Centaurea solstitialis (yellow starthistle)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Plant Type
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Hosts/Species Affected
- Host Plants and Other Plants Affected
- Biology and Ecology
- Latitude/Altitude Ranges
- Air Temperature
- Rainfall Regime
- Soil Tolerances
- Natural enemies
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Pathway Causes
- Pathway Vectors
- Plant Trade
- Impact Summary
- Economic Impact
- Environmental Impact
- Threatened Species
- Social Impact
- Risk and Impact Factors
- Uses List
- Similarities to Other Species/Conditions
- Prevention and Control
- Links to Websites
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Centaurea solstitialis Linnaeus
Preferred Common Name
- yellow starthistle
Other Scientific Names
- Calcitrapa solstitialis Lamarck
- Cyanus solstitialis Baumg.
- Leucantha solstitialis A. Löve & D. Löve
International Common Names
- English: Barnaby's thistle; golden starthistle; St. Barnaby's thistle; yellow centaury; yellow cockspur
- Spanish: abremanos; abrepuno amarillo
- French: centaurée du solstice
Local Common Names
- Germany: Sonnenwend Flockenblume
- Italy: spino giallo
- Netherlands: zomercentaurie
- South Africa: geeldissel; yellow centaurea
- CENSO (Centaurea solstitialis)
Summary of InvasivenessTop of page
Yellow starthistle is an invasive, exotic, seedy herbaceous annual. It invades disturbed areas such as roadsides, abandoned fields and waste places. It will occasionally invade crops, but is a serious rangeland weed, reducing forage quality and biodiversity. Greatest infestations are in areas of Mediterranean climate: cool, wet winters followed by hot, dry summers. It is listed as a noxious weed in 11 US states and two Canadian Provinces (Rice, 2003).
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Dicotyledonae
- Order: Asterales
- Family: Asteraceae
- Genus: Centaurea
- Species: Centaurea solstitialis
Notes on Taxonomy and NomenclatureTop of page
The currently accepted scientific name for yellow starthistle is Centaurea solstitialis. This designation was published by Linnaeus in 1753 (USDA-ARS, 2003). According to Ochsmann (2003), other names that have been proposed are Calcitrapa solstitialis (L.) Lam. (1778), Cyanus solstitialis (L.) Baumg. (1816) and Leucantha solstitialis (L.) Á. Löve & D. Löve (1961).
DescriptionTop of page
Rosettes bolt to form stems that are rigid, branched from near the base, bushy, woolly, greyish, and 150-750 mm tall. Stem leaves are alternate with cottony hairs, entire, stalked, narrow, sharp-pointed and lobeless. Upper leaves are narrow and short, 1.3-2.5 cm long (USDA, 1970).
Flower heads are terminal, urn-shaped with many inconspicuous bright-yellow, tubular flowers stiffly spreading above the narrow tip, the whole 1.8-2.5 cm high, without leafy bracts. Involucral bracts are numerous, closely overlapping, stiff and papery. Each of the middle bracts end in a stout, rigid, unbranched yellow spine, 1.2-2.5 cm long, with one or two pairs of very short spines at its base (USDA, 1970).
Flowers produce two kinds of glabrous achenes. One is smooth, dark brown, 2-3 mm long, oblong, and notched on one side, just above the base. Another is similar, but lighter coloured and has a pappus (plumed) of many white, thin, bristle-like spines of unequal length, 3-5 mm long. The plume helps the seed to disperse in the wind. More than 75% of the seeds are of the plumed type (USDA, 1970; Zouhar, 2002).
Plant TypeTop of page
DistributionTop of page
Yellow starthistle has spread throughout temperate parts of the world from its native range in Eurasia and the Mediterranean. It is native to Armenia, Azerbaijan, Georgia, Iran, Iraq, Lebanon, Syria, Tajikistan, Turkey, Turkmenistan, Ukraine, Algeria, Tunisia, Albania, Bulgaria, France, Greece, Italy, Spain and former Yugoslavia (USDA-ARS, 2003). It is now found throughout Europe, in South America and Africa and as far as the Asian steppes, but does not persist in cold northerly areas (Maddox et al., 1985; Roche et al., 1986).
Yellow starthistle is present in 41 of 50 US states and four Canadian provinces. In the eastern two-thirds of the USA, infestations are sporadic and localized, and populations fail to establish (Zouhar, 2002; USDA-NRCS, 2002). It is a particular problem in California, where over 6.9 million hectares are infested, and is increasing in Idaho, Oregon and Washington (Maddox et al., 1985; Maddox and Mayfield, 1985; Callihan et al., 1989; Duncan, 2001). A record for Vermont in previous versions of this datasheets could not be traced and has been deleted (12/09/12).
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.Last updated: 17 Feb 2021
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Botswana||Present||Introduced||Invasive||Original citation: Maddox et al. (1985)|
|South Africa||Present||Introduced||Invasive||Original citation: Maddox et al. (1985)|
|Federal Republic of Yugoslavia||Present||Native||Invasive|
|Canada||Present||Present based on regional distribution.|
|Trinidad and Tobago||Present||Introduced||Invasive|
|-Maryland||Present, Few occurrences||Introduced||Invasive|
|Australia||Present||Introduced||Invasive||Original citation: Maddox et al. (1985)|
|-New South Wales||Present||Introduced||Invasive||Original citation: Maddox et al. (1985)|
|-Queensland||Present||Introduced||Invasive||Original citation: Maddox et al. (1985)|
|-Victoria||Present||Introduced||Invasive||Original citation: Maddox et al. (1985)|
|New Zealand||Present||Introduced||Invasive||Original citation: Maddox et al. (1985)|
|Papua New Guinea||Present||Introduced||Invasive||Original citation: Maddox et al. (1985)|
|Argentina||Present||Introduced||Invasive||Original citation: Maddox et al. (1985)|
|Chile||Present||Introduced||Invasive||Original citation: Maddox et al. (1985)|
History of Introduction and SpreadTop of page
Risk of IntroductionTop of page
A major risk for introduction of C. solstitialis is through contaminated seeds (Zouhar, 2002).
HabitatTop of page
Yellow starthistle thrives in areas with cool, wet winters followed by hot, dry summers. It is found in well-drained soils, especially where fire, over-grazing, road construction or other causes have seriously disturbed the vegetation (Maddox, 1981; Roche et al., 1986; Thomsen et al., 1996b, Hierro et al., 2011).
It is often found as an invader of grasslands, where it forms dense stands and crowds out annual grasses. In California, USA, most of the infestation is in the Central Valley and adjacent foothills. It is less commonly found in desert, high mountain and moist coastal sites (Zouhar, 2002).
Habitat ListTop of page
|Terrestrial||Managed||Cultivated / agricultural land||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Managed forests, plantations and orchards||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Managed grasslands (grazing systems)||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Disturbed areas||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Rail / roadsides||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Natural / Semi-natural||Natural grasslands||Present, no further details||Harmful (pest or invasive)|
Hosts/Species AffectedTop of page
Host Plants and Other Plants AffectedTop of page
|Medicago sativa (lucerne)||Fabaceae||Main|
Biology and EcologyTop of page
Yellow starthistle is an invasive herbaceous annual or biennial of the Asteraceae family that spreads by seeds, forming dense stands in open, sunny areas (Maddox, 1981).
Yellow starthistle is diploid (2n=16), with a large amount of genetic variation (Sun, 1997; Sun and Ritland, 1998). Seeds from 34 distinct stands in California, Idaho and Oregon, USA, had wide biotypic variation for rosette area, plant height, number of branches, average number of flowers and seed production rate (Zouhar, 2002). Although hybrids with other Centaurea species have been found in Europe (Maddox et al., 1985), none have been found in California (Maddox, 1981).
Physiology and Phenology
Germination rate of plumed seed is higher in late autumn and winter, and that of plumeless seed is higher in spring. Over 90% of yellow starthistle seeds are able to germinate within a week after dispersal. Seeds germinate at both low and high temperatures, but emergence rate is nearly 100% between 10 and 20°C. Emergence is highest after early-season rainfall (Joley et al., 1997; Benefield et al., 2001). Although seeds can germinate in the dark, germination is greatly reduced in dark environments and appears to be stimulated by white light (Nolan and Upadhyaya, 1988; Joley et al., 1997).
Yellow starthistle has a very long life cycle for an annual plant, and it sometimes behaves as a biennial. It generally germinates from autumn through spring, but may not complete the life cycle until the following autumn or winter. The deep taproot extends below the zone of root competition of associated annual species and allows growth and flowering to occur well into the summer, long after other annual species have died (Prather, 1995).
Flowering and seed production times vary according to climate. Generally, plants bolt in the spring and flower in the summer. In California, yellow starthistle bolts from May to June. In June and July the flower buds form and spines appear at the tips of the bracts around the flower bud. Flowering takes place from June through August when the bright-yellow flowers open. In August, seeds are formed and leaves and stems begin to turn a straw colour. Seeds mature after the flowerhead fades from a bright yellow to a dull straw colour (Thomsen et al., 1994).
The small tubular florets produce two types of seed: light coloured, plumed seeds and darker, plumeless seeds. Florets in the centre of the head produce seeds with a ring of fine, white, thin bristles (plume). The outer circle of florets produce plumeless seeds. In general, the plants mature by late summer. By September or October, the plants dry out, lose leaves, and turn to silvery-grey skeletons with white, cottony terminal heads. In some places and under certain conditions, yellow starthistle survives over the winter, regrows in the spring, and dries out by early summer (Maddox, 1981).
When compared with the closely related congeners Centaurea calcitrapa and C. sulphurea which have a similar life history and have also been introduced to North America from Europe but have not become invasive there, C. solstitialis was found to grow faster, compete better and show faster evolution to larger seed size and seedling growth (Graebner et al., 2012).
Yellow starthistle is monoecious, generally self-incompatible and pollinator-dependent. Fertilization is mostly by outcrossing, but self-fertilization sometimes occurs. Bumblebees and European honeybees are the most important pollinators (Harrod and Taylor, 1995; Zouhar, 2002).
Reproduction is entirely through the production of numerous seeds. Seed output ranges from 700 to 10,000 seeds per plant, with averages of between 38 and 45 per seedhead (Maddox, 1981). Seed rain can be as dense as 29,000/m² (Callihan et al., 1993). Most seeds germinate within a year, but some can last 10 years or more in the soil (Callihan et al., 1993; Lanini et al., 1995; Thomsen et al., 1996b).
The plumed, lighter-coloured seeds disperse quickly after maturity, and the darker, plumeless seeds persist in the flowerhead until windy weather or other disturbances break them up (Roche, 1965). By December, the bracts are usually lost from the flower, and the white, cottony base of the head is revealed. Although most yellow starthistle seeds fall within 0.6 m of the parent plant (Roche, 1992), the plumed seeds can disperse 1.5 m by wind, and many seeds disperse over longer distances in fur, feathers or as a result of human activity. In California, human activity probably accounts for most of the seed dispersal (Callihan et al., 1989; Roche, 1992; Thomsen et al., 1996b).
Yellow starthistle is best adapted to open grasslands with an average annual precipitation of between 250 and 1500 mm, and most large infestations are found below 1500 m. It is generally associated with well-drained, deep silt loam and loam soils, but can tolerate shallow, rocky areas (Zouhar, 2002). The optimum environment is cool, wet winters followed by hot, dry summers. It can survive mean annual temperatures of between 4.3 and 18°C (Maddox et al. 1985; Climate, 2003). Limiting factors may be moisture during summer drought and light during winter growth (Zouhar, 2002). Hardiness, wide climate profile and abundant seed production of yellow starthistle make it potentially invasive everywhere it is introduced. However, it is uncommon in deserts and moist coastal sites (Zouhar, 2002).
At its northern limits, it appears on south-facing slopes where there is more heat and light. 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 (DiTomaso, 2001).
A study by Dukes et al. (2011) suggests that the prevalanece of yellow starthistle may increase in western North America with warmer climatic conditions and elevated CO2 and nitrate deposition levels.
Yellow starthistle is found in grasslands. In the Pacific Northwest, it is found with cheatgrass (Bromus tectorum). In Washington, it is found with bluebunch wheatgrass (Pseudoroegneria spicata) and Idaho fescue (Festuca idahoensis) (DiTomaso, 2001).
Latitude/Altitude RangesTop of page
|Latitude North (°N)||Latitude South (°S)||Altitude Lower (m)||Altitude Upper (m)|
Air TemperatureTop of page
|Parameter||Lower limit||Upper limit|
|Absolute minimum temperature (ºC)||-21|
|Mean annual temperature (ºC)||4||18|
|Mean maximum temperature of hottest month (ºC)||28||34|
|Mean minimum temperature of coldest month (ºC)||-21||5|
RainfallTop of page
|Parameter||Lower limit||Upper limit||Description|
|Dry season duration||6||number of consecutive months with <40 mm rainfall|
|Mean annual rainfall||250||1500||mm; lower/upper limits|
Rainfall RegimeTop of page
Soil TolerancesTop of page
Natural enemiesTop of page
|Natural enemy||Type||Life stages||Specificity||References||Biological control in||Biological control on|
|Bangasternus orientalis||Herbivore||Plants|Seeds||North America; Oregon|
|Chaetorellia australis||Herbivore||Plants|Seeds||USA; Oregon|
|Eustenopus villosus||Herbivore||Plants|Seeds||USA; Oregon|
|Urophora sirunaseva||Herbivore||Plants|Seeds||USA; Oregon|
Notes on Natural EnemiesTop of page
Means of Movement and DispersalTop of page
Natural Dispersal (non-biotic)
As the seedheads deteriorate, plumeless seeds drop to the soil near the parent plant (Callihan et al., 1989). Plumed seeds usually land no more than a metre away, and maximum wind dispersal is about 5 m (Roche, 1992).
Vector Transmission (biotic)
Plumed seeds stick to fur and clothing, resulting in dispersal through short to medium distances by humans and animals. Birds such as pheasants, quail, house finches and goldfinches feed heavily on yellow starthistle seeds and also are capable of long-distance dispersal (Roche, 1992).
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 (DiTomaso, 2001).
There have been multiple introductions of yellow starthistle to the USA, mainly through contaminated lucerne seed (Maddox et al., 1985; Luster et al., 2001). It is often spread as a contaminant in crop seed, but is also carried in hay or straw or by vehicles including construction or maintenance equipment (Maddox, 1981; Roche et al., 1986; Roche and Talbott, 1986; DiTomaso, 1998).
In California, 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 (DiTomaso, 2001).
As it is not an ornamental, C. solstitialis has probably never been intentionally introduced.
Pathway CausesTop of page
Pathway VectorsTop of page
Plant TradeTop of page
|Plant parts liable to carry the pest in trade/transport||Pest stages||Borne internally||Borne externally||Visibility of pest or symptoms|
|True seeds (inc. grain)||weeds/seeds||Yes|
|Plant parts not known to carry the pest in trade/transport|
|Fruits (inc. pods)|
|Growing medium accompanying plants|
|Stems (above ground)/Shoots/Trunks/Branches|
Impact SummaryTop of page
|Fisheries / aquaculture||None|
Economic ImpactTop of page
Yellow starthistle is toxic to horses in large amounts, causing equine nigropallidal encephalomalacia or 'chewing disease'. The first signs of poisoning are an inability to eat or drink as the muscles of the lips, face and tongue become stiff and swollen, giving the horse a fixed expression. Poisoning eventually results in permanent brain damage, and severely affected animals eventually die of thirst and starvation. Sheep, cows and other livestock are not affected (Kingsbury, 1964; Callihan et al., 1982; DiTomaso, 1998).
Millions of dollars in losses result from interference with livestock grazing and forage harvesting procedures, and lower yield and forage quality of rangelands (Callihan et al., 1982; Roché and Roché, 1988). Because of the spiny nature of yellow starthistle, livestock and wildlife avoid grazing in heavily infested areas. Livestock have slower weight gain, and reduced quality of meat, milk, wool and hides (DiTomaso, 2000).
Yellow starthistle is the most important roadside weed problem in much of central and northern California and has, on occasion, caused problems in dryland cereals, orchards, vineyards, cultivated crops and wastelands (Maddox et al., 1985).
It can also reduce land value and access to recreational areas (DiTomaso, 1998; Roche and Roche, 1988), reduce wildlife habitat and forage, displace native plants, and decrease native plant and animal diversity (Sheley and Larson, 1994). Dense infestations threaten natural ecosystems by fragmenting plant and animal habitats (Scott and Pratini, 1995).
Depletion of rangeland moisture has a significant impact on forage plants. Estimated losses due to water depletion are $16 to $56 million each year in California (DiTomaso, 2001).
Environmental ImpactTop of page
Existing vegetation is displaced wherever yellow starthistle invades. In dense stands, both large canopied, deep-rooted plants receiving full sunlight, and an understorey of smaller, shallow-rooted, shaded plants are present. Yellow starthistle can halt the germination and establishment of native plants by complete niche domination for example depleting soil moisture (Spencer et al., 2011). Native blue oak (Quercus douglasii) and purple needlegrass (Nassella pulchra) populations are threatened in North America (DiTomaso, 2001).
Threatened SpeciesTop of page
|Threatened Species||Conservation Status||Where Threatened||Mechanism||References||Notes|
|Baccharis vanessae (Encinitas baccharis)||NatureServe; USA ESA listing as threatened species||California||Competition - monopolizing resources||US Fish and Wildlife Service (2011a)|
|Bidens campylotheca subsp. pentamera (ko`oko`olau)||CR (IUCN red list: Critically endangered); USA ESA listing as threatened species||California; Oregon||Ecosystem change / habitat alteration||US Fish and Wildlife Service (2007c)|
|Brodiaea pallida (Chinese Camp brodiaea)||NatureServe; USA ESA listing as threatened species||California||Competition - monopolizing resources||US Fish and Wildlife Service (2012)|
|Camissonia benitensis (San Benito evening-primrose)||NatureServe; USA ESA listing as threatened species||California||Competition - monopolizing resources||US Fish and Wildlife Service (2006)|
|Grindelia fraxinipratensis (ash meadows gumplant)||NatureServe; USA ESA listing as threatened species||California; Nevada||Competition - monopolizing resources||US Fish and Wildlife Service (2007a)|
|Hesperolinon congestum (Marin dwarf-flax)||NatureServe; USA ESA listing as threatened species||California||Competition - monopolizing resources||US Fish and Wildlife Service (2011b)|
|Mirabilis macfarlanei||NatureServe; USA ESA listing as threatened species||Idaho; Oregon||Competition - monopolizing resources; Ecosystem change / habitat alteration||US Fish and Wildlife Service (2000)|
|Oenothera deltoides subsp. howellii (Antioch Dunes evening-primrose)||NatureServe; USA ESA listing as endangered species||USA; California||Competition - monopolizing resources||US Fish and Wildlife Service (2008)|
|Silene spaldingii (Spalding's catchfly)||USA ESA listing as threatened species||Idaho; Montana; Oregon; Washington||Competition - monopolizing resources||US Fish and Wildlife Service (2007b)|
|Speyeria callippe callippe (callippe silverspot butterfly)||USA ESA listing as endangered species||USA; California||Ecosystem change / habitat alteration||US Fish and Wildlife Service (2009a)|
|Zeltnera namophila||No Details||California; Nevada||Competition - monopolizing resources||US Fish and Wildlife Service (2009b)|
Social ImpactTop of page
Risk and Impact FactorsTop of page
- Invasive in its native range
- Proved invasive outside its native range
- Highly adaptable to different environments
- Highly mobile locally
- Has high reproductive potential
- Has propagules that can remain viable for more than one year
- Negatively impacts agriculture
- Negatively impacts animal health
- Negatively impacts tourism
- Reduced amenity values
- Reduced native biodiversity
- Competition - monopolizing resources
- Produces spines, thorns or burrs
- Highly likely to be transported internationally accidentally
- Difficult/costly to control
UsesTop of page
Uses ListTop of page
- Poisonous to mammals
Similarities to Other Species/ConditionsTop of page
There are two other Centaurea species in California with yellow flowers and long spines on the seedheads: Malta starthistle (Centaurea melitensis) and Sicilian starthistle (Centaurea sulphurea). The corolla of C. sulphurea is larger than 25 mm, whereas yellow starthistle is 10-20 mm. The central spikes of main floral bracts are smaller on C. melitensis, 5-10 mm, versus 10-25 mm on C. solstitialis. In other western states, bighead knapweed (Centaurea macrocephala) also has yellow flowers but does not have long, sharp spines on the flowerheads (DiTomaso, 2001).
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
Yellow starthistle responds best to integrated pest management (IPM) programmes that prevent seed formation and deplete the seed bank. Timing is critical. The grazing period should be timed to the palatable bolting stage in late May or June, before spines are formed (Thomsen et al., 1989, 1991). Stands should be mowed during early flowering when 2-5% of the plants show a bright yellow colour (Thomsen et al., 1997). Cultivation should be timed with autumn emergence, after the rains begin. Repeated cultivations are generally needed to control each new flush of seedlings. Cultivation will also bring deeply buried seeds to the surface where conditions are favourable for germination (Lanini et al., 1995; Thomsen et al., 1996b). Burning should be done at the end of the rainy season, but prior to the formation of viable seeds. According to Thomsen et al. (1996b), the best time to burn is probably when plants are in the early flowering stage, before seed formation.
Revegetation is critical for preventing weed infestations where the soil has been disturbed or the vegetation removed. Because of the aggressive nature of yellow starthistle, dense stands of perennial grasses, oats, wheat and legumes such as clover are needed (Woo et al., 1999).
Desirable plant species should be chosen with the site in mind, and a preliminary small-scale planting may be necessary. According to Thomsen et al. (1996b), more than a year will be required to find the species or mix of species most effective, because species that are excellent in the first year may be disappointing in subsequent years.
By depleting soil moisture and providing additional shade, early-growing perennial grasses can help to suppress yellow starthistle. 'Berber' orchardgrass (Dactylis glomerata) and Idaho fescue (Festuca idahoensis), which begin growth early, suppress thistle and other annual weeds more effectively than later growing perennial grasses such as intermediate Agropyron intermedium [Elytrigia intermedia subsp. intermedia] and tall A. elongatum [Elytrigia elongata] wheatgrasses (Borman et al., 1991).
Perennial grasses are slow to establish and most require at least 2 years to be competitive. Thomsen et al. (1994) recommended using a no-till drill for reseeding, as this allows seeding without turning the soil, and helps to keep deeply buried starthistle seeds from germinating.
Thomsen et al. (1996a, b) reseeded with dryland legumes. Subterranean clovers (Trifolium spp.) were chosen as pasture plants because they are palatable, self-seeding, and produce flowers and seeds below the bite of grazing animals. In addition, strong winter- and spring-growth make them useful as cover crops in vineyards and orchards.
Well-timed, heavy grazing by goats, sheep and cattle can reduce yellow starthistle seed production and biomass. Grazing should be timed to the bolting stage in late May or June, before spines are on the plant. Grazing earlier, at the rosette stage, favours yellow starthistle development by elimination of competitive plants which do not regrow as quickly. As most defoliated yellow starthistle will recover from one grazing, it is necessary to bring the animals back one to four times at about 2 week intervals under rotational grazing. Alternately, grazers can be left on site for 2-3 months under a continuous grazing regimen (Thomsen et al., 1993).
Hand-pulling, hoeing or weed whipping can be effective for isolated plants or small populations. The best time to remove plants is after they have bolted and before they have produced seeds (DiTomaso, 2001).
Mowing can significantly reduce the density of yellow starthistle, but mowing heights must be set correctly and mowings must be accurately timed. Plants mowed at ground level do not survive; however, in practice this is impractical because rocks and debris can interfere with mower blades. Mowing at a height of 10 cm gives the most effective reduction of yellow starthistle seedhead number and biomass (Benefield et al., 1999).
Late mowing is more effective than early mowing, but mowing should not be attempted after seeds are produced. Viable seeds are produced after the bright yellow flowers turn to a dull straw colour. Mowing during early flowering when 2-5% of the plants show a bright yellow colour gives the greatest reductions in both flower and seedling density (Thomsen et al., 1997).
Mowing success also depends on soil conditions. It is most effective when soil moisture is low, and one mowing might be enough. In most situations, one or two more mowings will be necessary and should be done once flowering resumes, approximately 4-6 weeks after the initial mowing. Since yellow starthistle growth varies according to site conditions and climactic factors, monitoring is necessary to time mowing correctly (Thomsen et al., 1997).
Starthistle can either grow upright or can develop an extensive low-branching growth habit. Starthistle that grows with a low-branching form is very difficult to control with mowing. For this reason, the integration of mowing with competing vegetation is important. Competing vegetation early in the year forces the starthistle to grow upright, and mowing is then more effective later in the season (Benefield et al., 1999). Competing vegetation also cuts in half the number of flowerheads per starthistle plant (Pitcairn et al., 1998).
Mowing before early flowering, soon after the bolting stage, will actually increase yellow starthistle densities. In addition, mowing should not be done if desirable vegetation is extensively flowering or setting seed, as this will favour starthistle (Lanini et al., 1995; Thomsen et al., 1997).
Cultivation can effectively control yellow starthistle. The equipment used depends on soil condition and plant stage. For loose soil and small plants, spike-tooth or spring-harrows are usually sufficient. For larger plants, a rotary hoe or disk is best (Thomsen et al., 1996b).
The best time to begin cultivation is when emergence begins in the autumn. Repeated cultivations are generally needed to control each new flush of seedlings. Cultivation will also bring deeply buried seeds to the surface where conditions are favourable for germination. Repeat cultivation then depletes the seedbank (Lanini et al., 1995; Thomsen et al., 1996b).
Herbicides alone are not a good way of dealing with starthistle in a rangeland situation. Most pre-emergent herbicides are not registered for rangeland, and repeated applications of post-emergent herbicides such as 2,4-D and others are needed to have any long-term effect. Use of clopyralid, which has both pre- and post-emergent activity, can lead to a build-up of other undesirable weed species such as medusahead (Taeniatherum caput-medusae) or barbed goatgrass (Aegilops triuncialis) (DiTomaso, 1996; DiTomaso et al., 1999). Repeated use of herbicides leads to yellow starthistle resistance (Fuerst et al., 1996).
If herbicides are used at all, they should be part of an IPM programme to establish competing vegetation (Woo et al., 1999). Generally, picloram and clopyralid are the most effective herbicides. They are best applied at the early rosette stage in the first or second year of a long-term IPM management plan. Spot applications of glyphosate at the bolting or early flowering stage can sometimes be helpful for isolated plants or small populations (DiTomaso, 2001).
Six insect natural enemies of yellow starthistle have been imported into the USA from Greece and are established as biological control agents (Rees et al.,1996). Three of these are weevils, including Bangasternus orientalis (brought into the USA in 1985), Eustenopus villosus (1990) and Larinus curtus (1992). The rest of the biocontrol agents are flies, including one gall fly Urophora sirunaseva (1985), the peacock fly Chaetorellia australis (1989) and the false peacock fly C. succinea (1989) (Maddox et al., 1991; Turner et al., 1994, 1995, 1996a, b; Balciunas and Villegas, 1999). The gall fly U. jaculata was imported, but did not establish (Turner et al., 1995; Pitcairn et al., 1998).
The false peacock fly (C. succinea) was introduced accidentally along with the peacock fly (C. australis). Ironically, the true peacock fly the USDA was trying to import did not establish well in California, but the false peacock fly is thriving at 415 survey sites in 44 counties. The false peacock fly is also more effective, infesting up to 50% of thistle heads in many sites, while the true peacock fly typically never infests more than 5% at a site (Balciunas and Villegas, 1999).
All six insects attack the seedhead and reduce seed production. They are all highly specific to yellow starthistle and do not attack commercially valuable or native plants. It is still too early to know their efficacy as control agents, but adult feeding damage by the hairy weevil E. villosus can be very extensive at release sites 3 years or older (Pitcairn et al., 1998). At one site, seedheads had 78% fewer seeds due to attack by false peacock fly (Balciunas and Villegas, 1999). In another area, a combination of E. villosus and C. succinea reduced seed production by 43-76% (DiTomaso, 2001).
There are few diseases of adult yellow starthistle in North America. However, seedling mortality due to disease can be extensive, particularly in high density settings. The most effective pathogen in North America may be Sclerotinia minor, but this microbe has too wide a host range to be useful as a biocontrol agent. So far, the rust fungus Puccinia jaceae var. solstitialis shows the greatest promise (Woods, 1998).
Though potentially serious, C. solstitialis can be controlled by an integrated programme of monitoring, biological control, proper grazing rotations, mulching, mowing, controlled burning and reseeding with competitive vegetation. Such an integrated approach can effectively control the weed without the use of toxic herbicides (Woo et al., 1999).
Cultivation should always be integrated with revegetation. Dense sowing of oats or wheat in autumn, after thoroughly tilling, mulching and fertilizing may completely eliminate the weed. One approach is to use a wheat straw mulch, at least 13 mm deep, into which 56 kg/ha of oats have been mixed (Dremann, 1992, 1996).
Grazing should be combined with other control methods such as competitive plantings and mowing. Animals must be confined with the starthistle by temporary electric fencing, and moved at times to adjust grazing pressure. If goats are used, exclosures may be needed to protect patches of native or desirable vegetation within the grazed area (Popay and Field, 1996).
Mowing should be integrated with revegetation. Early mowing enhances subclover performance because it reduces shading by tall annual grasses. Some varieties can form a dense network of interwoven stems and leafy canopies that reduce sunlight to yellow starthistle rosettes. But, whatever the variety used, competition from subclover by itself is insufficient to control yellow starthistle, and late spring or early summer mowings are required (Thomsen et al., 1996a).
Burning should also be combined with revegetation to prevent re-invasion. Burning should be done at the end of the rainy season, but before the formation of viable seeds. The best time to burn is the early flowering stage, prior to seed formation. Sowing grass seed the winter before a planned burn can provide fuel for the burns. Total combustion is not necessary. Plants can be killed by foliar scorching and stem girdling. A flamethrower can be used to scorch patches of accessible plants that do not burn (Hastings and DiTomaso, 1996; Thomsen et al., 1996b).
One successful burning programme in California produced a 90% reduction in relative starthistle cover, increased perennial grass cover by 300% and reduced the soil seed bank by over 99% (Hastings and DiTomaso, 1996).
ReferencesTop of page
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