Mediterranean region and Sinai.
Europe, Mediterranean region, western Asia, north America.
Weed of cultivation.
Annual.
Season and frequency of burning are important in determining yellow starthistle's response to fire. Three
consecutive years of burning at early flowering stage (late June to early July
in northern California) controlled yellow starthistle and
depleted the yellow starthistle soil seed bank [33] (see Fire Case Studies). Yellow starthistle plants
may remain green for up to 4 days following burning, possibly allowing seed to
mature if burning is conducted too late in the flowering stage [53]. Prescribed fires conducted early in the spring may not be hot
enough to kill yellow starthistle and may damage desirable plants.
For further information on prescribed fire use and yellow starthistle's response to fire, see this Research Project Summary: Changes in grassland vegetation following fire in northern Idaho.
Yellow starthistle in an invasive winter annual, or rarely a biennial or short-lived perennial forb. Yellow starthistle populations in the United States exhibit variations in phenology (bolting and flowering time) and morphology (plant size and leaf shape) [82,117,128]. Sun [142] found genetic diversity within yellow starthistle populations, and little divergence between populations. The following description provides characteristics of yellow starthistle that may be relevant to fire ecology and is not meant to be used for identification. Keys for identifying yellow starthistle are available [49,55,56,65,115,121].
Yellow starthistle stems are stiff and erect, 6 to 72 inches (15-200 cm) in height. Stems are openly branched, except in some very small plants [30,32]. Yellow starthistle produces rosette leaves that lie close to the ground [133]. Lower leaves are 2 to 6 inches (5-15 cm) long, and progressively smaller up the stem. Upper leaves are 0.4 to 1.2 inches (1-3 cm) long, narrow, and densely covered with cobwebby hairs later in the season.
Yellow starthistle inflorescences are borne in solitary flowerheads on stem tips, although vigorous plants may produce flowerheads in branch axils. The involucre is about 0.5 to 0.7 inches (1.2-1.8 cm) long. Phyllaries have 1 long central spine 0.4 to 1 inch (1.0-2.5 cm) long and 2 or more pairs of short lateral spines and are densely to sparsely covered with hairs. Yellow starthistle fruits are achenes of 2 types, both glabrous and about 2 to 3 mm long. Most of the achenes (seeds) (75-90%) have a short (2-5 mm), stiff pappus (plumed). Seeds at the periphery of the flowerhead are darker in color and have no pappus (plumeless).
Yellow starthistle has a large taproot that grows to soil depths of 3.3 feet (1 m) or more, allowing access to deep soil moisture during dry summer and fall months [30,116,130]. Hairs and waxy coating on mature yellow starthistle leaves reflect light, thus reducing the heat load and transpiration demand, while winged stems also dissipate heat. These qualities, along with deep roots, allow yellow starthistle to thrive under full sunlight in hot, dry conditions [30]. Vesicular arbuscular mycorrhizal fungi have been observed on yellow starthistle plants [50].
Once established, yellow starthistle can survive at high population densities, with estimates of 2 to 3 million plants per acre (5-7.5 million per hectare) reported. This dense cover reduces sunlight penetration to the soil surface, inhibiting germination and development of competing vegetation. Old yellow starthistle stalks are persistent and usually remain standing through the winter [20].
It has been suggested by several researchers that yellow starthistle might have an allelopathic effect on surrounding vegetation; however, there is no direct evidence to substantiate that claim [35,68].
Yellow starthistle is native to southern Europe and western Eurasia (Flora Europaea). Today, yellow starthistle can be found in most temperate areas around the world [82]. In North America, yellow starthistle now occurs in at least 41 of 50 US states and 4 Canadian provinces. Yellow starthistle is reported to be adventitious in Hawaii [138]. Infestations of yellow starthistle in the eastern two-thirds of the United States are sporadic and localized, but apparently populations fail to establish and persist on a year-to-year basis, possibly because of unfavorable growth conditions [49,82]. Infestations west of the Rocky Mountains are most severe in California, Washington, Oregon, and Idaho [20,38]. Infestations in California cover more area than those in all other states combined [82].
It is likely that there were multiple introductions of yellow starthistle to the United States [78], and that contaminated alfalfa seed (Medicago sativa) was the primary vehicle for these introductions [82]. Yellow starthistle seeds were found in adobe bricks from the period between 1824 and 1848 in California (Hendry 1931, as cited by [54]). The plant was first collected in Oakland, California in 1869 and was most likely introduced from Chile, while introductions from 1899 to 1927 appear to be from Turkestan, Argentina, Italy, France and Spain [32]. Introduction of yellow starthistle to other western states began in the late 1800s, and it was first reported outside of California near Bingen, Washington, around 1900 [30,133]. Yellow starthistle began spreading into grasslands in the Pacific Northwest in the 1920s. By 1985 yellow starthistle had spread to about 8 million acres (3.2 ha) in California [81]. Extensive road building, suburban development, and expansion in the ranching industry since the 1960s have contributed to the rapid and long-range dispersal of seed and the establishment of new satellite populations [45].
The following table reflects estimates of yellow starthistle acreage as reported by surveyed states or provinces in 1988 and again in 2000 (from [38]):
State/Province 1988 2000 Arizona not reported 3,000 California not reported 17,000,000 Colorado 10 100 Idaho 1,130,000 800,000 Kansas not reported 0 Montana 1 1 Nevada not reported 5,000 New Mexico not reported 500 North Dakota 0 400 Oregon 10,000 950,000 South Dakota 0 0 Utah 100 2,200 Washington 133,805 1,000,000 Wyoming 0 0 Alberta 0 0 British Columbia not reported 0 Total 19,761,201Although inventories are more common and more accurate in the year 2000 than in 1988, 50% of these states reported only 50% accuracy, while 31% reported 51% to 75% accuracy, and 2 states reported 75% to 100% accuracy [38].
The following biogeographic classification systems are presented as a guide to demonstrate where yellow starthistle might be found or is likely to be invasive, based on reported occurrence and biological tolerance to factors that are likely to limit its distribution. Precise distribution information is limited, especially in the southwestern, southern, central, midwestern, and eastern states. Therefore, these lists are somewhat speculative and probably not exhaustive.
Fire adaptations: Yellow starthistle is a winter annual that produces abundant seed crops (see Seed production). It can colonize a site after fire either from undamaged seed in the soil seed bank on site [33], or from seed brought to the site from an off-site source. Fire creates conditions that are favorable to the establishment of yellow starthistle such as increased sunlight at the soil surface, reduced competition, and areas of bare soil. If yellow starthistle seeds are present they are likely to germinate and establish seedlings in the postfire community.
FIRE REGIMES: Yellow starthistle occurs primarily in annual grasslands and oak woodlands in California, and perennial grasslands in the Pacific Northwest, where historic FIRE REGIMES have been dramatically altered. Yellow starthistle did not occur in these communities at the time in which historic fire regimes were functioning, but has established since fire exclusion began. Hastings and DiTomaso [53] suggest that invasion of California grasslands by yellow starthistle may be due, in part, to fire exclusion and reduced fire frequency in these ecosystems. This is supported by results from prescribed fire programs in these areas that have resulted in large reductions of yellow starthistle cover, and increased native plant diversity (see Fire Case Studies) [33,53]. Dense infestations of yellow starthistle may change the fuel characteristics of an invaded site and thus alter fire regime characteristics. In the Sugarloaf Ridge study, a site with dense yellow starthistle had insufficient fine fuel to carry fire when yellow starthistle was still green [53]. Conversely, Piper [100] suggests that dried skeletons of yellow starthistle can provide fuel for late summer wildfires.
The following table provides historic fire regime intervals for some communities and ecosystems in which yellow starthistle may be found. 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".
Community or Ecosystem Dominant Species Fire Return Interval Range (years) California chaparral Adenostoma and/or Arctostaphylos spp. sagebrush steppe Artemisia tridentata/Pseudoroegneria spicata 20-70 [99] basin big sagebrush A. tridentata var. tridentata 12-43 [126] mountain big sagebrush A. t. var. vaseyana 15-40 [4,18,86] Wyoming big sagebrush A. t. var. wyomingensis 10-70 (40**) [156,164] coastal sagebrush A. californica desert grasslands Bouteloua eriopoda and/or Pleuraphis mutica 5-100 plains grasslands Bouteloua spp. cheatgrass Bromus tectorum California montane chaparral Ceanothus and/or Arctostaphylos spp. 50-100 California steppe Festuca-Danthonia spp. western juniper Juniperus occidentalis 20-70 Rocky Mountain juniper J. scopulorum pinyon-juniper Pinus-Juniperus spp. 99] Jeffrey pine P. jeffreyi 5-30 Pacific ponderosa pine* P. ponderosa var. ponderosa 1-47 [3] interior ponderosa pine* P. p. var. scopulorum 2-30 [3,6,77] mountain grasslands Pseudoroegneria spicata 3-40 (10**) [2,3] California oakwoods Quercus spp. blue oak-foothills pine Q. douglasii-Pinus sabiana Oregon white oak Q. garryana 3] California black oak Q. kelloggii 5-30 [7] elm-ash-cottonwood Ulmus-Fraxinus-Populus spp. 36,157] *fire return interval varies widely; trends in variation are noted in the species summaryFire as a control agent: The effectiveness of fire in killing invasive plants or reducing their population growth depends on fire severity, time of burning, and prior and subsequent weather conditions [29], as well as the species present in the preburn community and represented in the soil seed bank. Sheley and others [133] state that burning is an ineffective method for controlling yellow starthistle; however, consecutive, annual prescribed fires have resulted in large reductions of yellow starthistle cover and seeds in the soil seed bank on some California and Oregon grassland sites [14,33,58,85]. Prescribed burning may also be a useful component in an integrated management approach [34,53,112,118].
DiTomaso [35] suggests that fire is an important component of most grassland ecosystems and that it can be used to control yellow starthistle as well as some of the invasive annual grasses with which yellow starthistle is often associated. Successful control requires timing the burn to the early flowering stage of yellow starthistle, and burning with sufficient heat to scorch the foliage and stem-girdle the plants. This heat may also stimulate seed germination, thereby facilitating seed bank depletion with repeated burning. The first year after burning, yellow starthistle plants emerge from seed stored in the soil. It is critical to remove these plants before they produce seed, as they are likely to be highly productive due to decreased competition [53]. Three consecutive years of burning in early July at Sugarloaf Ridge State Park in northern California reduced yellow starthistle cover by 90%, reduced yellow starthistle seeds in the soil seed bank by 99%, and increased total plant diversity and species richness [33] (see Fire Case Studies).
A similar prescription achieved somewhat different results when applied on a grassland site in the Lassen foothills of northern California [58]. In this area, yellow starthistle and medusahead (Taeniatherum caput-medusae) are both target species, and burns are timed in relation to the phenology of the dominant invasive species of individual burn units. The timing of burning for medusahead control (when it is cured, and just before seed dispersal) tends to coincide with the early flowering stage in yellow starthistle [14,58]. Grazing is deferred for 3 months prior to burning to allow a build-up of fuels. Slow-spreading backfires are used and are intended to be severe enough to kill medusahead seed. Because medusahead seed is not long-lived, eradicating it requires as little as 1 year of burning, whereas yellow starthistle requires 3 years of burning and follow-up monitoring, since seeds can remain viable for several years. Two consecutive years of burning were effective in reducing cover of invasives without reducing overall plant diversity. However, plant diversity decreased after the third year of burning, causing concern for several sensitive plant species in the area. The observed differences between this and the Sugarloaf Ridge site may be attributable to differences in soil type and climate; the Lassen site is a drier site [58].
Unlike mowed yellow starthistle plants, burned yellow starthistle plants do not usually resprout [14]. Minimal resprouting of scorched plants was observed following an early-spring burn where much of the vegetation was only lightly to moderately burned [85]. However, seeds on the soil surface are not typically damaged and may actually be stimulated to germinate following grassland fires, since the soil surface is only transiently heated to about 392 °F (200 °C) [35,53]. Because a single burn is likely to increase yellow starthistle plant size and seed production the following year, follow-up treatments (e.g., monitoring, repeated burning, hand-pulling) are critical to yellow starthistle control [35]. After several years of decreasing follow-up treatments, managers are able to maintain a 50-acre site free of yellow starthistle with 1 day's effort annually, in a Mediterranean annual grassland in southern Oregon [14].
One concern with consecutive annual burning is that there be sufficient fuels for prescribed burning to be effective [34]. This can be a problem where yellow starthistle populations are very dense and/or fuels are depleted with each successive burn. Prescribed fire will carry in starthistle of moderate density, but fire will not carry in very dense patches, since yellow starthistle is green at the optimal time of burning. To address this concern at Pinnacles National Monument in the Gabilan Mountains of the Central Coast Range in California, prescribed burning was coupled with seeding a sterile, annual wheat à wheatgrass hybrid in the fall to provide fuel for subsequent burns. The first year of burning resulted in a 98% reduction in yellow starthistle plants [85]. In areas where consecutive annual burning may be difficult or have negative impacts (e.g., areas with insufficient fuels, erosion concerns, or populations of sensitive species), prescribed burning may be more appropriate as part of an integrated management approach. For example, DiTomaso and others [34] suggest spraying with clopyralid the 1st year, to suppress legumes and stimulate grasses; and burning the 2nd year, when fuel loads would be higher and the previously suppressed legume populations may be stimulated by fire.
The time of burning for effective control of yellow starthistle in California is after annual grasses have cured and before yellow starthistle flowers. There may be air quality issues and a high potential for fire to escape at this time of year. Information about prescribed fire projects should be made available to local residents prior to any burning. Escape problems can be minimized by involving the local fire departments and the California Department of Forestry [34]. Burning in the fall or spring to reduce risk of fire escape may not as effectively control yellow starthistle [118]. However, Martin and Martin [85] burned earlier than is typically recommended, with 98% reduction of yellow starthistle. It is also possible to reduce risks by burning earlier in the season using a "brown and burn" technique, in which a nonselective herbicide such as glyphosate is applied, and the vegetation is allowed to cure before burning. The fire is contained more easily because surrounding vegetation is still green. The potential of fall burning to reduce weed competition for fall seeding is under investigation [118]. Yellow starthistle seedlings have also been controlled using winter or early spring flaming techniques that employ a gasoline and kerosene mixture to ignite the wet biomass and keep it burning long enough to blacken the plants [35,125]. This approach reduces risk of escaped fire and avoids some air quality issues, but it is somewhat non-selective and provides inconsistent control of yellow starthistle. It is most effective when followed by spring drought, but may be a complete failure when followed by a wet spring, especially if desirable species are suppressed [35].
Burning can also affect other control methods. Fire can have a serious impact on small animals and insects such as biocontrol agents and their larvae [34]. Burning to remove heavy annual plant debris improves herbicide efficacy by increasing contact with the target plants. Prescribed fire may also be used to prepare a seedbed for competitive perennials and seed broadcast in the ash [118]. In some cases, burning can lead to rapid invasion by other undesirable species from wind dispersed seeds and seeds in the soil seed bank [34].
Postfire colonization potential: Fire is likely create ideal conditions for establishment of yellow starthistle by reducing competition and litter, exposing the soil surface to light, releasing nutrients, and possibly even stimulating germination of yellow starthistle seed. Asher and others [5] cite a case in the Ishi Wilderness Area of northern California in which yellow starthistle provides an example of "severe postfire weed spread and impacts". No details were given.
General precautions should be followed to prevent yellow starthistle establishment after fire. The USDA Forest Service's "Guide to noxious weed prevention practices" [154] provides several fire management considerations for weed prevention in general that can be applied to yellow starthistle. Wildfire managers might consider including weed prevention education and providing weed identification aids during fire training; avoiding known weed infestations when locating firelines, monitoring camps, staging areas, and helibases, to be sure they are kept weed free; taking care that equipment is weed free; incorporating cost of weed prevention and management into fire rehabilitation plans; and acquiring restoration funding. Careful postfire vigilance to identify and record the establishment of new populations is critical. About 1 month after fire, survey for signs of new or resprouting weeds. Repeated surveys will be needed, with the frequency and intensity guided by local conditions [5].
Potential weed problems must be addressed during prefire planning of prescribed burns, and following both wild and prescribed fires. When planning a prescribed burn, preinventory the project area and evaluate cover and phenology of any yellow starthistle present on or adjacent to the site and evaluate the potential for increased yellow starthistle populations in the area [5]. Avoid ignition and burning in areas at high risk for yellow starthistle establishment or spread, and/or plan for follow-up treatments in succeeding years. Avoid creating soil conditions that promote weed germination and establishment. Discuss weed status and risks in burn rehabilitation plans [154].
To prevent infestation, re-establish vegetation on bare ground as soon after fire as possible, using either natural recovery or artificial techniques as appropriate to site conditions and objectives. When reseeding after wildfires and prescribed burns, use only certified weed-free seed. Monitor the burn site and associated disturbed areas after the fire and the following spring for emergence of yellow starthistle, and treat to eradicate any emergent yellow starthistle plants. Regulate human, pack animal, and livestock entry into burned areas at risk for weed invasion until desirable site vegetation has recovered sufficiently to resist weed invasion. Additional guidelines and specific recommendations and requirements are available [5,47,154].Yellow starthistle is most often found on roadsides and abandoned fields and pastures, waste places, recreational areas and disturbed grassland or woodland [32,49,55,158]. It is a problem primarily in moderately warm, exposed areas on relatively dry, fertile soils [32].
Yellow starthistle is best adapted to open grasslands with average annual precipitation between 10 and 60 inches (250-1500 mm). It is generally associated with deep, well-drained soils. Although populations can occur at elevations from sea level to as high as 8,000 feet (2,400 m), most large infestations are found below 5,000 feet (1,500 m) [30,82]. The optimum environmental conditions for yellow starthistle appear to be in northern California's mediterranean-type climate (cool, wet winter and hot, dry summer), which enable yellow starthistle to grow during winter, bolt in spring, and escape summer drought [81,139]. Yellow starthistle is uncommon in deserts and moist coastal sites [32].
Yellow starthistle is capable of establishment and at least short-term persistence in any of the major plant communities in Washington below subalpine, but demonstrates its maximum potential in the bluebunch wheatgrass (Pseudoroegneria spicata)-Idaho fescue (Festuca idahoensis) zone of the steppe region of the Columbia Basin Province. In this community, on the southern slopes of the foothills of the Blue Mountains, it appears to reach its northern limits. Occasional small populations are found farther north, but the farther north the more they are restricted to steep south-facing slopes [121,145]. Here it is most competitive on deep silt loam soils on south slopes of the higher natural grassland types. In other areas, compensating factors for moisture or heat were found, or yellow starthistle was not competitive, producing only a few heads on a dwarfed plant [119]. Although yellow starthistle more readily invades and dominates south slopes and disturbed sites in southwestern Oregon, it is not restricted to them. In the absence of competitive perennial vegetation, yellow starthistle often forms dense stands on valley floors and invades openings in the conifer transition zone above oak woodlands [118]. In north-central Idaho, yellow starthistle infestations thrive in disturbed areas with high sunlight exposure and well-drained soils that receive 10 to 30 inches (254-762 mm) of precipitation per year [20].
On annual rangelands where yellow starthistle occurs with cheatgrass, population dominance oscillates between yellow starthistle and cheatgrass [130]. Yellow starthistle invades and dominates annual grasslands by using the deep soil moisture that remains after shallow-rooted annual grasses die in early summer [118]. Areas with deep soil and years with moderate to heavy spring rainfall give yellow starthistle the greatest advantage [74]. In dry spring conditions (and shallow soils), early maturing annual grasses have the advantage [130], although yellow starthistle can also survive at extremely low soil water potential (<-6.0 MPa) as compared to annual grasses (>-2.1 MPa) [35]. Growth rates of cheatgrass and yellow starthistle appear to depend primarily on plant density, soil depth, and available soil moisture [132]. Sheley and Larson [130] suggest that intraspecific competition is more important for both yellow starthistle and cheatgrass than is interspecific competition, citing evidence of resource partitioning via root depth, allowing for greater niche occupation on a site.
Yellow starthistle seedlings are more likely to dominate in deep silt loam and loam soils with few coarse fragments [74,145], but they can also establish on shallow, rocky soils [79,82]. In deep soils, yellow starthistle can reduce soil moisture reserves to depths greater than 6 feet (~2 m), and in 3-foot-deep (~1 m) foothill soils it can extract soil moisture from fissures in bedrock [35,45]. A Washington study suggested a relationship between yellow starthistle cover and soil depth or total moisture-holding capacity of the soil on south-facing aspects. Here, yellow starthistle may be dependent on the ability of the soil to retain sufficient moisture for the maturation of the plant during the summer drought period [116,117,145]. Studies in southeastern Washington natural grasslands indicated that the dual requirements of soil moisture for reproduction during the summer drought period, and light for winter growth may be critical limiting factors for yellow starthistle [116].
At its northern limit in Washington state (48° 45' north latitude), yellow starthistle is restricted to south-facing slopes [116]. Yellow starthistle's conspicuous absence on north aspects in this area appears to be related to heat and light [116,117]. This may explain why it has not persisted in British Columbia [115], and why it has gradually disappeared in northern areas of Romania (46° 47' north latitude) (Prodan (1930) as cited by [115,116]). A critical factor at the northern limits of yellow starthistle appears to be radiation during the winter [116]. Yellow starthistle needs sunlight at the soil surface from fall through spring to grow roots long enough to tap water stored deep in the soil during summer drought [118]. Yellow starthistle seedlings can survive extended frost periods, but mature plants rarely survive the winter in cold climates. Cold tolerance appears to be lost during the transition from vegetative to reproductive phases [30].
When yellow starthistle seed from 34 distinct stands in California, Idaho, Oregon, and Washington was grown in a uniform nursery at Pullman, Washington, there was wide biotypic variation for the following variables: rosette area, decurrent leaf width, plant height, stature, number of branches, shape and form, growth stage on four dates, number of buds produced on 2 dates, flowering rate, date of first flower and first seed, average number of flowers, and seed production rate. None of the variables correlated with precipitation, elevation, or latitude of the population source [113].
Cattle, sheep and goats will graze on yellow starthistle in early spring, and up to the bolting stage. These ruminants will not graze yellow starthistle plants after the spines emerge [82,133,139,148,149,150]. Yellow starthistle provides forage for ruminants in late spring and early summer when other green forage is generally unavailable [150]. Yellow starthistle is toxic to horses, causing a neurological disease called equine nigropallidal encephalomalacia, or "chewing disease" with prolonged ingestion. Poisoning is most likely when yellow starthistle is the only feed available (e.g. on poor condition range) or when yellow starthistle is a substantial contaminant of dried hay. In some cases, horses acquire a taste for yellow starthistle and seek it out even when other forage is available [98]. Other animals (e.g. mules and burros) are not susceptible to the toxic effects [24,30].
Yellow starthistle foliage may be eaten by grasshoppers, and yellow starthistle seeds are consumed by several species of birds including ring-necked pheasants, California quail, house finches, and American goldfinches [32,117].
Yellow starthistle is a facultative winter annual (sometimes biennial, from a tap root) that relies on abundant seed production for population persistence and spread [80]. The reproductive biology [80,143] and life history [131] of yellow starthistle have been explored.
Breeding system: Yellow starthistle is monoecious, pollinator-dependent and facultatively xenogamous [52,80,143]. Most yellow starthistle plants are self-incompatible [80].
Pollination: European honeybees are an important pollinator of yellow starthistle and may be responsible for up to 57% of seed set [8,80]. Bumblebees are also important floral visitors, and several other insects contribute to the fertilization of yellow starthistle ovules [8,52].
Seed production: The number of flowerheads and seeds produced by yellow starthistle plants varies with soil moisture and intensity of competition. Plants can produce 1 to 1,000 flowerheads per plant and 30 to 80 seeds per flowerhead [11,74,79]. Large plants can produce nearly 75,000 seeds, and Roche [117] calculated a potential 148,928 seeds produced by a nursery plant. The number of seeds per unit area produced by an infestation of yellow starthistle has been measured by several authors, with reports ranging from 14 to 100 million seeds per acre (35-250 million seeds per hectare) [23,33,74]. The number of seeds produced on an annual basis is dependent on the size of the infestation and the amount of spring precipitation [131,133]. Sheley and Larson [131] measured seed output in yellow starthistle at 1,940 and 470 seeds per square foot (21,600/m² and 5,200/m²) under moist and dry spring conditions, respectively. The percentage of plumed seeds ranges from about 70% to over 90% [11,74,79,117]. In heavy infestations, yellow starthistle populations produce far more seeds than are necessary to reinfest the area year after year [30].
Seed dispersal: Distribution of yellow starthistle seedlings is predominantly in and near the previous year's yellow starthistle debris, suggesting a relatively slow rate of spread [117]. Plumeless seeds usually remain in seedheads until fall and winter (between November and February [133]) and fall to the soil just below the parent plant as seedheads deteriorate [20]. The size of the pappus on plumed seeds is small relative to seed size and wind moves seeds only short distances, roughly equal to the height of the plant [1,114]. Roche [114] recorded 92% of seeds falling within 2 feet (0.6 m) of the parent plant, and 48% within 1 foot (0.3 m). A combination of gusty wind and dry conditions maximizes wind dispersal. Roche also measured maximum wind dispersal at about 16 feet (<5 m) over bare ground with wind gusts of 25 miles per hour (40 km/hr). While not an effective long-distance dispersal mechanism, wind dispersal can serve to increase the area of an infestation by persistently advancing the perimeter. Yellow starthistle seed can also be transported over short to medium distances by animals and humans. The pappus bristles are covered with stiff, microscopic barbs that readily adhere to clothing and hair [20,32]. Long-distance dispersal of yellow starthistle seed is often directly related to human activities and occurs by movement of livestock, vehicles, equipment, and contaminated hay and crop seed [32,35]. Birds such as ring-necked pheasants, California quail, house finches, and American goldfinches feed heavily on yellow starthistle seeds [114,117]. While some (e.g., ring-necked pheasants) may be responsible for long distance dispersal of yellow starthistle seed, most seeds consumed by birds are lost to the regeneration pool [114].
Seed losses have been recorded by researchers as a discrepancy between total seed production and number of seeds dispersed. In Washington, plumed seed was dispersed in August with a 30% loss between crop production and dispersal, and plumeless seed was released in December with a 65% loss [74]. Sheley and Larson [130] observed a 41% loss of seed production at the time of dispersal. Further losses are observed between dispersal and seedling establishment. Roche [114] found 290 seedlings per square foot (3,230/m²) and 1.8 seeds per square foot (20/m²) in May, in an area where about 2,630 seeds per square foot (29,200/m²) were dispersed the previous fall. He speculated that the remaining seeds may have been consumed or moved by birds, rodents, insects or whirlwinds.
Seed banking: Yellow starthistle seed may remain viable in the soil for as long as 10 years [23]. In heavily infested areas, the soil seed bank of yellow starthistle approaches 13% of annual seed production and consists primarily of plumeless seeds [131,133]. However, evidence for seed banking in yellow starthistle varies. Induced dormancy, seed type, depth of burial, site fire history, and other site conditions may be responsible for observed differences in seed longevity.
On a bunchgrass site in California where yellow starthistle was present, no yellow starthistle seeds germinated from soil samples taken from 0 to 5-inch (0-13 cm) depths in October [83]. On a bunchgrass range site in Washington state that was dominated by cheatgrass and yellow starthistle (density of 16 to 21 adults per square foot (180-236/m²)), the number of yellow starthistle seeds found in soil samples ranged from about 252 to 378 per square foot (2,800-4,200/m²) [74,131]. On a California grassland site, seed density measurements in the soil under yellow starthistle in the fall varied from year to year in untreated, control plots from 309 to 911 per square foot (3,438-10,127/m²) [33].
After-ripening may prevent premature germination of yellow starthistle in dry habitats. Many yellow starthistle seeds undergo secondary dormancy after exposure to high temperatures and low moisture within 1 month of dispersal. These seeds do not then germinate even under adequate light and moisture conditions [30]. Joley and others [64] observed some evidence of after-ripening in both buried and dry-stored seed, although results were inconsistent. Benefield and others [11] report 87.6% to 95.2% viability and 84% to 87.6% germination rates at the dispersal stage at 2 California sites. Thus, nearly all viable seed was able to germinate suggesting that yellow starthistle at these locations may not have an innate or induced dormancy mechanism or an ecologically significant after-ripening period.
Callihan and others [23] harvested plumed and plumeless yellow starthistle seeds from a site in Idaho, and buried them in a sandy loam soil under an annual grass community in southeast Washington at depths of 1, 2, and 6 inches (2.5, 5, and 15 cm). They detected no effects of burial depth on seed longevity, but did find that viability decreased over time. Average longevity of plumed and plumeless seeds was 10 and 6 years, respectively [23,96]. Joley and others [64] buried plumed and plumeless seeds at 2 inches (5 cm) for 72 months in the field in a sandy loam soil under a fallow orchard in northeastern California. They found no effects of seed type on germination. In another study, plumed seeds were buried at several depths. After 13 months, germinable seeds increased with depth, with 0.5%, 3.9%, 63.1%, and 88.1% germinability at 0.2-, 0.4-, 1-, and 2-inch (0.5, 1, 2.5, and 5 cm) depths, respectively. The loss of germinable seeds at shallow depths is attributed primarily to germination, as indicated by split seedcoat.
One year of prescribed burning in Sonoma County, California, significantly reduced the seed bank of yellow starthistle (P<0.05) and 3 consecutive years of burning, with no further seed recruitment, further depleted the seed bank. Prior to burning, yellow starthistle seed density ranged from 309 to 911 seeds per square foot (3,438-10,127/m²). Seed density declined to 240 seeds per square foot (2,673/ m²) after 1 year of burning, 38 seeds per square foot (421/m²) after 2 years of burning, and 5 to 11 seeds per square foot (52-127/m²) after 3 consecutive years of prescribed burning. Corresponding decreases in seedling density the following springs were also observed [33]. Similarly, Joley and others [64] reported over 83% depletion in the seed bank 1 year after preventing seed rain by mowing or clipping mature plants, and after 3 years, only 3.9% of the original seeds remained. Benefield and others [11] sowed 1,000 seeds 0.4 inch (1 cm) deep in weed-free soil containers in California. They observed that after 1 year, germination and seed recovery accounted for 69% and 57% of the original 1,000 seeds sown, for plumed and plumeless seeds, respectively. Of the seeds recovered, over 80% were damaged or degraded by microbial or insect activity. Unrecovered seed was speculated to have been lost to bird or rodent predation. These results support the contention that, under California soil and climatic conditions and average field conditions where seeds are predominantly dispersed on the soil surface, yellow starthistle seeds are relatively short-lived. Microbial degradation and predation of yellow starthistle seeds probably contribute to the rapid depletion of the soil seed bank [11,30,64].
Two to 3 years of effective control can dramatically reduce yellow starthistle infestation and presence in the soil seed bank [33,64]. Yet the survival of even a small percentage of seeds can potentially lead to reinfestation of a site, even in the absence of off-site seed recruitment [23,33,64].
Germination: The mean number of viable and germinable yellow starthistle seeds increases consistently with advancing phenological stage, with some germinable seed developing as early as the late flowering stage [80] and late senescence stage (see Seasonal Development) [11]. Plumeless seeds mature earlier than plumed seeds, and germination of plumed seeds is slightly greater than plumeless seeds [35]. Dimorphic seeds may be associated with the success of yellow starthistle in exploiting variable soil moisture and temperature regimes in semiarid environments [75]. The different seed types tend to have different optimums with regard to temperature and moisture in the lab [75,117]. In the field, the germination rate of plumed seed is higher in late fall and winter, and that of plumeless seed is higher in spring [11].
Over 90% of yellow starthistle seeds are germinable 1 week after seed dispersal [11]. Yellow starthistle seeds germinate at both low and high temperatures, allowing for both fall and spring germination when moisture is adequate [20]. In the field, seeds usually germinate in late fall or early winter, when sufficient soil moisture is present [79,133], though germination can continue throughout winter and into spring in northern California [63]. Germination is closely correlated with winter and spring rainfall events [11,79], with germination occurring throughout the rainy season, and emergence highest after early-season rainfall events [11]. The extended period of germination increases the difficulty of controlling yellow starthistle populations with late winter and early spring treatments, as germination subsequent to control efforts can result in continued infestations.
Joley and others [63] found that temperature, light, seed type, collection date, dormancy and storage, and interactions of these factors all affected yellow starthistle germination in the lab. Nearly 100% germination occurs when seeds are exposed to moisture, light and constant temperatures of 50, 59, or 68 °F (10, 15, or 20 °C), or alternating temperatures of 59:41 or 68:50 °F (15:5 or 20:10 °C). Sheley and others [128,134] found that under similar conditions of moisture and temperature and no light, germination occurred rapidly, with nearly all seed germinating within 96 hours. Total germination is reduced at temperatures above 86 °F (30 °C) and below 43 °F (6 °C) [122]. Although seeds can germinate in the dark, germination is greatly reduced in dark environments [63] and appears to be stimulated by white light [92].
Seedling establishment/growth: Yellow starthistle seedlings first allocate resources to root extension and then to leaf expansion. Yellow starthistle root growth is vigorous and can extend deeper than 3.3 feet (1 m) during early seedling establishment [116].
In exposed areas, high germination can result in extremely dense seedling populations [74,145]. Near Walla Walla, Washington, yellow starthistle seedling populations reached winter (mid-January) densities approaching 2,500 plants per square foot (26,875/m²). Subsequent frost heaving reduced seedling populations by about 40%, and yellow starthistle density was further reduced by 75% during the juvenile phase (late May to mid-June) [131,133]. Similarly, Roche [117] observed a 58% reduction in yellow starthistle density from April to July. Adult density of yellow starthistle at a particular site can be closely associated with soil depth and thus, late season water storage capacity. Roche and others [116] observed a significant relationship (P< 0.001) between the number of yellow starthistle plants per unit area and total soil moisture from 19 May to 29 August in southeastern Washington.
Asexual regeneration: There is no evidence for asexual regeneration in yellow starthistle.
Yellow starthistle does not survive well in shaded areas, and is less competitive in areas dominated by shrubs, trees, tall perennial forbs and grasses, or late-season annuals. Yellow starthistle infestations are nearly always restricted to disturbed sites or open grasslands dominated by annuals. In areas dominated by yellow starthistle, the level of competition for light can be intense, resulting in a low rate of seedling survival through self-thinning [30,32]. The rosette growth form is particularly vulnerable to shading by overtopping vegetation (Regehr and Bazzaz 1976, as cited by [116]). Yellow starthistle is dependent on light on the soil surface for winter rosette and taproot development [117].
In an experiment in southeastern Washington, shade was indicated as a major stress factor for yellow starthistle in unclipped perennial grass stands. Yellow starthistle did not invade areas of undisturbed perennial grasses; and clipped bunchgrasses were more susceptible to yellow starthistle invasion than were clipped sod-forming grasses. Yellow starthistle seedlings in plots shaded by grass litter and standing grass plants appeared weak and spindly with erect yellowish-green leaves compared to vigorous blue-green leaves of prostrate rosettes grown in grass-free control plots. Reduced light levels also reduced root growth and flower production. Plants grown under 6% full sunlight failed to flower; and the average number of flowers increased with increasing light [116].
Because yellow starthistle plants can germinate over an extended time period, the canopy of an infestation may be composed of yellow starthistle plants in several stages of development. In dense stands of yellow starthistle, the population consists of both large-canopied, deep-rooted plants receiving full sunlight, and an understory of smaller, shallow-rooted, shaded plants. In this way yellow starthistle maximizes niche occupation on a site [30], and can halt native plant succession by not allowing germination and establishment of natives.
In some areas, control of one weed may lead to increase of another. Mosley and others [90] suggest that cheatgrass control may lead to replacement with yellow starthistle.
Centaurea solstitialis, the yellow star-thistle, is a species of thorny plant in the genus Centaurea, which is part of the family Asteraceae. A winter annual, it is native to the Mediterranean Basin region and invasive in many other places. It is also known as golden starthistle, yellow cockspur and St. Barnaby's thistle (or Barnaby thistle).[1]
Centaurea solstitialis is an annual herb from the family Asteraceae. During the vegetative stage it forms a rosette of non-spiny leaves, between 5–20 centimetres (2–8 inches) in diameter. As the summer approaches, it produces a flowering stem up to 1 metre (3+1⁄2 feet) in height.[2] The leaves at the base are lobed and range between 5–8 cm (2–3 in) in length, while the ones on the stem are unlobed and smaller.[2] Between May and October, the stem produces numerous spinous flower heads (capitula) about 12–16 millimetres (1⁄2–5⁄8 in) across, containing between 10–50 yellow flowers, with spines between 1–2.5 cm (3⁄8–1 in).[2] Flowers within capitula are pollinated by insects and each capitula produces 10–50 seeds, some with and some without a pappus. It is an annual semelparous species, and will die after reproduction is completed, normally by the end of the summer.
Similar species include purple star-thistle (Centaurea calcitrapa), sulphur star-thistle (C. sulphurea), Maltese star-thistle (C. melitensis), and rough star-thistle (C. aspera).[3] Bachelor's button (C. cyaneus) is a relative.[2]
Centaurea solstitialis is a weed also on its native European range (e.g., Turkey, Greece, Italy, France, Spain), consequently, it inhabits highly disturbed ruderal habitats, being typically found on roadsides and cereal crop margins. After introduction in several parts of the world as an exotic species (e.g., Australia, Argentina, Chile, the United States),[4] it has developed local adaptations to the different habitats colonized,[5][6] and an incipient level of reproductive isolation between native and non-native ranges has been detected[7]—a case of ecological speciation.
Star-thistle is a valuable source of pollen, thus nectar for pollinators. Yellow star-thistle, a noted pest plant, is a major nectar source for many central valleys and foothill butterflies.[8] Star-thistle populates ground that has been abused: dry, compacted, or scraped clean. A plant with a taproot system, it has a crucial role in restoring the soil by bringing up vital micronutrients. Similar to many plants classified as 'weeds', they (in the words of Mark Schonbeck) "quickly establish in, protect, and restore soil that has been left exposed by natural and human-caused disturbances".[9]
C. solstitialis grows as a balanced part of the ecosystems in Eurasia, where it is kept in check by an assortment of natural herbivore enemies and other plants that have co-evolved with it in its native habitat. However, it has been introduced in several parts of the world, including Australia, Argentina, Chile, and the U.S.[4] In many of these non-native regions, where the particular array of natural biological controls found in the native regions do not exist, it has become an invasive species and noxious weed.
The yellow star-thistle plant has the ability to create monotypic stands and habitats in the cultivated soil of fields, graded dirt sites, and disturbed natural ecosystem lands. Its colonization eliminates and prevents other plant species from growing, terminating the habitat's biodiversity. Extensive spreading monotypic fields of yellow star-thistle are not uncommon. Its growth plasticity, competitiveness, preference for the Mediterranean climate, and a lack of natural herbivore enemies and co-evolved species, make it a very successful invader. The plant is an invasive pest in field crops, degrades native plant habitats and natural ecosystems, prevents the grazing of domestic animals in rangelands, and is a physical barrier to indigenous animal movement in wildlands.[10]
The introduction of C. solstitialis in North America probably occurred in California sometime after the start of the California Gold Rush, as a fodder seed contaminant in imported Chilean-harvested alfalfa seed, also known as Chilean clover (Trifolium macraei).[11] Star-thistle has been introduced throughout North and South America, Africa, and Europe.[12]
In California, yellow star-thistle was dispersed into agricultural fields and immediately took hold in the state's areas with a Mediterranean climate. Human factors, such as mowing, land grading for development and roads, domestic animal grazing, and disturbance of the soil surface for agricultural tillage and wildland firebreaks have and continue to contribute to the successful thriving and spread of this plant. Yellow star-thistle is now a very common sight in vacant lots and fields, along roadsides and trails, in pastures and ranch lands, and in parks, open-space preserves and natural areas.
After the turn of the 20th century, Spain, France, Italy, and perhaps Turkestan were also likely sources of the invasion's seed in California.[11] Since its introduction to California in the mid-19th century,[11] it has become a large-scale invasive species (noxious weed or invasive exotic) throughout 23 U.S. states. It currently dominates over 15,000,000 acres (61,000 square kilometres) in California alone.[13][14]
By 1970,[14] yellow star-thistle had reached 23 U.S. states.[11] According to the U.S. Forest Service, as of 2006 the plant has been reported present in 41 of the 48 contiguous U.S. states, with the only exceptions being Maine, Vermont, and five of the Deep South states (Arkansas, Louisiana, Mississippi, Alabama, and Georgia).[15] The plant is considered an invasive species in six of the 41 states: California, Oregon, Washington, Idaho, Utah, and New Jersey.
Most herbicides used for controlling yellow star-thistle are registered for range lands, right-of-way, and other non-crop areas. Many auxin-like or growth-regulator herbicides are used for post-emergence control, including 2,4-D, aminopyralid, clopyralid, dicamba, picloram and triclopyr. Alternatively, glyphosate may be used. Pre-emergence herbicides used for yellow star-thistle control include chlorsulfuron and sulfometuron. Pre-emergence and post-emergence herbicides may be used effectively together to kill growing plants as well as any new seedlings that may emerge from the copious soil seed bank often produced by yellow star-thistle. Controlled burning may also be used in conjunction with clopyralid application as an effective integrated approach to yellow star-thistle management.[16]
Aminocyclopyrachlor + chlorsulfuron, aminopyralid, chlorsulfuron, clopyralid, clopyralid + 2,4-D, dicamba, diflufenzopyr + dicamba, picloram, and triclopyr + clopyralid for the Pacific Northwest of North America.[17]
A yellow star-thistle biotype resistant to picloram was discovered in a pasture near Dayton, Ohio, in 1988. This biotype was determined to have cross resistance to other auxin-like herbicides, especially clopyralid.[18] Resistance was discovered to be conveyed by a single nuclear recessive gene.[19]
Picloram-resistant C. solstitialis has been observed which had cross-resistance to clopyralid, dicamba, and fluroxypyr, but not triclopyr or 2,4-D.[20]
Yellow star-thistle is sometimes resistant to removal methods such as mowing and burning, because of its long root system and the seeds' ability to withstand fire. The plant has been the target of biological pest control programs with positive results. Seven types of seed-feeding insects have been released (one accidentally) to control the plant.[21][22]
Three species of weevil in the beetle subfamily Cleoninae effectively reduce seed production in the yellow star-thistle.
Four species of tephritid fruit fly also attack the seedheads of yellow star-thistle.
A variety of the rust fungus Puccinia jaceae var. solstitialis, first released in July 2003 on a ranch in the Napa Valley, has shown promise as an agent against yellow star-thistle,[32][33] dramatically damaging leaves and hampering growth.[33] The rust causes widespread pathology in the leaves of the plant and slows its dispersal. The fungus Synchytrium solstitiale (Synchytrium of phylum Chytridiomycota) is also being considered as an agent of biological control.[34]
Grazing by goats, cattle, or sheep can be effective in controlling yellow star-thistle.[35] Goats will eat star-thistle even in its spiny stage.[36] Because yellow star-thistle growth is particularly difficult to inhibit in canyon rangelands since its remoteness limits control options, goats and other herbivores have become an excellent option to curb the plant's spread. According to one study, grazing has decreased yellow star-thistle presence by 58% when compared to the study's controls. Subplots also showed a 94% decrease in seed heads after only three years of experimentation.[37]
California researchers (Thomsen et al., 1996) tested mowing, controlled sheep grazing and subterranean clover plantings to control star-thistle growth. According to the researchers, subclover would help fill the void left by star-thistle populations.[38]
They compared subclover seeding, grazing, two mowings, grazing + mowing without subclover – and an untouched control without grazing, subclover, or mowing. Thistle seed production was 130 times higher where only one mowing was done, and 1,720 times higher where nothing was done, as compared to the area that had been grazed and mowed twice. Excellent yellow star-thistle control can be achieved with the combination of competing plants, mowing, and rotational grazing.[39]
Although these biocontrol agents have proven somewhat effective at controlling Centaurea solstitialis, there is interest in finding additional agents to further control the species. Two weevils, including Larinus filiformis, a flowerhead feeding weevil, and Ceratapion basicorne (Illiger) (Coleoptera: Apionidae), which develops in the root crown of rosettes have been evaluated and proposed for introduction.[40] Also under evaluation are a stem-feeding flea beetle Psylliodes chalcomera (Illiger) (Coleoptera: Chrysomelidae) (Cristofaro et al. 2004a), a lacebug Tingis grisea Germar (Heteroptera: Tingidae), and a mite Aceria solstitialis de Lillo (Acari: Eriophyidae).[41][42]
Grazing of the plant by horses can cause nigropallidal encephalomalacia or "chewing disease", a neurological condition. The disease generally follows consumption of 60–200% of the horse's body weight over an extended period of a month or more, or 2.3–2.6 kilograms (5.1–5.7 pounds) of star-thistle per 100 kg (220 lb) body weight per day. Though star-thistle is most dangerous when it is the only plant available or is delivered as a contaminant in dried hay, horses may develop a taste for it and seek it out. Many other grazing species, including mules and burros, are not affected.[43]
Although the spines make the plant a pain to deal with, it produces a light honey.[2]
Centaurea solstitialis, the yellow star-thistle, is a species of thorny plant in the genus Centaurea, which is part of the family Asteraceae. A winter annual, it is native to the Mediterranean Basin region and invasive in many other places. It is also known as golden starthistle, yellow cockspur and St. Barnaby's thistle (or Barnaby thistle).
