.S., Conn, J.S. 2011. Control of Orange Hawkweed (Hieracium aurantiacum) in Southern Alaska. Journal of Invasive Plant Science and Management. 4(1):87-94.

Interpretive Summary: Land managers are often faced with the task of selecting a weed control treatment from among a wide variety of options. Frequently there are people with a wide variety of concerns, opinions, and ideas that will want to question or even legally challenge the manger’s decision. Orange hawkweed is a non-indigenous invasive plant species that is spreading rapidly in Alaska and there are few options for its control. Chemical control methods for orange hawkweed have not been studied in Alaska. In addition, there are concerns about impacts on native plant species and hopes that reduced rates of herbicides will result in acceptable control with minimal impacts on other plant species. The results of this research indicate that higher rates of both aminopyralid and clopyralid are needed to effectively control orange hawkweed. In a pasture system, where grasses are preferred to forbs and shrubs, aminopyralid has an advantage as it will control many other species compared to clopyralid and there will be an increase in grass productivity. In a field where the aesthetics of multiple forb species mixed with grass and willows is preferred, clopyralid will have a reduced impact many more of these species than will aminopyralid.

Technical Abstract: Orange hawkweed is a perennial European plant and an escaped ornamental that has colonized roadsides and grasslands in south central and southeast Alaska. This plant is forming near monotypic stands, reducing plant diversity and decreasing pasture productivity. A replicated greenhouse study was conducted in 2006 and repeated in 2007 to determine efficacy of six herbicides (aminopyralid, clopyralid, picloram, picloram +chlorsulfuron, picloram+metsulfuron, and triclopyr) for orange hawkweed control. Based on results of the greenhouse trials, replicated field studies were conducted at two sites each year in 2007 and 2008 with three rates each of aminopyralid and clopyralid to determine efficacy of orange hawkweed control and impacts on non-target native vegetaIn the field, only aminopyralid at 105 g ai/ha and clopyralid at 420 g ai/ha controlled orange hawkweed consistently, with peak injury observed one year after treatment. Control with clopyralid was slightly less than with aminopyralid at all observation times except at Homer in 2007, where there was a near monotypic stand of orange hawkweed and clopyralid did not remove all orange hawkweed plants. Aminopyralid controlled clover, wild celery, daisy, brittlestem hempnettle, and willow from the treated areas. Other plant species such as grasses and some annual forbs recovered or increased following control of the hawkweed. Clopyralid had less impact on non-target species with most recovering the year after treatment. In a pasture system, where grasses are preferred to forbs and shrubs, aminopyralid has an advantage as it controls a broader array of forbs compared to clopyralid. In natural areas, where the desire to retain biodiversity and the aesthetics of multiple forb species mixed with grass and willows is preferred, clopyralid will leave greater species diversity than aminopyralid. Preliminary greenhouse trials for determining the efficacy of aminopyralid and clopyralid for control of orange hawkweed was a useful predictor of results obtained in the field.