Author: Joe DiTomaso, Global Invasive Species Team, The Nature Conservancy
- 1 Overview
- 2 Taxonomy
- 3 Stewardship summary
- 4 Natural history
- 5 Condition
- 6 Management/Monitoring
- 7 Economic Uses
- 8 Resources
- 9 Information sources
- 10 Images from Bugwood.org
- This annual plant can grow 0.25-3 ft. (0.07-0.9 m) in height.
- Leaves that are located near the base are petiolate, dissected, 2-6 in. (5-15.2 cm) long. These certain leaves are normally absent during the time of flowering. Leaves further up the stem are 0.4-4 in. (1-10 cm) long.
- Flowering occurs from June-October. Yellow flowers appear in heads at the tips of the branches. Bracts that extend the flower head have a small cluster of spines.
- This seed type represents between 10 and 25% of the total seed and often remains in the seed-heads until late fall or winter. The central flowers produce glossy, gray, or tan to mottled cream-colored and tan seeds with a short, stiff, unequal, white pappus 0.08-0.2 in. (2-5 mm) long.
- Ecological Threat
- Centaurea solstitialis invades woodlands, fields, pastures and roadsides.
Centaurea comes from the Greek word Centaur that means spearman or piercer. The Latin term solstitium refers to the summer solstice or the longest day of the year, and –alis means "pertaining to." Thus, the specific epithet solstitialis means pertaining to the longest day of the year. This is in reference to the ability of C. solstitialis to flower very late into the summer.
Centaurea solstitialis is a winter annual that can form dense impenetrable stands that displace desirable vegetation in natural areas, rangelands, and other places. It is best adapted to open grasslands with deep well-drained soils and average annual precipitation between 10 and 60 inches (25 and 150 cm) per year. Centaurea solstitialis originated from southern Europe but was introduced from Chile to California during the gold rush. It has spread rapidly since the mid-1900s and is now estimated to infest 15-20 million acres (6-8 million ha) in California and a couple of additional million acres in other western states.
Control of C. solstitialis cannot be accomplished with a single treatment or in a single year. Effective control requires suppression of seed production. An integrated approach using several methods is the most ecologically sound strategy for long-term management of C. solstitialis.
Mechanical, cultural, biological and chemical control options are available for management of C. solstitialis. Mowing can be used as a mechanical option for C. solstitialis control provided it is well timed and used on plants with a high branching pattern. Cultural control options include grazing, prescribed burning, and re-vegetation with competitive species.
Sheep, goats or cattle are effective in reducing C. solstitialis seed production when grazed after plants have bolted but before spines form on the plant. Goats will eat starthistle even in the spiny stage.
In California, burning is best performed at the end of the rainy season when flowers first appear. Centaurea solstitialis should be green at this time and will require desiccated vegetation to burn. Most annual vegetation other than C. solstitialis, particularly grasses, should have dried and shed their seeds by this time. Burning can also increase the recovery and density of perennial grasses.
Re-vegetation programs using perennial grasses or legumes can be effective for management of C. solstitialis but establishment may be difficult in areas without summer rainfall.
Six biological control agents of C. solstitialis have been imported from Europe and are well established in the western United States. Of these the most effective are the hairy weevil (Eustenopus villosus) and the false peacock fly (Chaetorellia succinea). These insects attack the flower/seed head, and directly or indirectly reduce seed production by 43 to 76%. They do not, by themselves, provide sustainable management of C. solstitialis, but can be an important component of an integrated approach.
Clopyralid and picloram (not registered in California) are the most effective herbicides for full season control of C. solstitialis. Unlike most postemergence herbicides, they provide both foliar and soil activity. The best timing for application is when C. solstitialis is in the early rosette stage. Clopyralid gives one season of control and is generally used at 1.5 oz a.e./acre, 4 oz product/acre (110 gm a.e./ha; 290 gm product/ha). Picloram has longer soil residual activity than clopyralid (two to three years) and is applied at 0.25 lb and 0.375 lb a.e./acre (0.28 kg and 0.42 kg a.e./ha). Glyphosate is a non-selective herbicide that is also effective on C. solstitialis. It will control bolted plants at 1 lb a.e./acre, 0.33 gal product/acre (1.1 kg a.e./ha; 9.4 liters product/ha) or 1% solution and can be used as a late season spot treatment to small infestations or escaped plants.
Invasive erect winter annuals (sometimes biennials) mostly to 1 m tall (occasionally to 2 m tall) with spiny yellow-flowered heads.
Cotyledons oblong to spatulate, base wedge-shaped, tip +/- squared, glabrous, 6-9 mm long, 3-5 mm wide. First few leaves typically oblanceolate. Subsequent rosette leaves oblanceolate, entire to pinnate-lobed. Later rosette leaves 15 cm long, typically deeply lobed +/- to midrib. Lobes mostly acute, with toothed to wavy margins. Terminal lobes largest, triangular to lanceolate. Leaves of rosettes under reduced light levels are larger and more erect. Surfaces +/- densely covered with fine cottony hairs that +/- hide stiff thick hairs and leaf surfaces.
Stems stiff, openly branched from near or above the base or sometimes not branched in very small plants. Stem leaves alternate, mostly linear or +/- narrowly oblong to oblanceolate. Lower stem leaves sometimes +/- deeply pinnate-lobed. Margins smooth, toothed, or wavy. Leaf bases extend down the stems (decurrent) and give stems a winged appearance. Largest stem wings typically to approximately 3 mm wide. Foliage grayish- to bluish-green, densely covered with fine white cottony hairs that +/- hide thick stiff hairs and glands. Rosette leaves typically withered by flowering time.
Taproots grow vigorously early in the season to soil depths of 1 m or more, giving plants access to deep soil moisture during the dry summer and early fall months.
Plants flower from May to December in California, but have a narrower flowering period in more northern states with shorter seasons. Flowerheads are ovoid, spiny, solitary on stem tips, and consist of numerous yellow disk flowers. Vigorous individuals of yellow starthistle may develop flower heads in branch axils. Involucre (phyllaries as a unit) is approximately 12-18 mm long. Phyllaries are palmately spined, with one long central spine and 2 or more pairs of short lateral spines. Phyllaries are more or less densely to sparsely covered with cottony hairs or with patches at the spine bases. The central spine of the main phyllaries are 10-25 mm long, stout, yellowish to straw-colored throughout. Lateral spines occur typically in 2-3 pairs at the base of the central spine. The corollas are yellow, and mostly 13-20 mm long. The number of flowers per head varies and depends upon growing conditions, but generally ranges between 30 and 100. The flowers are insect-pollinated, and are mostly self-incompatible.
Flowerheads produce two types of achenes (seeds), both glabrous, approximately 2-3 mm long, with broad bases. Achenes are +/- barrel-shaped, +/- compressed, and laterally notched at the base. Flowers at the periphery of the flowerheads produce dull dark brown, often speckled with tan, achenes that are darker and have no pappus. This seed type represents between 10 and 25% of the total seed and often remain in the seedheads until late fall or winter. The central flowers produce glossy, gray or tan to mottled cream-colored and tan seeds with a short stiff, unequal, white pappus (2-5 mm long). This represents the majority of seed produced (75-90%), and dispersal occurs soon after dried flower remnants are detached from heads.
Plants usually senesce in late summer or fall. Heads shed the central spines, but tightly retain a ball of dense fuzzy gray hairs (chaff) on the receptacle. Often a dense layer of thatch develops on heavily infested sites.
Centaurea solstitialis is best adapted to open grasslands with average annual precipitation between 10 and 60 inches (25 to 150 cm) per year. It is generally associated with deep well-drained soils. Although populations can occur at elevations at high as 8,000 ft (2,400 m), most large infestations are found below 5,000 ft (1,500 m).
Biology and ecology
Centaurea solstitialis typically begins flowering in late May and continues through September, sometimes into December or later. There are very low levels of self-fertilization in C. solstitialis. Honeybees play an important role in the pollination of C. solstitialis, and can account for 50% of seed set. Bumblebees are the second most important floral visitor to flowers, but several other insects also contribute to fertilization of the ovules.
On average, seedheads require 21 days to progress from pre-bloom to petal abscission. The time period from flower initiation to the development of mature viable seed is only 8 days. To prevent seed production, it is most practical to gauge timing of late season control practices around flower initiation, as this stage is easily recognizable. To prevent new seed recruitment, late-season control options such as tillage, mowing, prescribed burning, and herbicides should be conducted before approximately 2% of the total spiny heads have initiated flowering.
Average seed production per seedhead ranges from about 35 to over 80 seeds , depending upon the site. Large plants can produce over 100,000 seeds. Centaurea solstitialis infestations can produce 50-100 million seeds per acre (20-40 million seeds/ha). Of the total seeds produced, between 75% and 90% are pappus-bearing and 10% to 25% are non-pappus-bearing.
The pappus-bearing seeds are usually dispersed soon after flowers senesce and drop their petals. However, non-pappus-bearing seeds can be retained in the seed head for a considerable period of time, extending into the winter. These seeds have no wind dispersal mechanism and most fall to the soil just below the parent plant. With pappus-bearing seed, the pappus is not an effective long distance wind dispersal mechanism. About 92% of C. solstitialis seed fall within 2 feet (60 cm) of the parent plant, with a maximum dispersal distance of 16 ft (4.9 m) over bare ground even at wind gusts of 25 miles/hr (40 km/hr). By comparison, birds such as pheasants, quail, house finches, and goldfinches feed heavily on C. solstitialis seeds and are capable of transporting seed greater distance. Human influences including vehicles, contaminated crop seed or hay, road maintenance, and moving livestock can also contribute to rapid and long distance spread of the seed.
Germination and dormancy
Over 90% of C. solstitialis seeds are germinable one week after seed dispersal. Maximum germination of C. solstitialis seeds (nearly 100%) occurs when seeds are exposed to moisture, light and temperatures of 10, 15, or 20oC. At temperatures above 30oC germination is dramatically reduced. When exposed to light and moisture germination occurs rapidly (typically by 24 h) with nearly all seed germinating within 96 hours. However, with increasing exposure to higher temperatures and low moisture (within 1 month of dispersal), as would occur in later summer, many seeds undergo secondary dormancy and do not germinate under adequate light and moisture conditions. This ensures that all seed do not germinate following an occasional late summer thunderstorm, where subsequent seedling mortality would occur when no additional moisture is received over an extended time period.
Although germination occurs throughout the rainy season (October to June), emergence is highest after early fall rainfall events. The extended timing of germination increases the difficulty of controlling C. solstitialis populations during the late winter and early spring, as subsequent germination often results in significant infestations.
In a study conducted in Idaho, the average longevity of non-pappus-bearing and pappus-bearing seeds was six and ten years, respectively. Even after six years of burial, 9% of the pappus-bearing seed germinated. However, in other studies conducted in California, over 95% of the seed either emerged or were damaged two or three years after natural dispersal to the soil surface. This suggests that C. solstitialis seeds may be relatively short-lived under normal field conditions where seeds are predominantly dispersed on the soil surface. Furthermore, microbial degradation and invertebrate predation of C. solstitialis seeds contribute significantly to the rapid depletion of the soil seedbank.
Growth and establishment
Seedling establishment, root and shoot growth In exposed areas, high germination can result in extremely dense seedling populations. Seedlings are more likely to establish in soils with deep silt loam and loam with few coarse fragments. In many areas, a significant amount of self-thinning occurs and only a small fraction of seedlings reach reproductive maturity ( Sheley and Larson 1994a). Thus, in heavily infested areas, C. solstitialis populations produce far more seeds than are necessary to re-infest the area year after year.
Following germination, C. solstitialis allocates resources initially to root growth, secondarily to leaf expansion, and finally to stem development and flower production. Root growth during the winter and early spring is rapid and can extend well beyond 3 feet (1 m) in depth. Centaurea solstitialis roots elongate at a faster rate and to greater depths than potentially competitive species, including weedy annual grasses and clovers. Rapid germination and deep root growth in C. solstitialis extends the period of resource availability into late summer, long after seasonal rainfall has ended and shallow-rooted annual grasses have senesced. By extending the period of resource availability, competition is reduced at the reproductive stage.
Shading of young rosettes can have a dramatic affect on root growth. Reduced root growth is correlated with increased shading (DiTomaso, unpublished data). Since C. solstitialis plants germinate over an extended time period beginning with the first fall rains and ending with the last spring rain event, the resulting canopy is often composed of plants in several stages of development. In dense stands of C. solstitialis, the population consists of both large canopied plants receiving full sunlight and an understory of smaller shaded plants. Thus, light suppression is likely a significant factor regulating root growth. The roots of larger plants exposed to full sunlight quickly grow to great depths, while roots of shaded plants underneath the C. solstitialis canopy occupy shallower depths for longer periods of time. Under these conditions, soil moisture is rapidly depleted from all depths in the soil profile and C. solstitialis strongly competes with other shallow-rooted desirable species, as well as many deep-rooted perennials.
Seedlings that germinate following autumn rains overwinter as basal rosettes. Rosettes develop slowly in the early spring. Bolting typically occurs in late spring or early summer and by mid-summer spines appear on developing seedheads. At the more mature stages of development, the hairs and waxy grayish coating on the foliage of C. solstitialis reflect a considerable amount of light. This reduces the heat load and transpiration demand during the hot and dry summer months. The winged stems add surface area and also act to dissipate heat like a radiator. These characteristics, as well as a deep root system, allow C. solstitialis to thrive under full sunlight in hot and dry conditions. Vigorous shoot growth coincides with increased light availability as neighboring annual species senescence and desiccate. Moreover, the presence of spines on the bracts surrounding the seedhead provides protection against herbivory. This is particularly important during the vulnerable flowering and seed development stages.
Centaurea solstitialis plants are insensitive to photoperiod and lack a vernalization requirement. This allows late germinating plants to flower and set seed within one year provided adequate moisture is available. Flowering continues until newly developing buds are killed by frost. In climates with milder winters, plants can act as biennials. However, in colder climates, mature plants rarely survive the winter. In contrast, seedlings can survive extended frost periods. Cold tolerance (hardiness) appears to be lost during the transition from vegetative to reproductive phases.
Senesced stems can contain the non-pappus-bearing seeds for about a month until the spiny bracts fall off. The receptacles of the flowerheads contain abundant amounts of fine chaff giving the old seedheads a cotton-tip appearance. Stems of C. solstitialis degrade slowly and may remain erect for at least one year.
Water, light and temperature
Heavy infestations of C. solstitialis in grasslands with loamy soils can use as much as 50% of annual stored soil moisture (Gerlach, unpublished data). In deep soils, C. solstitialis can significantly reduce soil moisture reserves to depths greater than six feet (1.8 m).
Seasonal moisture can influence competition between C. solstitialis and annual grasses. Under dry spring conditions, early maturing annual grasses have an advantage over late season annuals, like C. solstitialis, as they utilize the available moisture and complete their life cycle earlier. In contrast, under moderate or wet conditions, C. solstitialis has an advantage by continuing its growth later into the summer and fall and producing more seed. Thus, in grassland systems, the greatest advantage for C. solstitialis occurs in areas 1) dominated by annual grasses, 2) with deep soil, and 3) in years with moderate to heavy spring rainfall. Under these conditions, C. solstitialis matures later, has increased seed production, and has little competition for deep soil moisture.
Centaurea solstitialis rosettes are very susceptible to light suppression, and will produce short roots, larger leaves, more erect rosettes, and fewer flowers than plants in full sunlight. Consequently, C. solstitialis does not survive well in shaded areas, and is less competitive in areas dominated by shrubs, trees, taller perennial forbs and grasses, or late season annuals. For this reason, infestations are nearly always restricted to open grasslands dominated by annuals or disturbed sites. Even in areas dominated by C. solstitialis, the level of competition for light can be so intense that seedlings will vigorously compete with each other, accounting for the low rate of seedling survival through self-thinning.
Introduction, spread, and distribution
The center of origin of C. solstitialis is believed to be Eurasia, where it is native to Balkan-Asia Minor, the Middle East and southcentral Europe. Its introduction in North America probably occurred sometime after 1849 as a seed contaminant in Chilean-grown alfalfa seed, also known as Chilean clover. Historical records indicate that alfalfa was first introduced to Chile from Spain in the 1600s and from Chile to California at the time of the gold rush. Despite its origin from Spain, the source of alfalfa in California before 1903 was only from Chile. After 1903, it was likely that alfalfa was also introduced from Spain, France, Italy, and perhaps Turkestan.
The spread of C. solstitialis into California occurred through a multiple step process. Before the 1870s alfalfa was grown primarily along river levees near Sacramento, Marysville and San Francisco. At this time, C. solstitialis infestations that accompanied alfalfa stands were fairly localized and found only in California. From 1870 to about 1905 much of the surrounding areas previously consisting of dry-farmed wheat and barley fields were converted to both dryland and irrigated alfalfa fields. During this period, C. solstitialis established as dense local populations in these areas and along adjacent roadsides. Introduction of C. solstitialis to other western states occurred in the 1870s and 1880s (Roché 1965). The first report outside of California was in Bingen, Washington. These first introductions were likely through contamination of alfalfa seed.
The use of tractors and other equipment spread seed to other locations, including grain fields. During the 1920s, C. solstitialis expanded rapidly in grasslands within the Pacific Northwest states. At the same time, C. solstitialis infestations in California probably decreased between 1920 and 1940, most likely due to changes in crop production techniques and the widespread use of inorganic herbicides, such as sodium arsenite and sodium chlorate, along roadsides. However, around the 1930s or 1940s C. solstitialis began to invade the foothill grasslands in California. Thus, C. solstitialis now became a part of the grazed rangeland system. By 1958, it was estimated to have invaded over 1 million acres (400,000 ha) of California.
Since the 1960s three factors greatly contributed to its further spread, including extensive road building programs, increased suburban development, and an expansion in the ranching industry. Over the past 40 years, C. solstitialis has spread exponentially to infest rangeland, native grasslands, orchards, vineyards, pastures, roadsides, and wasteland areas. Infestations reached nearly 8 million acres (3 million ha) in California by 1985. In the mid-1980s C. solstitialis was estimated to occupy 280,000 acres (113,000 ha) in Idaho, 135,000 acres (55,000 ha) in Oregon, and 148,000 acres (60,000 ha) in Washington. In 1989, Callihan et al. estimated that C. solstitialis was expanding in rangelands by 7,000-20,000 acres/year (2800-8000 ha/yr) in the west. By 1994, the rate of spread was estimated to be twice as rapid.
Today, C. solstitialis has been estimated to infest over 15 million acres (6 million ha) in California, and can be found in 56 of the 58 counties in the state. Nationally, the weed is found in 23 of the 48 contiguous states, extending as far east as New York. It has also extended into Canada from British Columbia to Ontario. Globally, C. solstitialis is found in most of the temperate areas around the world.
Although no economic assessments have been conducted for C. solstitialis, millions of dollars in losses probably occur from interference with livestock grazing and forage harvesting procedures, and lower yield and forage quality of rangelands. Because of the spiny nature of C. solstitialis, livestock and wildlife avoid grazing in heavily infested areas. Thus, infestations can greatly increase the cost of managing livestock. Although the nutritional component of C. solstitialis leaves is highly digestible by ruminants during the growing season , its nutrient value declines as the plants mature. Centaurea solstitialis in the pre-spiny stage contains between 8 to 14% protein. However, an analysis of the nutritional status of cattle manure in the fall indicated that C. solstitialis-infested pastures contain considerably less crude protein and total digestible nutrients compared to uninfested pastures.
Other non-crop areas
In addition to rangeland, pastures and grasslands, C. solstitialis is also an important weed problem along roadsides, and an occasional problem in dryland cereals, orchards, vineyards, cultivated crops, and wastelands. It can also reduce land value and reduce access to recreational areas. In addition, C. solstitialis infestations can reduce wildlife habitat and forage, displace native plants, and decrease native plant and animal diversity. Dense infestations not only displace native plants and animals, but also threaten natural ecosystems and nature reserves by fragmenting sensitive plant and animal habitat. Centaurea solstitialis invasions on the Agate Desert Preserve in southwest Oregon threatens Lomatium cookei, a globally rare plant species.
Centaurea solstitialis significantly depletes soil moisture reserves in annual grasslands in California  and in perennial grasslands in Oregon (Borman et al 1992). Because of its high water usage, C. solstitialis threatens both human economic interests as well as native plant ecosystems. Gerlach estimated  that C. solstitialis might cause an annual economic loss of $16 to $56 million in water conservation costs in the Sacramento River watershed alone.
Toxicity to horses
When ingested by horses, C. solstitialis causes a neurological disorder of the brain called nigropallidal encephalomalacia or "chewing disease." Continued feeding results in brain lesions and ulcers in the mouth. There is no known treatment for horses that have been poisoned by C. solstitialis. In most cases poisoning destroys the animal’s ability to chew and swallow and death occurs through starvation or dehydration.
The poisoning is a chronic condition affecting the horse primarily after the animal has ingested fresh or dried plant material over an extended period, typically 30 to 60 days, at cumulative fresh weight of 60 to 200% their body weight. Cheeke and Shull (1985) reported the lethal dose to be 2.3 to 2.6 kg C. solstitialis per 100 kg of body weight per day. The clinical signs of poisoning include drowsiness, difficulty in eating and drinking, twitching of the lips, tongue flicking, and involuntary chewing movements.
Centaurea solstitialis poisoning is generally most dangerous when it is the only feed available or when it is a significant contaminant of dried hay. In some cases, however, horses acquire a taste for C. solstitialis and seek it out even when other forage is available. In northern California in 1954, it was estimated that at least 100 cases of horse poisoning by C. solstitialis occurred annually. Because the toxicity and identification of C. solstitialis is better understood today, cases of poisoning in horses are now relatively uncommon. It appears that only horses are affected by ingesting C. solstitialis. Other animals, including mules and burros are not susceptible to the toxic effect of the weed. However, all grazing animals can sustain damage to their eyes from the plant’s long, sharp spines.
Mechanisms of spread
Human activities are the primary mechanisms for the long distance movement of C. solstitialis seed. Seed is transported in large amounts by road maintenance equipment and on the undercarriage of vehicles. The movement of contaminated hay and uncertified seed are also important long distance transportation mechanisms. Once at a new location, seed is transported in lesser amounts and over short to medium distances by animals and humans. The short, stiff, pappus bristles are covered with microscopic, stiff, appressed, hair-like barbs that readily adhere to clothing and to hair and fur. The pappus is not an effective long distance wind dispersal mechanism as wind moves seeds less than a few feet (less than a meter).
The goal of any management plan should be not only controlling the invasive weed, but also improving the degraded community, enhancing the utility of that ecosystem, and preventing reinvasion or invasion by other weed species. This usually requires a long-term integrated management plan.
It is important to consider the advantages and disadvantages of each approach and to judge how each option may best fit into a long-term program. It is possible that several different strategies can prove successful in a given location. The consistent components of a successful program should include persistence, flexibility, and, most importantly, preventing new seed recruitment. A list of management options for the control of C. solstitialis can be seen at http://wric.ucdavis.edu.
Mechanical control options for C. solstitialis typically include hand pulling, hoeing, weed whipping, tillage or mowing.
Hand pulling, hoeing or weed whipping: Manual removal of C. solstitialis is most effective with small patches or in maintenance programs where plants are sporadically located in the grassland system. This usually occurs with a new infestation or in the third year or later in a long-term management program. These methods can also be an important in steep or uneven terrain where other mechanical tools (e.g., mowing) are impossible to use. To ensure that plants to not recover it is important to detach all above ground stem material. Leaving even a 2 inch (5 cm) piece of the stem can result in recovery if leaves and buds are still attached to the base of the plant. The best timing for manual removal is after plants have bolted but before they produce viable seed (i.e., early flowering). At this time, plants are easy to recognize and some or most of the lower leaves have senesced. Hand removal is particularly easy in areas with competing vegetation. Under this condition, C. solstitialis will develop a more erect slender stem with few basal leaves. These plants are relatively brittle and easy to remove. In addition, they usually lack leaves at the base and, consequently, rarely recover even when a portion of the stem is left intact.
Tillage: Tillage is effective, and is occasionally used on roadsides. It is also often used in agricultural lands which probably accounts for the uncommon occurrence of C. solstitialis as a cropland weed. In wildlands and rangelands, tillage is usually not appropriate because it can damage important desirable species, increase erosion, alter soil structure, and expose the soil for rapid re-infestation if subsequent rainfall occurs.
Mowing: Mowing may be an alternative strategy for small landowners that do not wish to use herbicides. It is a popular control technique in recreational areas and has less impact on the environment than tillage. A few land managers have successfully controlled C. solstitialis using continuous mowing over multiple years. However, since mowing is a late season management tool it is best employed in the later years of a long-term management program or in a lightly infested area. This gives the land manager the ability to assess the level of infestation and the flexibility of choosing the most appropriate and cost effective option, which can include mowing. If only a few plants are present, hand pulling may be a better choice than mowing.
Although mowing can be a cost-effective control method, it is not feasible in many locations due to rocks and steep terrain. Even when mowing is employed, it is not always successful and can decrease the reproductive efforts of insect biocontrol agents, injure late growing native forb species , and reduce fall and winter forage for wildlife and livestock. In addition, its success depends on proper timing and the growth form of the plant. Mowing too early or late will usually increase the C. solstitialis problem. Plants with an erect, high-branching growth form are effectively controlled by a single mowing at the early flowering stage, while sprawling low-branching plants cannot be controlled even with repeated mowings at the proper timing. Despite its limitations, mowing conducted at the early flowering stage, before viable seed production, can be very effective for C. solstitialis control.
Properly timed (May and June) intensive grazing by cattle, sheep or goats can reduce growth, canopy cover, survivability, and reproductive capacity of C. solstitialis. Grazing should be conducted after the stems bolt but before spiny seedheads develop. Cattle and sheep avoid C. solstitialis once the buds produce spines, whereas goats continue to browse plants even in the flowering stage. For this reason, goats have become a more popular method for controlling C. solstitialis in relatively small infestations.
Grazing the weed during the bolting stage could provide palatable high protein forage (8 to 14%). This can be particularly useful in late spring and early summer when other annual species have senesced. Grazing alone will not provide long-term management or eradication of C. solstitialis, but can be a valuable tool in an integrated management program.
Properly timed prescribed burning will control some important noxious annual grasses, such as barbed goatgrass (Aegilops triuncialis), medusahead (Taeniatherum caput-medusae) and ripgut brome (Bromus diandrus), as well as late flowering broadleaf species such as C. solstitialis.
Burning should be timed to coincide with the very early C. solstitialis flowering stage. At this time C. solstitialis has yet to produce viable seed, whereas seeds of most desirable species have dispersed and grasses have dried to provide adequate fuel. Fire has little if any impact on seeds in the soil.
In addition to controlling C. solstitialis, burning will reduce the thatch layer, expose the soil, and recycle nutrients held in the dried vegetation. In the first growing season after the burn, plant diversity will often increase, particularly native perennial grasses and forbs.
Despite its effectiveness, air quality issues can be a significant problem when burns are conducted adjacent to urban areas. A major risk of prescribed burning is the potential of fire escapes. This risk is greatest when burns are conducted during the summer months. In some areas, burning can lead to rapid invasion by other undesirable species with wind-dispersed seeds, particularly members of the sunflower family.
The ability to use repeated burning depends on climatic and environmental conditions. In areas where resources are ample and total plant biomass is abundant, two or three consecutive years of burning may be practical. However, in other environments or years, fuel loads may not be sufficient to allow multiple year burns. Consequently, prescribed burning may be a more appropriate option as part of an integrated approach.
In addition to summer burning, C. solstitialis seedlings have been controlled using winter or early spring flaming techniques. This technique is somewhat non-selective and the control of C. solstitialis is inconsistent. When spring drought follows a flaming treatment, control of C. solstitialis can be excellent. In contrast, a wet spring can lead to complete failure and increased C. solstitialis infestation, particularly since competing species may be dramatically suppressed.
Re-vegetation programs for C. solstitialis control generally rely on re-seeding with native or high forage non-native perennial grasses.  Re-vegetation with desirable and competitive plant species can be the best long-term sustainable method of suppressing weed invasions, establishment, or dominance, while providing high forage production.
Because of the ecological diversity within most grassland ecosystems, no single species or combination of species will be effective under all circumstances. Unfortunately, few studies have been conducted on the restoration of C. solstitialis infested grasslands using a wide diversity of species, particularly natives.
In western states, competitive grasses used in re-vegetation programs for C. solstitialis management include non-native perennial grasses such as crested wheatgrass (Agropyron desertorum), intermediate wheatgrass (Elytrigia intermedia (=Agropyron intermedium)), pubescent wheatgrass (Thinopyrum intermedium), Bozoisky Russian wildrye (Psathyrostachys juncea), sheep fescue (Festuca ovina), tall oatgrass (Arrhenatherum elatius), or orchardgrass (Dactylis glomerata), as well as the native perennial grasses including big bluegrass (Poa ampla) and thickspike wheatgrass (Elymus lanceolatus subsp. lanceolatus (=Agropyron dasystachyum)). These species provide good livestock forage and a sustainable option for rangeland maintenance.
In those parts of California with a Mediterranean climate, re-vegetation programs for control of C. solstitialis are more difficult that those in other western states where summer rainfall is critical to the establishment and survival of native perennial grasses.
In addition to perennial grasses, non-native crimson clover (Trifolium incarnatum) and subterranean clover (Trifolium subterraneum) were used for re-seeding programs in foothill ranges of Oregon and California. Used as a sole control option, however, T. subterraneum did not provide adequate seasonal control of C. solstitialis.
Re-vegetation projects for C. solstitialis control nearly always rely on integrated strategies. In most cases, it is difficult to establish desired plants without the management of competing vegetation, including C. solstitialis and annual grasses. The goal of these re-vegetation projects is to develop sustainable high quality range conditions and improved wildlife habitat capable of providing long-term C. solstitialis control without the need for continued herbicide treatments.
Six insects have become established for the control of C. solstitialis in the western United States. These include three species of weevils (seed-head weevil [Bangasternus orientalis], flower weevil [Larinus curtus], and the hairy weevil [Eustenopus villosus]), and three species of flies (seed-head fly [Urophora sirunaseva], peacock fly [Chaetorellia australis], and the false peacock fly [Chaetorellia succinea]). All six insects attack the flower heads of C. solstitialis and produce larvae that develop and feed within the seedhead.
Of the four insects that are well established in California  only two, Eustenopus villosus and Chaetorellia succinea, have any significant impact on reproduction. The combination of these two insects reduces seed production by 43 to 76%. Although this level of suppression is not sufficient to provide long-term C. solstitialis management, the use of biological control agents can be an important component of an integrated management approach. A more successful biological control program will likely require the introduction of plant pathogens or other insects capable of severely damaging or feeding on roots, stems, or foliage. Biocontrol researchers continue to search for such insects or pathogens in C. solstitialis' native range.
The most widely studied pathogen for C. solstitialis control is the Mediterranean rust fungus Puccinia jaceae. It can attack the leaves and stem of C. solstitialis, causing enough stress to reduce flowerhead and seed production. It is well suited to environmental conditions found in California and other areas of infestation in North America. The organism is currently under investigation and has not been released for use.
Clopyralid (Transline®, Stinger®) and picloram (Tordon®) provide postemergence control of C. solstitialis seedlings and rosettes, as well as soil residual activity for at least one season. These compounds give the best control of C. solstitialis and are the least injurious to grasses. Picloram is not registered in California.
Clopyralid gives excellent control of C. solstitialis at very low rates (1.5 to 4 oz a.e./acre; 100-280 g a.e./ha). The timing for application is broad, usually ranging between January and May. Clopyralid is a very selective herbicide and does not injure grasses or most broadleaf species. However, depending on the timing of application, it does damage or kill many species in the legume family (Fabaceae), as well as the sunflower family (Asteraceae). It can also cause some injury is members of the nightshade (Solanaceae), knotweed (Polygonaceae), carrot (Apiaceae), and violet (Violaceae) families. Clopyralid is also effective on plants in the bolting and bud stage, but higher rates (4 oz a.e./acre; 280 g a.e./ha) are required. Applications made after the bud stage will not prevent the development of viable seed. When clopyralid is used to control seedlings a surfactant is not necessary. However, when treating older plants or plants exposed to moderate levels of drought stress, surfactants can enhance the activity of the herbicide. A combination of clopyralid and 2,4-D amine (Curtail®) has also been used for C. solstitialis control in western states other than California. It can be used at 0.25 to 1 pint/acre (0.3-1.2 liter/ha) after the majority of C. solstitialis rosettes have emerged but before bud formation.
Picloram is the most widely used herbicide to control C. solstitialis in western states other than California. It acts much like clopyralid, but gives a broader spectrum of control and has much longer soil residual activity. Picloram is applied (usually with a surfactant) at a rate between 0.25 lb and 0.375 lb a.e./acre (0.28-0.42 kg a.e./ha) in late winter to spring when plants are still in the rosette through bud formation stages. This treatment can provide effective control for about two to three years. Although well developed grasses are not usually injured by labeled use rates, young grass seedlings with less than four leaves may be killed.
A limited number of postemergence herbicides are registered for use in rangelands, pastures, and wildlands. They include 2,4-D (many trade names), dicamba (Banvel®, Vanquish®), triclopyr (Garlon 3A®, Garlon 4®, Remedy®), and glyphosate (Roundup®). These postemergent herbicide treatments generally work best on seedlings. They are not effective for the long-term management of C. solstitialis when used in spring, as they have no soil residual activity and will not control plants germinating after application.
The most effective way to use postemergence compounds for C. solstitialis control is to incorporate them into later stages of a long-term management program. In particular, they are effectively used to spot-treat escaped plants or to eradicate small populations in late season when C. solstitialis is easily visible but has yet to produce viable seed.
2,4-D (0.5 to 0.75 lb a.e./acre; 0.56-0.84 kg a.e./ha), dicamba (0.25 to 1.0 lb a.e./acre; 0.28-1.1 kg a.e./ha) and triclopyr (0.5 or 1.5 lb a.e./acre; 0.56-1.7 kg a.e./ha) are growth regulator herbicides that can provide acceptable control of C. solstitialis when applied at the rosette growth stage. Amine forms are as effective as ester forms at the small rosette growth stage, but amine forms reduce the chance of off-target movement. Glyphosate controls C. solstitialis at 1 lb a.e./acre (1.1 kg a.e./ha). Good coverage, clean water, and actively growing C. solstitialis plants are all essential for adequate control. Unlike the growth regulator herbicides, glyphosate is non-selective and controls most plants, including grasses. A 1% solution of glyphosate also provides effective control and is used at this concentration for spot treatment of small patches. Glyphosate is a very effect method of controlling C. solstitialis plants in the bolting, spiny, and early flowering stages at 1 to 2 lb a.e./acre (1.1-2.2 kg a.e./ha). However, it is important to use caution when desirable perennial grasses are present. In late season treatments, except with glyphosate and ester formulations, a surfactant should be added to the herbicide formulation.
A number of non-selective preemergence herbicides will control C. solstitialis to some level, including simazine, diuron, atrazine, imazapyr, imazapic, metsulfuron, sulfometuron, chlorsulfuron, bromacil, tebuthiuron, oxyfluorfen and prometone. All these compounds are registered for use on right-of-ways or industrial sites (although not all in California), but few can be used in rangeland, pastures, or wildlands. In rangeland, only metsulfuron (Escort®) (not registered in California) and to some degree chlorsulfuron (Telar®) (not registered for pastures or rangeland in any state) provides selective control of C. solstitialis without injuring desirable grasses. Both these compounds are used at 1 to 2 oz a.i./acre (70-140 g a.i./ha). Chlorsulfuron and metsulfuron do not have postemergence activity on C. solstitialis and therefore, must be used in combination with 2,4-D, dicamba, or triclopyr to provide effective control of C. solstitialis in grasslands.
Most often a single method is not effective in the sustainable control of C. solstitialis and other range weeds. A successful long-term management program should be designed to include combinations of mechanical, cultural, biological, and chemical control techniques. There are many possible combinations that can achieve the desired objectives, and choices will have to be tailored to the site, economics, and management goals. Sometimes the control techniques must be in a particular sequence to be successful. The most effective sequence includes early season control strategies in the first year or two of a management program, followed by late season options in the later years.
Not every aspect of C. solstitialis is detrimental. It is regarded as an important honey source plant in California and other western states.
- Zouhar, Kris. 2002. Centaurea solstitialis. Fire Effects Information System, U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory
- Global Invasive Species Database. 2011. Invasive Species Specialist Group (ISSG) of the IUCN Species Survival Commission
- USDA Forest Service, Forest Health Staff, Newtown Square, PA Weed of the Week
- Oregon Department of Agriculture Plant Programs, Noxious Weed Control
- Plant Conservation Alliance's Alien Plant Working Group
- University of California, Jepson Flora Project
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- ↑ Harrod, R.J. and R.J. Taylor. 1995. Reproduction and pollination biology of Centaurea and Acroptilon species, with emphasis on C. diffusa. Northwest Science 69(2):97-105. 2.0 2.1
- ↑ Maddox, D.M., D.B. Joley, D.M. Supkoff, and A. Mayfield. 1996. Pollination biology of yellow starthistle (Centaurea solstitialis) in California. Canadian Journal of Botany 74:262-267. 3.0 3.1
- ↑ Barthell, J.F., J.M. Randall, R.W. Thorp, and A.M. Werner. 2001. Promotion of seed set in yellow star-thistle by honey bees: Evidence of an invasive mutualism. Ecological Applications (in press). 4.0 4.1
- ↑ Benefield, C.B., J.M. DiTomaso, G.B. Kyser, and A. Tschohl. 2001. Reproductive biology of yellow starthistle (Centaurea solstitialis): Maximizing late season control. Weed Science 49:83-90. 5.0 5.1 5.2 5.3 5.4 5.5
- ↑ Maddox, D.M. 1981. Introduction, phenology, and density of yellow starthistle in coastal, intercoastal, and central valley situations in California. ARR-W-20, pp. 1-33. USDA-ARS. 6.0 6.1 6.2 6.3
- ↑ DiTomaso, J.M., G.B. Kyser, and M.S. Hastings. 1999a. Prescribed burning for control of yellow starthistle (Centaurea solstitialis) and enhanced native plant diversity. Weed Science 47: 233-242. 7.0 7.1 7.2
- Roche, B. 1965. Ecologic studies of yellow star-thistle (Centaurea solstitialis L.). 1965. Ph.D. Dissertation, University of Idaho.
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- ↑ Roche, B.F., Jr. 1992. Achene dispersal in yellow starthistle (Centaurea solstitialis L.). Northwest Science 66:62-65. 10.0 10.1 10.2
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- ↑ Joley, D.B., D.M. Maddox, B.E. Mackey, S.E. Schoenig, and K.A. Casanave. 1997. Effect of light and temperature on germination of dimorphic achenes of Centaurea solstitialis in California. Canadian Journal of Botany 75(12):2131-2139. 15.0 15.1 15.2
- ↑ Callihan, R.H., F.E. Northam, J.B. Johnson, E.L. Michalson, and T.S. Prather. 1989. Yellow starthistle. Biology and management in pasture and rangeland. Current Info. Series No. 634, 4 pp. Univ. Idaho, College of Agriculture. 16.0 16.1
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