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Ecology and Management of Invasive Hawkweeds [ Hieracium spp]

Invasive Species Technical Note Number MT-16

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Invasive Species Technical Note MT-16 (PDF; 271 KB)

October 2007
By Jim Jacobs, Invasive Species Specialist, NRCS, Bozeman, Montana
Jessie Wiese, Research Associate, Montana State University, Bozeman, Montana

Abstract

The invasive hawkweeds (Hieracium spp; Asteraceae taxonomic family) are creeping perennial forbs with fibrous roots, creeping stolons, milky sap, and showy orange or yellow dandelion-like flowers (see Figure 1). There are four species known to grow in Montana; orange hawkweed (Hieracium aurantiacum) being the most common, infesting approximately 51,000 acres and meadow hawkweed (H. caespitosum; considered a complex of three highly hybridized species), also called yellow hawkweed, occupying an estimated 6,000 acres.

Figure 1. A large orange and yellow hawkweed infestation in a mountain meadow.
Figure 1. A large orange and yellow hawkweed infestation in a mountain meadow.

Native to Europe, orange hawkweed was introduced into the United States in the early 1800s as an ornamental due to its bright flashy flowers. Orange and meadow hawkweeds were first recorded in Montana from Flathead County in 1961 and 1993, respectively. As of 2007, 16 Montana counties, predominantly in the northwestern part of the state, had reported hawkweed infestations. They are listed as Category 2 noxious weeds, meaning they have been recently introduced into Montana and/or are rapidly spreading from current infestations. Management criteria for Category 2 weeds include awareness and education, prevention of movement into non-infested areas, monitoring and containment of known infestations, and eradication where possible.

Hawkweeds reproduce by seed and vegetatively by aboveground runners (stolons). Seeds are produced both sexually by pollination and asexually without pollination. Hawkweeds are very invasive due to their spread by seed initially and then local population expansion by stolons. Once established, hawkweed rosettes and seedlings can form a solid mat excluding desired species. Hawkweeds thrive on low productivity soils and are found in mountain meadows, deforested areas, high alpine hillsides, along roads and waterways, and in lawns.

Applications of nitrogen, phosphorous, and potassium fertilizer (300 pounds/acre 15:15:15) have been used to reduce hawkweed infestations on low productivity sites by increasing the competitiveness of desirable plants. Growth regulating herbicides such as 2,4-D, aminopyralid, clopyralid, and picloram can provide short-term control when applied in the spring or early summer. Surfactants in the spray solution will improve herbicide contact with the hairy stems and leaves and increase herbicide efficacy. Hawkweeds are excellent candidates for integrated weed management using combinations of fertilizer, herbicide, and seeding competitive plants. Sheep and rabbits eat the flowers of hawkweeds but have little impact on populations, and are more likely to utilize perennial grasses reducing their competitiveness with hawkweeds.

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Last Modified: 08/18/2008