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Hieracium aurantiacum

• INTRODUCTORY
• DISTRIBUTION AND OCCURRENCE
• BOTANICAL AND ECOLOGICAL CHARACTERISTICS
• FIRE EFFECTS AND MANAGEMENT
• MANAGEMENT CONSIDERATIONS
• APPENDIX: FIRE REGIME TABLE
• REFERENCES

INTRODUCTORY

AUTHORSHIP AND CITATION FEIS ABBREVIATION NRCS PLANT CODE COMMON NAMES TAXONOMY SYNONYMS LIFE FORM
	Photo by UAF Cooperative Extension Archive, University of Alaska-Fairbanks, Bugwood.org

Orange hawkweed may hybridize with other hawkweeds (Hieracium) [79], most notably tall hawkweed (H. piloselloides) [56,102].

In this report, the term "invasive hawkweeds" refers to species in the subgenus Pilosella that are nonnative to North America, including meadow hawkweed (H. caespitosum) and tall hawkweed.

DISTRIBUTION AND OCCURRENCE

• GENERAL DISTRIBUTION
• HABITAT TYPES AND PLANT COMMUNITIES

GENERAL DISTRIBUTION:
Orange hawkweed is native to northern and central Europe, occurring primarily in mountain meadows and hillsides [108]. It was introduced in Vermont by 1875 as a garden ornamental [102] and has been planted many times subsequently. Orange hawkweed has repeatedly escaped cultivation [108].

As of this writing (2010), orange hawkweed has a wide but discontinuous distribution in North America. Its distribution is most continuous from southern Canada and New England south to the mid-Atlantic states and west through the Great Lakes region to the Pacific Northwest. Its native distribution in Europe and current distribution in the United States suggest that orange hawkweed has the greatest potential for establishment in cool, subhumid to humid sites in the northern regions of the United States [108]. Orange hawkweed is also invasive in Australia [105] and New Zealand [48,51]. Plants Database provides a distributional map of orange hawkweed.

HABITAT TYPES AND PLANT COMMUNITIES:
Orange hawkweed occurs in a variety of plant communities within its North American distribution, including grasslands and other open areas, forests, and wetlands. A management guide suggests that sites most vulnerable to orange hawkweed establishment in the Pacific Northwest are disturbed areas, including roadsides, pastures, hay fields, abandoned farmland, mountain meadows, logged areas, and forest clearings [108].

Open plant communities: Floras commonly report orange hawkweed occurring in fields [24,26,27,60,67,76,79,85,102,110], pastures [76,102], meadows [26,67], and open hillsides [67]. On Prince Edward Island, orange hawkweed occurred in old fields regenerating into red spruce (Picea rubens) forest and was not found in forest interiors [14]. An extension service publication from Montana reports that orange hawkweed is closely associated with habitats that support oxeye daisy (Leucanthemum vulgare), sulfur cinquefoil (Potentilla recta) and spotted knapweed (Centaurea maculosa) [75]. Distribution records from Washington document orange hawkweed occurring on a dry gravel roadside with common St Johnswort (Hypericum perforatum), red alder (Alnus rubra), broom (Cytisus), pearly everlasting (Anaphalis), and field horsetail (Equisetum arvense) [74].

As of this writing (2010), there were few descriptions of open, native plant communities with orange hawkweed. On Nantucket Island, Connecticut, orange hawkweed was found on sand dunes [4]. It occurred infrequently in sand barren plant communities in southeastern Ontario. Sand barrens were dry openings surrounded by and interspersed with jack pine (Pinus banksiana) forest [13]. Orange hawkweed was one of several nonnative species to dominate upland bracken fern-grasslands in northern Wisconsin. Bracken fern-grasslands contained western bracken fern (Pteridium aquilinum), slender wheatgrass (Elymus trachycaulus), arctic brome (Bromus kalmii), poverty oatgrass (Danthonia spicata), and roughleaf ricegrass (Oryzopsis asperifolia) [15]. In another Wisconsin upland bracken-grassland on ridges and hillsides, orange hawkweed, poverty oatgrass, and lichens dominated the understory [100]. Orange hawkweed was infrequently encountered in heavily grazed pastures and ungrazed, native tallgrass prairie in north-central Oklahoma. Native tallgrass prairie contained indiangrass (Sorghastrum nutans), big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparium), paintbrush bluestem (Andropogon ternarius), and silver bluestem (Bothriochloa saccharoides) [52]. Orange hawkweed was also documented in the Palouse Prairie region of western Idaho [5]. A nonnative species guide to Alaska reported that orange hawkweed was rapidly expanding within a native forb and fern meadow on Camp Island in the Kodiak National Wildlife Refuge [35].

Forested plant communities: Orange hawkweed occurs in forested plant communities throughout its North American distribution. Forested plant community descriptions below are organized by forests that are primarily coniferous, primarily deciduous, or mixed coniferous-deciduous.

Coniferous forests: Orange hawkweed occurs in coniferous forests in the Northeast, Great Lakes region, and Northwest.

In southeastern New Brunswick, orange hawkweed was found in 3- to 8-year-old black spruce (Picea mariana) plantations (25% frequency). Surrounding forest contained red spruce, white spruce (P. glauca), balsam fir (Abies balsamea), sugar maple (Acer saccharum), red maple (A. rubrum), yellow birch (Betula alleghaniensis), and paper birch (B. papyrifera) [99]. In Maine, orange hawkweed occurred in large, recent (<15-year-old) clearcuts in red spruce forests [17]. In the southern Adirondack Mountains of New York, orange hawkweed occurred in forest openings in old-growth eastern white pine-eastern hemlock (Pinus strobus-Tsuga canadensis) forest [65]. In southeastern Ontario, orange hawkweed occurred at 10% to 45% frequency in approximately 70-year-old coniferous forests containing white spruce, eastern white pine, balsam fir, and black spruce [61].

In northern Michigan, orange hawkweed was detected infrequently in mesic forests dominated by white spruce and balsam fir [62]. It was common 2 years after prescribed fire in a 35-year-old jack pine forest in northern Lower Michigan [1]. In northern Wisconsin, orange hawkweed was found along the edges and in the interior of jack pine and red pine (P. resinosa) plantations [21]. In northern Wisconsin, orange hawkweed was detected with low frequency (5%) in jack pine barrens consisting of open, shrubby areas with scattered young jack pine and infrequent older red pine. Scattered patches of regenerating or mature aspen (Populus spp.), oak (Quercus spp.), and maple (Acer spp.) were also present [8].

Distribution records from Montana documented orange hawkweed occurring in subalpine fir/dwarf huckleberry (A. lasiocarpa/Vaccinium caespitosum), grand fir/queencup beadlily (Abies grandis-Clintonia uniflora), and subalpine fir/queencup beadlily-beargrass (Xerophyllum tenax) plant communities [74]. It was detected in successional lodgepole pine (Pinus contorta) forests in northwestern Montana [43]. In British Columbia, orange hawkweed occurred at low levels in mixed Douglas-fir (Pseudotsuga menziesii)-lodgepole pine forests that had undergone single seed-tree retention harvests 5 to 8 years prior to sampling [87]. Orange hawkweed was scarce in 6- to 10-year-old Engelmann spruce (Picea engelmannii)-subalpine fir plantations in southeastern British Columbia. [50].

Deciduous forests: Orange hawkweed occurs in deciduous forests in the Southeast, Northeast, Great Lakes region, and the Northwest.

A flora reports orange hawkweed occurring in openings in oak-hickory (Quercus-Carya) forests in North and South Carolina [72].

On Prince Edward Island, orange hawkweed occurred in interior upland hardwood forests, forest edge, and adjacent developed communities. Hardwood forests were dominated by yellow birch, sugar maple, and American beech (Fagus grandifolia) [59]. In northeastern New Brunswick, orange hawkweed occurred in the understory of 7- to 25 -year-old mixed-hardwood forests. Orange hawkweed cover ranged from 2% to 14%, with no clear pattern of cover relative to stand age. Mixed-hardwood forests contained a mixture of pin cherry (Prunus pensylvanica), red maple, quaking aspen (Populus tremuloides), and paper birch [58]. In central New York, orange hawkweed occurred in the interior of sugar maple forests adjacent to residential areas. American beech, basswood (Tilia americana), eastern hemlock, and white ash (Fraxinus americana) were other common overstory trees [68].

Orange hawkweed occurred in several aspen (Populus) plant associations in northern Lower Michigan. It occurred in seral forests dominated by bigtooth aspen (P. grandidentata), paper birch, and quaking aspen; hardwood forests with pin cherry, paper birch, red maple, American beech, sugar maple, and bigtooth aspen; and lowland or bog forests containing quaking aspen and bigtooth aspen [25]. In northern Michigan, orange hawkweed occurred on a sandy ridge in a pin cherry plant association. The forest was dominated by short (<33 feet (10 m) in height) pin cherry trees but also contained red maple and small amounts of black spruce [20]. On the Upper Peninsula of Michigan, orange hawkweed occurred in northern hardwood forests dominated by sugar maple, with lesser amounts of yellow birch, basswood, eastern hemlock, red maple, and eastern hophornbeam (Ostrya virginiana) [10]. In the same region, orange hawkweed dominated the understory of a dry, open area near a quaking aspen forest. It also occurred in the understory of a floodplain with speckled alder (Alnus incana subsp. rugosa), black ash (F. nigra), balsam poplar (P. balsamifera), and quaking aspen [86].

Distribution records from Idaho document orange hawkweed occurring in a "weedy aspen grove" [74].

Mixed coniferous-deciduous forests: In a beach-dune complex on the shores of Lake Michigan in eastern Wisconsin, orange hawkweed was infrequent in ridge forests codominated by red maple, northern whitecedar (Thuja occidentalis), eastern white pine, red pine, eastern hemlock, and paper birch [97]. In northern Wisconsin and the Upper Peninsula of Michigan, orange hawkweed was detected frequently in upland forests dominated by eastern white pine, eastern hemlock, and/or sugar maple [104].

Wetland plant communities: Orange hawkweed occurs in wetland plant communities with variable tree canopy cover. In north-central Massachusetts, orange hawkweed occurred in a small (500 feet (150 m) in diameter), herbaceous wetland adjacent to mixed-mesophytic forests of red maple, American beech, paper birch, gray birch (B. populifolia), white ash, northern red oak (Q. rubra), eastern white pine, and eastern hemlock [63]. Orange hawkweed occurred with high cover in a rich fen in southwestern New York [23]. Orange hawkweed was found in a bog mined for peat in southern Ontario. Quaking aspen and birch (Betula spp.) dominated disturbed areas, while open stands of birch were found on relatively undisturbed areas [44]. On the Upper Peninsula of Michigan, orange hawkweed occurred in a sedge (Cyperaceae) fen. Broadleaf deciduous forests dominated upland areas, while a mixture of sedges, grasses, and low shrubs dominated low wetland areas [49]. In Wisconsin, orange hawkweed occurred in sedge meadows used for commercial cranberry production. Sedge meadows were dominated by sedges, asters (Asteraceae), knotweeds (Polygonaceae), ferns (Osmundaceae, Polypodiaceae), and roses (Rosaceae) [45].

Orange hawkweed is described as a dominant groundlayer species in the following vegetation classification from Wisconsin:

• upland western bracken fern-grasslands; slender wheatgrass, arctic brome, poverty oatgrass, and roughleaf ricegrass are characteristic grass dominants [15]

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

• GENERAL BOTANICAL CHARACTERISTICS
• SEASONAL DEVELOPMENT
• REGENERATION PROCESSES
• SITE CHARACTERISTICS
• SUCCESSIONAL STATUS

Orange hawkweed is a rhizomatous [12,16,26,51,107,108], and/or stoloniferous perennial herb [107] that exudes a milky sap [108]. Plants have a basal rosette of leaves covered in trichomes. Each rosette produces 10 to 30 flowering stems that are 10 to 36 inches (25-91 cm) tall. Each inflorescence consists of 5 to 39 orange flower heads. The seeds are tiny, with a tawny tuft of barbs at one end [108]. Flowering stems may also arise from spreading stolons [16,26,72,85]. Plants produce from 4 to 12 leafy stolons that can reach a length of 4 to 12 inches (10-30 cm) [108]. Stolons have been described as slender [26,108] or stout [16].
		Photo by Michael Shephard, USDA Forest Service, Bugwood.org

Orange hawkweed roots are shallow and fibrous [12,51,107,108]. A fact sheet from the Pacific Northwest describes orange hawkweed rhizomes as short, resembling a root crown [12].

Orange hawkweed may be allelopathic. Laboratory studies suggest that orange hawkweed is pollen-allelopathic, limiting the sexual reproduction of other plants by inhibiting pollination, germination of seeds, or seedling growth [69]. In western bracken fern-grasslands in northern Wisconsin, one author observed that orange hawkweed produced a "potent antibiotic" that was detrimental to other plants except western bracken fern [15]. One weed management guide reported that orange hawkweed may have allelopathic tendencies [35].

A weed management guide reports that orange hawkweed seeds lack an after-ripening period and may germinate as soon as they are released from the parent plant [108] (see Germination). Stolons elongate through the summer, forming daughter rosettes at their tips. The stolons die as roots anchor daughter rosettes, and the young plants become independent of the mother plant [12]. After flowering, the supporting rosette dies [82]. New plants sprout each year from rhizomes [12].

Pollination and breeding system: Orange hawkweed is mostly apomictic. In New Zealand, most (93.8%) orange hawkweed seed was produced without fertilization [48]. It occasionally produces seed through pollination [12]. Orange hawkweed is pollinated by wind [33] and is visited by insects. On the Upper Peninsula of Michigan, orange hawkweed was an important source of pollen for bees (Megachile relativa and M. inermis) [86]. In Maine, orange hawkweed was visited by bumblebees (Bombus spp.), though the author noted that orange hawkweed offered minute quantities of nectar [34]. It was visited by nectar-collecting yellowbanded bumble bees (B. terricola) in old fields in eastern Ontario [71]. Orange hawkweed was visited by several butterfly species in Michigan [101], and was identified as a nectar species for the Karner blue butterfly in Wisconsin [47].

Seed production: Orange hawkweed may produce many seeds. An extension service publication reported that a single orange hawkweed flower can produce 12 to 50 tiny black seeds [75]. A nonnative species guide from Alaska reported that flowering stems produce hundreds of seeds [35]. An Australian government publication reports that an 11-foot² (1 m²) mat of orange hawkweed may produce up to 40,000 seeds per year [105].

An extension service publication suggests that seed production is density dependent, with production decreasing as population density increases. Most seed production occurs on the periphery of orange hawkweed populations, where plant density is typically lower than in population interiors [75].

In successional old fields in southern Michigan, daughter rosettes usually did not flower the year of establishment but flowered the spring of the following year [82]. In greenhouse experiments in east-central Minnesota, 35% of 3-month-old orange hawkweed seedlings flowered [37].

Seed dispersal: Orange hawkweed seeds mostly disperse over short distances, though long-distance dispersal may occur. In successional old fields in southern Michigan, most orange hawkweed seeds (94.9% and 82.7% in 2 different years) fell within 3 feet (1 m) of the parent plant [82]. However, several sources report that orange hawkweed seed may be dispersed long distances. A fact sheet from the Pacific Northwest reports that minute barbs along the seeds' ribs enable seeds to stick to hair, fur, feathers, clothing, and vehicles [12]. An Australian government publication reported that road maintenance equipment, hikers, and ski equipment have been implicated in long-distance orange hawkweed dispersal [105]. An extension service publication suggests that seeds may be transported in contaminated landscaping soil [75]. Orange hawkweed seeds and plant fragments may also be transported downslope by water [105].

Seed banking: Orange hawkweed apparently maintains a persistent seed bank, though the longevity of orange hawkweed seeds is not clear. Several secondary sources [32,35,75,105] report that orange hawkweed seeds may survive for up to 7 years in the soil seed bank, but a primary reference for this information was not identified.

Soil seed bank studies detected few or no orange hawkweed seedlings emerging from soil samples. Orange hawkweed seeds were found at a density of 34.8 seeds/m² in soil samples from a mixed-hardwood woodlot in southern Ontario. Two months after collection, orange hawkweed seedlings had not emerged from soil samples placed in a greenhouse [9]. One orange hawkweed seedling emerged from near-surface (1 to 2 inches (2-4 cm)) soil samples taken in mid-June from a jack pine community in eastern Ontario [103]. In mowed and undisturbed successional old fields in Germany, orange hawkweed was present at low levels in the extant vegetation, but seedlings did not emerge from soil samples [18].

Germination: Orange hawkweed seeds do not have an after-ripening period and can germinate immediately after dropping from the plant. Fall-germinated seedlings usually do not survive [12]. In germination trials, orange hawkweed seeds were gathered from successional old fields in southern Michigan and were not exposed to cold stratification. Germination rates declined through the season; for seeds gathered in June, July, August, and September, total germination was 66%, 28%, 18.5%, and 13% respectively. Cold stratification improved germination of late-season seeds but had little effect on early-season seeds. Small seeds had lower germinability than large seeds [82]. Orange hawkweed plants collected in New Zealand had high seed set (91%), and most seeds (71%) germinated on nutrient agar within 4 days. A few seeds produced multiple seedlings from individual seeds [48].

Seedling establishment and plant growth: A fact sheet from the Pacific Northwest suggests that most new populations of orange hawkweed establish via seed and then expand vegetatively [12]. In successional old fields in southern Michigan, few orange hawkweed seedlings established within patches of orange hawkweed, and those that established outside of patches did not survive their first winter [82].

The ability of orange hawkweed seedlings to establish may be limited by the presence of other vegetation. In Japan, the presence of other vegetation seemed to be the most important factor inhibiting establishment; it was difficult for orange hawkweed seedlings to establish in pastures with dense vegetation [88]. In experiments in old fields in east-central Minnesota, legume, moss, and leaf litter cover was negatively correlated with the size of orange hawkweed plants (P=0.001), while graminoid, forb, and bare ground cover was not [37]. In northern Wisconsin, shading by western bracken fern was reported as detrimental to orange hawkweed plants [15].

One study from Canada found that increased light and nutrients promoted orange hawkweed growth. In growth chamber experiments, orange hawkweed plants showed improved growth and net assimilation rates when grown in high (1,100 µmol/m²/s PAR) versus low (200 µmol/m²/s PAR) light conditions. An increase in nutrients also improved growth and net assimilation rates [64].

Once established, orange hawkweed populations are persistent [76] and may occur at high density or in a monoculture [74,82,88,105]. Orange hawkweed occurred in pastures in Japan at a density of > 1,000 plants/m² [88]. Distribution records from Montana document orange hawkweed populations occurring as both scattered plants and as monocultures [74]. In jack pine barrens in northern Wisconsin, orange hawkweed cover averaged 4.2% but ranged from 0.1% to 30%. [8].

Patches of orange hawkweed may cover extensive areas. An Australian government publication suggests that under ideal conditions, an orange hawkweed plant can form a 5-foot² (0.5 m²) mat in its first year. One population in Australia consisted of 20 to 50 plants covering 160 feet² (15 m²) [105]. In Maine, orange hawkweed covered a 915-foot² (85 m²) area [34]. In Washington, an orange hawkweed patch covered 200 feet² (20 m²) [74].

Vegetative regeneration: Orange hawkweed spreads vegetatively via stolons [16,26,38,51,72,85,108] and rhizomes [16,26,51,107,108]. Orange hawkweed sprouted following herbicide application in Japan [88].

In greenhouse experiments in east-central Minnesota, 91% of 3-month-old orange hawkweed seedlings produced stolons [37].

A fact sheet from the Pacific Northwest suggests that most orange hawkweed spread is accomplished vegetatively once a population is established [12]. An extension service publication suggests that stolon production is density dependent; thus, most vegetative spread occurs on the periphery of established populations where plant density is typically low [75].

Soil: Several weed management summaries report that soils that are well drained, textured, and moderately low in organic matter are most susceptible to establishment by invasive hawkweeds [12,19,75].

Orange hawkweed occurs on a variety of soil textures. In New Brunswick, orange hawkweed occurred on gravelly silt loam with large shale fragments [29]. On Nantucket Island, it occurred on sand dunes [4]. In central New York, orange hawkweed was abundant on fine sandy loam, silt loam, and silt clay loam, and scarce on marly silt [92]. Herbarium records from Wisconsin document orange hawkweed occurring on sand, sandy loam, and mixed sand and gravel [96]. An invasive plant guide for the Upper Midwest states that invasive hawkweeds prefer sandy or gravelly soil [16]. Distribution records from Montana document orange hawkweed occurring on silt loam amongst scree [74].

Several sources report that orange hawkweed occurs on well-drained soil [12,19,35,75,76]. In old fields in east-central Minnesota, soil moisture had a significant negative effect on size of orange hawkweed (P=0.013) [37]. Herbarium records from Wisconsin document orange hawkweed occurring on dry and moist soils [96]. Orange hawkweed occurs in several wetland plant communities. See Habitat Types and Plant Communities for detailed descriptions of wetland plant communities where orange hawkweed occurs.

Orange hawkweed may tolerate low-nutrient, acidic, or disturbed soils. In Japan, it usually establishes on "poor" soils [88]. A weed management guide suggests that invasive hawkweeds tolerate low-productivity soils [108]. An invasive plant guide for the Upper Midwest states that invasive hawkweeds prefer slightly acidic conditions [16]. In Japan, orange hawkweed was usually found on acidic soils, but acidic soils were not always "preferred" [88]. One flora reports that orange hawkweed occurs on disturbed soil [66].

Researchers in Japan suggested that the presence of other vegetation was more important than soil characteristics in determining orange hawkweed establishment [88].

Climate: In North America, orange hawkweed occurs in climates ranging from the humid, continental climate of central New York [68] to the subboreal climate of southeastern British Columbia [50]. Orange hawkweed's distribution in Europe and current distribution in the United States suggest that it is of greatest threat in cool, subhumid to humid sites in the northern regions of the United States [108]. However, one study found that nonnative populations of orange hawkweed occupied different climatic niches than native populations. Populations in eastern North America occurred in warmer, wetter conditions than those of the native range. Populations in western North America also had a broad climatic niche, generally occurring in warmer conditions than native populations [3].

Elevation: Orange hawkweed occurs at a wide range of elevations in North America.

Disturbance appears to favor orange hawkweed. An Australian government publication reports that orange hawkweed is most likely to establish along roads, hiking trails, and other areas experiencing human disturbances [105]. A weed management guide reports that orange hawkweed spread is enhanced by mechanical disturbances—including control efforts and machinery use—as well as local disturbances such as livestock, wild ungulate, or rodent activity [108]. In west-central Wisconsin, orange hawkweed established in high abundance on heavily disturbed construction remediation sites [47]. In a study examining the effects of disturbance created by commercial cranberry (Vaccinium macrocarpon) production on wetland plant communities in Wisconsin, the importance value of orange hawkweed occurring in sedge meadow communities decreased with distance from disturbance (P=0.007) [45]. Orange hawkweed also occurs in areas that have been mowed [84], grazed [52], logged [17,39,43,50,87,99], burned [1,29,43,70], logged and burned [1,30,43,80], and mined for peat [44].

FIRE EFFECTS AND MANAGEMENT

• FIRE EFFECTS
• FUELS AND FIRE REGIMES
• FIRE MANAGEMENT CONSIDERATIONS

Immediate fire effect on plant: As of this writing (2010), there was no published information on the immediate effects of fire on orange hawkweed. Orange hawkweed is likely top-killed by fire; belowground rhizomes may survive. As of 2010, no information was available regarding fire effects on or heat tolerance of orange hawkweed seeds.

Postfire regeneration strategy [83]:
Surface rhizome and/or a chamaephytic root crown in organic soil or on soil surface
Rhizomatous herb, rhizome in soil
Geophyte, growing points deep in soil
Ground residual colonizer (on site, initial community)
Initial off-site colonizer (off site, initial community)
Secondary colonizer (on- or off-site seed sources)

Fire adaptations and plant response to fire:
Fire adaptations: Orange hawkweed exhibits some characteristics that make it adapted to surviving and establishing after fire. It is rhizomatous [16,26,51,107,108], and it is likely that rhizomes below the soil surface survive fire. It reportedly sprouts from stolons and rhizomes after mechanical disturbance [105], and sprouted following herbicide application in Japan [88]. Orange hawkweed seeds have the potential for long-distance seed dispersal, and seeds reportedly persist in the seed bank for at least 7 years. Seedling establishment and plant growth seem to be favored by disturbance and high-light conditions (see Successional Status).

Plant response to fire: Several studies document orange hawkweed occurring in areas burned by wildfire [43,80,107] or prescribed fire [1,29,30,70,90], but to date (2010), a lack of details about fire characteristics, pre- and postfire vegetation, and orange hawkweed response limit the inferences that can be made from these studies. The limited information available suggests that fire may facilitate orange hawkweed establishment and spread.

It is not clear whether fire increases the abundance of orange hawkweed. An Australian government publication reports that existing orange hawkweed populations spread following a January wildfire [105]. Two years after prescribed fire in logged jack pine-red pine forest in northern Lower Michigan, orange hawkweed was one of several forbs commonly found in the understory [1], but its abundance prior to the fire was not reported.

Some studies have documented orange hawkweed establishing after fire in sites where it was not found previously. In eastern Ontario, orange hawkweed was found in a jack pine clearcut within 2 years of prescribed burning. It was not detected in the stands prior to fire [90]. In New Brunswick, a mixed deciduous-coniferous woodlot was clearcut, left untreated for a year, then burned in April for 2 consecutive years to encourage low sweet blueberry (Vaccinium angustifolium) and velvetleaf blueberry (V. myrtilloides) production. June vegetation surveys showed that orange hawkweed was not present prior to treatments, in the year between clearcutting and burning, or in the 1st year after fire. Four orange hawkweed stems were detected 2 months after the 2nd fire. The author suggested that orange hawkweed established via wind-dispersed seed [29].

Orange hawkweed was detected in surveys of 3 managed coniferous forest stands in British Columbia. Prior to treatment, the forests largely contained hybrid spruce (P. engelmannii × P. glauca). Stands were clearcut in the winter and burned the following autumn. Fire was of moderate severity and generally consumed all fine fuels but exposed little mineral soil. Stands were planted with lodgepole pine and/or hybrid spruce the year after fire. Vegetation was sampled prior to treatments and for 10 years afterward. Orange hawkweed was not detected prior to burning or in surveys 1, 2, 3, and 5 years after fire, but it was detected at low levels (0.02% mean cover) 10 years after fire on all 3 sites [30].

One study documented orange hawkweed in burned areas but not in unburned plots. It was detected after both single and multiple low-severity spring prescribed fires in a red pine and eastern white pine plantation in Michigan's southwestern Lower Peninsula. Fire conditions are available in the Research Project Summary of this study. In study plots burned once, the average cover of orange hawkweed 4 growing seasons after fire was 0.33%. In study plots burned biennially, its average cover 2 growing seasons after the 2nd fire was 0.07%, and it did not occur the first growing season after the 3rd fire. It was not present in unburned plots [70].

The combination of fire and other disturbances may increase the potential for orange hawkweed establishment. In northwestern Montana, orange hawkweed was studied in mountain meadows and 3 types of successional lodgepole pine forests: 1) logged, 2) burned by wildfire [43] 12 years previously, and 3) burned by wildfire 12 years previously [42] and then salvage-logged. Descriptions of the wildfire or logging treatments were not reported. Orange hawkweed had the highest probability of detection on burned and salvage-logged sites (P=0.002), which the authors attributed to relatively low levels of "competitive" interactions with other plants and high levels of sunlight and disturbance. However, it was not clear that disturbance was the only factor influencing orange hawkweed establishment, because orange hawkweed had a higher likelihood of establishment in mountain meadows than the 2 other disturbed cover types [43].

Fire regimes: It is not known what fire regime orange hawkweed is best adapted to. In North America, orange hawkweed occurs in a wide variety of plant communities, and consequently, a range of fire regimes. See the Fire Regime Table for further information on fire regimes of vegetation communities in which orange hawkweed may occur. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".

Preventing postfire establishment and spread: Because of its potential for long-distance seed dispersal and the suggestion that orange hawkweed established on burned sites via wind-dispersed seed [29], monitoring areas in close proximity to known populations of orange hawkweed for potential establishment is advised. Orange hawkweed establishment may occur soon after fire (e.g., 1 year after wildfire in British Columbia [80], 2 years after prescribed fire in Ontario [90] and New Brunswick [29], 2 and 4 growing seasons after prescribed fire in Michigan [70], and 4 years after wildfire in Australia [105]), or may be delayed (e.g., 10 years after wildfire in British Columbia [30]). These observations, combined with orange hawkweed's persistence in the seed bank, suggest that long-term monitoring may be necessary to limit orange hawkweed's establishment in burned areas.

• Incorporate cost of weed prevention and management into fire rehabilitation plans
• Acquire restoration funding
• Include weed prevention education in fire training
• Minimize soil disturbance and vegetation removal during fire suppression and rehabilitation activities
• Minimize the use of retardants that may alter soil nutrient availability, such as those containing nitrogen and phosphorus
• Avoid areas dominated by high priority invasive plants when locating firelines, monitoring camps, staging areas, and helibases
• Clean equipment and vehicles prior to entering burned areas
• Regulate or prevent human and livestock entry into burned areas until desirable site vegetation has recovered sufficiently to resist invasion by undesirable vegetation
• Monitor burned areas and areas of significant disturbance or traffic from management activity
• Detect weeds early and eradicate before vegetative spread and/or seed dispersal
• Eradicate small patches and contain or control large infestations within or adjacent to the burned area
• Reestablish vegetation on bare ground as soon as possible
• Avoid use of fertilizers in postfire rehabilitation and restoration
• Use only certified weed-free seed mixes when revegetation is necessary

MANAGEMENT CONSIDERATIONS

• FEDERAL LEGAL STATUS
• OTHER STATUS
• IMPORTANCE TO WILDLIFE AND LIVESTOCK
• OTHER USES
• IMPACTS AND CONTROL

Orange hawkweed flowers are visited by a variety of insects, including bees [22,34,86,101] and butterflies [47,101]. Though leaves are high in nutrients, trichomes limit insect herbivory [78]. Orange hawkweed was damaged by meadow spittle bugs (Philaenus spumarius) in old fields in east-central Minnesota [37].

Cover value: No information is available on this topic.

Impacts: A variety of fact sheets, government publications, and weed management guides suggest that orange hawkweed populations form dense mats that exclude other plants [35,75], including native plants [3,12,75,105] and economically valuable forage species [12,40,75,108]. A 2007 Australian government publication reported that orange hawkweed could cause an estimated $48 million in agricultural production losses [105]. Orange hawkweed may also be a problem in lawns and gardens [75].
		Photo by Richard Old, XID Services, Inc., Bugwood.org

Orange hawkweed's ability to dominate an area is attributed to fast vegetative spread [12,75,108], allelopathy [15,35,69], and tolerance of low-nutrient sites [35,75].

Control: Control of orange hawkweed is complicated by its ability to sprout from stolons [12,108] and rhizomes [12] following control treatments. An extension service publication suggests control efforts should concentrate on the periphery of established populations, where most sexual and vegetative reproduction occurs [75]. Control effectiveness may depend on a program that integrates multiple management procedures such as fertilizers, herbicides, seeding, and other techniques that decrease orange hawkweed spread and favor desired species [35].

In all cases where invasive species are targeted for control, no matter what method is employed, the potential for other invasive species to fill their void must be considered [7]. Control of biotic invasions is most effective when it employs a long-term, ecosystem-wide strategy rather than a tactical approach focused on battling individual invaders [57].

Prevention: A fact sheet from British Columbia suggests orange hawkweed establishment may be prevented by minimizing soil disturbance and promptly revegetating disturbed areas. Consumers should avoid purchasing wildflower seed mixes that contain invasive hawkweeds [40].

It is commonly argued that the most cost-efficient and effective method of managing invasive species is to prevent their establishment and spread by maintaining "healthy" natural communities [57,81] (e.g., avoid road building in wildlands [93]) and by monitoring several times each year [41]. Managing to maintain the integrity of the native plant community and mitigate the factors enhancing ecosystem invasibility is likely to be more effective than managing solely to control the invader [36].

Weed prevention and control can be incorporated into many types of management plans, including those for logging and site preparation, grazing allotments, recreation management, research projects, road building and maintenance, and fire management [95]. See the Guide to noxious weed prevention practices [95] for specific guidelines in preventing the spread of weed seeds and propagules under different management conditions.

Cultural control: In Japan, orange hawkweed establishment and persistence were limited by other vegetation and it was not able to establish via seed in pastures with dense vegetation [88]. A weed management guide suggests that cultural methods are useful for controlling orange hawkweed in pastures or rangelands. Where perennial grasses, legumes, or other preferred forbs are present, fertilizers may be applied to increase nutrient availability to the desired species [75,108]. A fact sheet from the Pacific Northwest reports that, depending on soil productivity and grass condition, a single nitrogen application may be sufficient for grasses to suppress invasive hawkweed growth for 3 to 5 years. Good grazing management may extend this period. Integrated management techniques may increase the effectiveness of cultural methods in controlling orange hawkweed.

Physical or mechanical control: A weed management guide from Australia reports that any activities that disturb orange hawkweed plants, such as digging or grazing, can stimulate the growth of new plants from fragmented roots, stolons and rhizomes [105]. Physical disturbance by machinery may spread orange hawkweed across fields, and local disturbances such as activity by livestock, wild ungulates, or rodents may enhance its spread [108].

A fact sheet from the Pacific Northwest suggests that small infestations of orange hawkweed may be eradicated by carefully digging out rosettes; for this technique to be effective, all stolons, rhizomes, and roots must be collected to prevent on-site sprouting [12]. A nonnative species guide from Alaska also reports that hand-removal of plants may be effective if all parts are removed. Hand removal from lawns is more effective if used in conjunction with fertilizer [35].

Several sources report that mowing does not control orange hawkweed because basal rosettes are too low to be impacted by mowing blades [12,35,105,108]. Mowing may prevent or limit seed production, but it may also encourage vegetative spread [12,105,108]. In northeastern Georgia, dozens of orange hawkweed plants were reported from a frequently mowed, grassy area. [84]. Livestock consume orange hawkweed, but grazing encourages vegetative spread just as mowing does [75].

A weed management guide states that orange hawkweed does not appear to survive in annually tilled cropland [108]. A summary of invasive hawkweeds reports that they do not persist with tillage, particularly where herbicide use and tillage are combined [19].

Biological control: Both insects [106,108] and fungi [108] were being evaluated for use in controlling orange hawkweed and other invasive hawkweeds as of 2010.

Biological control of invasive species has a long history that indicates many factors must be considered before using biological controls. Refer to these sources: [98,109] and the Weed control methods handbook [91] for background information and important considerations for developing and implementing biological control programs.

Chemical control: Herbicides may control orange hawkweed [12,35,40,75,108], though orange hawkweed sprouted following herbicide application in Japan [88]. An invasive plant guide for the Upper Midwest states that herbicides have effectively controlled orange hawkweed while in the rosette stage, though a surfactant may be needed to enhance contact with hairy leaves [16]. Integrated management techniques may increase the effectiveness of herbicide application in controlling orange hawkweed. See the following weed management guides for information about using herbicides to control orange hawkweed: [12,16,35,40,75].

Herbicides are effective in gaining initial control of a new invasion or a severe infestation, but they are rarely a complete or long-term solution to weed management [11]. See the Weed control methods handbook [91] for considerations on the use of herbicides in natural areas and detailed information on specific chemicals.

APPENDIX: FIRE REGIME TABLE

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