Orange hawkweed is a perennial European plant 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 the 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 nontarget native vegetation. In the field, only aminopyralid at 105 g ae ha⁻¹ (0.1 lb ae ac⁻¹) and clopyralid at 420 g ae ha⁻¹ controlled orange hawkweed consistently, with peak injury observed 1 yr after treatment. Control with clopyralid was slightly less than that provided by aminopyralid at all observation times, except at Homer, AK, in 2007, where there was a near-monotypic stand of orange hawkweed, and clopyralid did not remove all orange hawkweed plants. Aminopyralid controlled clover (Trifolium spp.), seacoast angelica (Angelica lucida), arctic daisy (Chrysanthemum arcticum), common hempnettle (Galeopsis tetrahit), and willow (Salix spp.) in 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 nontarget species with most recovering the year after treatment. In a pasture system, where grasses are preferred to forbs and shrubs, aminopyralid has an advantage because it controls a broader array of forbs compared with clopyralid. In natural areas, where the desire to retain biodiversity and the aesthetics of multiple forb species mixed with grasses and willows is preferred, clopyralid will leave greater species diversity than aminopyralid.

Nomenclature: Aminopyralid; chlorsulfuron; clopyralid; metsulfuron; picloram; triclopyr; Orange hawkweed, Hieracium aurantiacum L. HIEAU; arctic daisy, Chrysanthemum arcticum L. CHYAR; clover, Trifolium spp.; common hempnettle, Galeopsis tetrahit L. GAETE; seacoast angelica, Angelica lucida L. ANLU; willow, Salix spp

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 nonindigenous, 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 because it will control many other species compared with 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 effect on many more of these species than will aminopyralid.