Botanical description: This description covers characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identification are available (e.g., [22,37,43,70,73,75]).
Tall hawkweed is a perennial herb that exudes a milky sap when broken. Plants have a basal rosette and flowering stems that reach 12 to 42 inches (30-107 cm) in height. Inflorescences are 2 or more clusters of yellow flowers at the end of stems . Tall hawkweed seeds are small (1.5-2 mm) achenes with a row of bristles . One flora states that tall hawkweed may be variable in form .
Tall hawkweed has short [13,22,61], stout rhizomes  but lacks stolons [13,73]. In old fields in north-central New Jersey, one author observed that tall hawkweed had shallow roots . In central New York, tall hawkweed roots penetrated approximately 7 inches (17 cm) into a substrate of finely ground limestone .Photo courtesy of Richard Bauer, University of Wisconsin-Stevens Point
Density of tall hawkweed populations is variable. In a revegetating limestone quarry in central New York, the density of adult rosettes ranged from 34.0 plants/m² to >100 plants/m² . In a field in southeastern Ontario, a nearly pure population of tall hawkweed covered 300 acres (125 ha), with an average density of 860 plants/m² *. In an abandoned pasture in southeastern Ontario, tall hawkweed populations were patchy but not dense .
*Some information in General Botanical Characteristics comes from Johnson and Thomas , who tentatively identified the hawkweed studied as tall hawkweed, but noted that it differed morphologically from other tall hawkweed populations in Ontario. The authors reported that the species had been identified in the past as either H. florentinum , now considered a synonym of tall hawkweed [10,51,65], or H. praeltum var. decipiens .
Potential for postfire establishment and spread: Tall hawkweed possesses several traits that make it adapted to surviving and/or establishing after fire (see Fire adaptations), and postfire establishment has been documented (see Plant response to fire). Disturbances associated with fire (e.g., bulldozing firelines ) may also encourage tall hawkweed establishment.
Preventing postfire establishment and spread: Because of its potential for long-distance seed dispersal and the possibility that tall hawkweed established on burned sites via wind-dispersed seed , monitoring areas in close proximity to known tall hawkweed populations is advised.Preventing invasive plants from establishing in weed-free burned areas is the most effective and least costly management method. This may be accomplished through early detection and eradication, careful monitoring and follow-up, and limiting dispersal of invasive plant propagules into burned areas. General recommendations for preventing postfire establishment and spread of invasive plants include:
Fire adaptations: Tall hawkweed exhibits some characteristics that make it likely to survive and/or establish after fire. It is rhizomatous [13,22,61], and it is likely that rhizomes below the soil surface survive fire. It sprouted from rhizomes and/or root crowns within 100 days of a severe fire in alvar woodlands near Ottawa, Ontario . Tall hawkweed also sprouts from adventitious root buds [41,42,61]. Though not documented in the available literature, it is likely that tall hawkweed root buds could survive and sprout after fire. Tall hawkweed seeds have the potential for long-distance dispersal. The authors of one study from northern Lower Michigan suggested that tall hawkweed established in burned areas via wind-dispersed seeds . A row of bristles on tall hawkweed seeds  may facilitate other means of dispersal (e.g., attachment to clothing, fur, or feathers) into burned areas. The emergence of a few tall hawkweed seedlings from soil samples collected in Ontario  suggests that it is possible for tall hawkweed to establish in burned areas via soil-stored seed (see Seed banking). Though tall hawkweed is frequently documented in early-successional plant communities or disturbed areas (see Site Characteristics), it is not clear which characteristics of these sites favor tall hawkweed.
Plant response to fire: Information on the response of tall hawkweed to fire was sparse in the literature as of this writing (2010). Tall hawkweed has been documented in burned areas in Michigan  and Ontario [10,11,12]. The scarcity of studies and lack of details about fire characteristics, pre- and postfire vegetation, and tall hawkweed response limit the inferences that can be made regarding fire effects on tall hawkweed.
One study reported tall hawkweed surviving fire. A severe fire occurred in late June in alvar woodlands near Ottawa, Ontario. The fire burned 376 acres (152 ha), moving up to 50 feet (15 m)/minute, with flames reaching >100 feet (30 m) in height. One hundred days after the fire, vegetation surveys were conducted in 5 areas where all vegetation was top-killed or killed by fire. Tall hawkweed sprouted from rhizomes and/or root crowns in all 5 sampling areas, with an average frequency of 6.4 % (range 2-14%) .
Though tall hawkweed may be frequent on some burned sites, no pattern of change through time was evident in the literature. In northern Lower Michigan, tall hawkweed occurred in successional forests burned by both wildfire (1 fire: 1911) and prescribed fires (4 fires: 1936, 1948, 1954, 1980). Treatment information was limited to the most recent fire, which was described as similar to the 3 previous fires. The forest was clearcut in the fall of 1979 and spring of 1980, then burned in August 1980. The fire was patchy and left approximately 15% of the site unburned. Vegetation sampling was conducted in the same stand for 3 years following the 1980 fire and for 1 year (1981) in 4 other stands of various ages. Prefire vegetation was not reported. Tall hawkweed showed no clear response to time since fire. In the single stand where it was monitored for 3 postfire years, its frequency increased between the 1st and 2nd year after fire but decreased slightly the 3rd year. In the 5 stands monitored in 1981, tall hawkweed was present in stands of all ages; however, it was most frequent in the stand burned 27 years prior to the survey and least frequent in the stand burned 1 year prior to the survey .Tall hawkweed frequency (% of sampling areas where the species was detected) in upland pine-hardwood forests at various stages after fire in northern Lower Michigan  Same stand Four different stands Sampling year 1981 1982 1983 1981 1981 1981 1981 Years since fire 1 2 3 27 33 45 70 Tall hawkweed frequency 8 20 16 66 40 32 Present
The authors hypothesized that tall hawkweed established in burned areas via wind-dispersed seeds .
One study showed tall hawkweed was more frequent on bulldozed than burned sites. In a study investigating the effects of disturbance in alvar woodlands in eastern Ontario, tall hawkweed was detected most frequently in woodlands sampled 465 days after bulldozing compared to areas sampled 100 days and 1 year after a high-severity fire and an adjacent undisturbed area. Because the bulldozed areas were related to fire-control efforts, fire suppression may have introduced tall hawkweed or produced conditions favoring its increase .Frequency (%) of tall hawkweed in areas with different disturbances or different times since disturbance in alvar woodlands in eastern Ontario  Days since disturbance Frequency Undisturbed Not applicable 9 Burned 100 6.4 Burned 365 11.3 Bulldozed 465 27
Tall hawkweed establishes on sites with a range of soil and climate conditions and does not appear to have specific elevation requirements. Floras report tall hawkweed occurring primarily in disturbed areas, including fields [22,37,43,70,75], pastures [22,43], meadows , roadsides [22,37,43,70,75], gravel pits, and along railroads . Floras also report tall hawkweed establishing in dry woods, sandy and rocky openings, even, wet ground , and open woodlands . See Habitat Types and Plant Communities for detailed descriptions of plant communities where tall hawkweed occurs.
Soil: As of this writing (2010), the soil preferences of tall hawkweed were unknown, though an invasive plant guide for the Upper Midwest reports that invasive hawkweeds prefer sandy or gravelly and slightly acidic soils . Tall hawkweed occurred on glacial outwash sands  and sandy or gravelly hillsides  in Michigan and on rocky soil clearings in New Hampshire .One study from central New York reported that it commonly established on calcareous soils in the Northeast and documented it establishing on coarse limestone fragments with pH ranging from 7.8 to 8.3 . In the Great Lakes region, tall hawkweed occurred in several alvar plant communities with limestone or dolostone bedrock [10,11,12,46].
Tall hawkweed appears to prefer moist soil (see Germination and Seedling establishment and plant growth), though it may establish in areas where soils are dry and experience high summer temperatures [45,46]. In central New York, tall hawkweed established in an abandoned limestone quarry where surface temperatures commonly reached 118 °F (48 °C) in the growing season .
Tall hawkweed appears to tolerate poor soils, though such conditions may not be favorable (see Seedling establishment and plant growth). It may establish in soils with low organic matter , low nutrients , or high salinity . In laboratory tests, it tolerated soils with low levels of heavy metal contamination . In Ontario, it was a common species on acidic mine tailings with high concentrations of heavy metals .
Climate: The climatic preferences of tall hawkweed are unknown. In North America, it occurs in areas with continental climates like those found in New Hampshire  and Connecticut . In places like northern Wisconsin, tall hawkweed populations are exposed to a short growing season and moderately long winters with persistent snow cover and frequent extreme cold temperatures. Average annual temperatures range from 35 °F to 50 °F (2-10 °C) . Tall hawkweed is not reported from sites in North America with low annual precipitation.Average annual precipitation of sites with tall hawkweed within its North American distribution Location Precipitation (mm) Michigan 770 , 800  New Hampshire 1,132  New Jersey 1,150  Wisconsin 610 to 1,150  Ontario 900 
Elevation: Tall hawkweed occurs at a range of elevations in North America.Elevation of sites with tall hawkweed within its North American distribution Location Elevation (feet) Connecticut 550  Montana 3,700  Wisconsin 761 to 1,506  Ontario 1,076 
Impacts: No information is available on this topic.
Control: Information on the control of tall hawkweed was lacking in the literature as of this writing (2010). Control of tall hawkweed is complicated by the presence of rhizomes and adventitious root buds (see Vegetative regeneration) that may sprout following control treatments. For information on the control of other invasive hawkweeds, see FEIS reviews for orange and meadow hawkweeds. It should be noted that other invasive hawkweeds may possess botanical traits (e.g., stolons) that differ from tall hawkweed.
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 . 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 .
Prevention: 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 [35,55] (e.g., avoid road building in wildlands ) and by monitoring several times each year . 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 .
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 . See the Guide to noxious weed prevention practices  for specific guidelines in preventing the spread of weed seeds and propagules under different management conditions.
Fire: For information on the use of prescribed fire to control this species, see Fire Management Considerations.
Cultural control: An invasive plant guide for the Upper Midwest states that invasive hawkweeds can be controlled by repeated cultivation or through "competition" with more aggressive native species .
Physical or mechanical control: An invasive plant guide for the Upper Midwest states that hand digging may control invasive hawkweeds. Mowing may prevent seed formation but does not prevent vegetative spread .
Biological control: No information is available on this topic.
Chemical control: No information is available on this topic.Integrated management: No information is available on this topic.
As of this writing (2010), no information was available regarding the importance of tall hawkweed to wildlife and livestock.
Palatability and/or nutritional value: No information is available on this topic.
Cover value: No information is available on this topic.
Tall hawkweed occurs in a variety of plant communities within its North American distribution, including grasslands and other open plant communities, shrublands, woodlands or savannas, and forests.
Open plant communities:
Tall hawkweed often establishes in old fields or other open areas created by human disturbance. In north-central New Jersey, it was present throughout succession in 1- to 60-year-old fields. Species composition changed over time, but fields generally contained a mixture of goldenrod (Solidago), bluestem (Andropogon), and aster (Asteraceae) . Tall hawkweed was uncommon (<5% cover) in two 6-year-old fields in New Jersey. Dominant species included meadow hawkweed, Virginia strawberry (Fragaria virginiana), rice button aster (Symphyotrichum dumosum var. dumosum), and hairy white oldfield aster (S. pilosum var. pilosum) in one field and narrowleaf plantain (Plantago lanceolata) and common cinquefoil (Potentilla simplex) in the other . In southeastern Ontario, tall hawkweed occurred in an abandoned pasture in an early stage of secondary succession. Frequent species in the pasture included Canada bluegrass (Poa compressa), timothy (Phleum pratense), black medick (Medicago lupulina), common St Johnswort (Hypericum perforatum), and Queen Anne's lace (Daucus carota) . In central New York, tall hawkweed was the primary colonizer of a limestone quarry abandoned 40 years previously. Plant cover on the site was approximately 15 to 20%, with tall hawkweed comprising about 80% of the cover. Associated plants included hawkweed oxtongue (Picris hieracioides), common viper's bugloss (Echium vulgare), common St Johnswort, and Queen Anne's lace .
Tall hawkweed establishes in other open plant communities, including wetlands, lakeshores, and coastal areas. It occurred in constructed wetlands along the Delaware River in New Jersey . In northern Ohio, tall hawkweed occurred in a man-made freshwater nontidal marsh dominated by common reed (Phragmites australis) . Along the northern shores of Lake Michigan, it occurred in a freshwater shoreline plant community dominated by American beachgrass (Ammophila breviligulata) on dry dunes and rushes (Juncus and Eleocharis) on wet, cobblestrewn beaches . On a barrier island along the southern shore of Long Island, New York, tall hawkweed was frequent on dry sandflats and ruderal sites . It occurred in several coastal sandplain communities in Massachusetts. It was most frequent in "weedy" plant communities characterized by wrinkleleaf goldenrod (Solidago rugosa), Kentucky bluegrass (Poa pratensis), common velvetgrass (Holcus lanatus), and sweet vernalgrass (Anthoxanthum odoratum). It also occurred in grasslands with little bluestem (Schizachyrium scoparium), fescue (Festuca spp.), beach pinweed (Lechea maritima), sickleleaf silkgrass (Pityopsis falcata), and bentgrass (Agrostis spp.); little bluestem plant communities with bentgrass, rice button aster (Symphyotrichum dumosum), and goldentop (Euthania spp.); yellow sedge (Carex pensylvanica) plant communities; heath (Ericaceae) grasslands with toothed whitetop aster (Sericocarpus asteroides), poverty oatgrass (Danthonia spicata), and low sweet blueberry (Vaccinium angustifolium)); high-diversity native sandplain grasslands with flaxleaf whitetop aster (Ionactis linariifolius), toothed whitetop aster, coastal plain blue-eyed grass (Sisyrinchium fuscatum), and kinnikinnick (Arctostaphylos uva-ursi); and hairgrass (Deschampsia) plant communities with wavy hairgrass (D. flexuosa) and kinnikinnick .
In Isle Royale National Park, Michigan, tall hawkweed was abundant in beaked hazelnut-serviceberry-chokecherry (Corylus cornuta subsp. cornuta-Amelanchier spp.-Prunus virginiana) shrublands occurring on ridges and rocky summits. Characteristic species included black crowberry (Empetrum nigrum) and northern mountain-ash (Sorbus decora) .
Woodlands or savannas:
Tall hawkweed establishes in several woodland or savanna plant communities. It occurred in a montane eastern redcedar (Juniperus virginiana var. virginiana)-hardwood woodland occurring on steep south-facing rock outcrops in eastern North Carolina. Pignut hickory (Carya glabra var. glabra), mockernut hickory (C. tomentosa), and chestnut oak (Q. prinus) were common associates . In southwestern Connecticut, tall hawkweed was infrequent on dry rocky ledges on the west side of a basalt ridge system. An eastern redcedar-yellow sedge (J. virginiana-Carex pensylvanica) plant community dominated basalt ridges . Tall hawkweed was abundant in sand barren plant communities in southeastern Ontario. Sand barrens were dry openings surrounded by and interspersed with jack pine (P. banksiana) forest . In Isle Royale National Park, tall hawkweed was abundant in poverty oatgrass-Canada bluegrass herbaceous plant communities occurring on well-drained rocky granite summits and ridge slopes with exposed granite bedrock. Characteristic species included reindeer lichen (Cladina spp.) and xanthoparmelia lichen (Xanthoparmelia spp.). Sparse tree cover was mostly white spruce (Picea glauca) and quaking aspen (Populus tremuloides) .
Tall hawkweed occurs in several alvar
plant communities in the Great Lakes region. It occurred in the nonvascular pavement plant community found throughout the Great Lakes region and the shagbark hickory/common pricklyash (Carya ovata/Zanthoxylum americanum) savanna-alvar, found only in southern Ontario. The nonvascular pavement plant community occurred on exposed, flat limestone or dolostone pavement and was characterized by sparse cover of lichens and mosses (e.g., cup lichen (Cladonia pocillum), blackthread lichen (Placynthium nigrum), tortella moss (Tortella spp.)), forbs (e.g., tall hawkweed, gray goldenrod (Solidago nemoralis), red columbine (Aquilegia canadensis)), and occasional small shrubs and trees (e.g., common snowberry (Symphoricarpos albus), riverbank grape (Vitis riparia), common juniper (Juniperus communis), northern whitecedar (Thuja occidentalis), paper birch (Betula papyrifera), eastern redcedar, and butternut (Juglans cinerea)). The shagbark hickory/common pricklyash savanna-alvar was sparsely treed (10-25% tree cover). Associated trees included bur oak (Q. macrocarpa), chinkapin oak (Q. muehlenbergii), white ash (Fraxinus americana), and rock elm (Ulmus thomasii). Tall hawkweed occurred in grassy patches in the understory with poverty oatgrass, Philadelphia panicgrass (Panicum philadelphicum), yellow sedge, Canada bluegrass, and gray goldenrod . It occurred in dry alvar woodlands near Ottawa, Ontario. Woodlands were dominated by northern whitecedar, quaking aspen, balsam fir (Abies balsamea), white spruce, and eastern white pine (Pinus strobus) [11,12]. In eastern Ontario, tall hawkweed had 58% frequency in transitional areas between alvar shrublands and grasslands. Quaking aspen and white spruce saplings provided some canopy cover .
Tall hawkweed establishes in deciduous, coniferous, and mixed forests in the Northeast and Great Lakes regions of North America.
On Prince Edward Island, tall hawkweed occurred along the edge of an upland hardwood forest characterized by yellow birch (B. alleghaniensis), sugar maple (Acer saccharum), and American beech (Fagus grandifolia) . In northern Lower Michigan, tall hawkweed occurred at low levels in both recently clearcut (3- to 12-year-old) and mature (55- to 80-year-old) dry-mesic forests dominated by bigtooth aspen (Populus grandidentata) . On Michigan's Upper Peninsula, tall hawkweed occurred in northern hardwood forests dominated by sugar maple [7,27] and American beech . In northern Wisconsin, tall hawkweed occurred at low levels along roadsides in logged forests. All sites were dominated by sugar maple and basswood (Tilia americana), with varying levels of yellow birch, white ash, black ash (F. nigra), American elm (U. americana), and red maple (A. rubrum) .
Tall hawkweed was uncommon (2.5% frequency; 1.6% cover) in the interior of 8- to 10-year-old jack pine plantations regenerating after clearcutting in northern Wisconsin .
In a revegetating limestone quarry in central New York, tall hawkweed plants established in the spring and developed small rosettes during their first growing season. First-season growth was described as slow due to the harsh microclimatic conditions of the quarry floor. Young plants overwintered as green leafy rosettes and continued vegetative growth during their 2nd and 3rd growing seasons. Some plants flowered during their 4th growing season, producing a single, leafless flowering stem with one to numerous heads in May or June . Tall hawkweed flowering dates range from May to September in different parts of its North American range.
In New York, tall hawkweed seeds matured in mid-June . In an abandoned pasture in southeastern Ontario, seeds matured from late June to early July. Flowering stems died in August and were replaced by new vegetative stems arising from the axil buds of the oldest leaves on rhizomes .
In Wisconsin, a variety of insects visited tall hawkweed flowers, including bees (Andrena spp., Bombus spp., Halictidae) and syrphid flies (Syrphidae) . Butterflies  and 36 species of bees  visited tall hawkweed flowers in Michigan.
Tall hawkweed reproduces via seed and also spreads vegetatively. Two sources suggest that vegetative spread is a more common means of regeneration than seed production. Observations from Ontario suggested that seedlings were rare in an established population of tall hawkweed, and most young plants were sprouts from the roots of existing plants . In a revegetating limestone quarry in central New York, the number of tall hawkweed seedlings was low, representing <4% of young plants .
The little available information available (2010) suggests that moisture favors tall hawkweed establishment, growth, and survival. However, few studies addressed this topic, and most information came from either a population only tentatively identified as "tall hawkweed"  or a site (revegetating limestone quarry ) where field conditions may not resemble those of other plant communities where tall hawkweed occurs.
In southeastern Ontario, most "tall hawkweed" seedlings emerged in the days following relatively high (0.7-3.8 inches (18-97 mm)) rainfall. Most seedling mortality occurred after periods of low rainfall in July and August. Seedling emergence and mortality were higher on bare soil microsites compared to goldenleaf campylium moss (Campylium chrysophyllum) microsites. In artificially seeded field plots, 1-year seedling survival was higher on moss than on bare soil (14.2% versus 6.3%, respectively). The authors suggested that moss microsites retained the moisture needed for "tall hawkweed" seedlings to survive .
Tall hawkweed plant growth may be inhibited by poor soil conditions, including low moisture, low nutrients, or heavy metal contamination. In central New York, tall hawkweed plants growing in a revegetating limestone quarry allocated more resources to roots than aboveground parts compared to populations in nearby meadows, a pattern the author attributed to moisture and nutrient deficiencies in the quarry . In field experiments in Ontario, contamination by low levels of heavy metals (copper, nickel, cadmium, and zinc) was correlated with low tall hawkweed leaf production, low plant mass, and delayed phenological development .
One study suggests that once established, tall hawkweed populations persist in the short term. In a revegetating limestone quarry in central New York, established tall hawkweed rosette survival was high (90% to 100%), and survival and density did not change significantly over 3 years of study .
Tall hawkweed hybridizes with orange hawkweed (H. aurantiacum) in Michigan .
In this review, the term "invasive hawkweeds" refers to species in the subgenus Pilosella that are nonnative to North America (e.g., tall hawkweed, orange hawkweed, meadow hawkweed (H. caespitosum)).
Tall hawkweed has rhizomes [13,22,61] and sprouts from adventitious root buds [41,42,61]. One study reported that rhizomes were not important for vegetative reproduction . Observations from Ontario suggested that most young tall hawkweed plants had sprouted from the roots of existing plants. Young individuals dug up at the outer edge of an expanding population were often connected to a larger plant by roots . In an abandoned pasture in southeastern Ontario, root buds were common but not abundant. Buds arose from roots near the surface of the soil at varying distances from the parent plant, resulting in a patchy, but not dense, stand structure . One laboratory study found that seedlings produced leaves and a well-developed root system within several weeks of germination. Many of the roots had already produced buds, which developed leaves and eventually produced a root system. Laboratory studies confirmed that root fragments (excised root tips) could produce lateral roots and buds .
It is not clear what factors promote vegetative growth or survival. In greenhouse experiments, fertilization increased root bud production; nonflowering tall hawkweed plants exposed to high nitrogen levels (210 mg/L) produced 6.92 root buds/plant, while plants exposed to low nitrogen levels (15 mg/L) produced 0.87 root bud/plant . Low light levels may keep tall hawkweeds in a vegetative state and limit flowering. In one greenhouse study, tall hawkweed seedlings were maintained in a vegetative state by providing seedlings with 8 hours of light and 16 hours of darkness . Survival of tall hawkweed plants of vegetative origin appears to be high; in a revegetating limestone quarry in central New York, the survival of vegetative juveniles ranged from 82.6% to 100% .