Yellow Starthistle Information

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Yellow Flowered Starthistle Fact Sheet

Produced by
Evelyn Healy and Joseph DiTomaso

Yellow starthistle [Centaurea solstitialis L][CENSO][CalEPPC: A1][CDFA list: C]
Malta starthistle [Centaurea melitensis L.][CENME][CalEPPC: B]
Sicilian starthistle [Centaurea sulphurea Wild.][CDFA list: B]

SYNONYMS:
yellow starthistle: golden starthistle, yellow cockspur, St. Barnaby's thistle
Malta starthistle: tocalote, Napa thistle
Sicilian starthistle: sulphur-colored Sicilian thistle, misidentified as Centaurea sicula L. in some California references.

GENERAL DESCRIPTION: Noxious erect winter annuals (sometimes biennials) with spiny yellow-flowered heads, mostly to 1 m tall. Refer to the table Comparison of yellow-flowered starthistles for a quick review of important differences.
yellow starthistle: Annual, sometimes biennial, to 2 m tall. Plants are highly competitive and typically develop dense, impenetrable stands that displace desirable vegetation in natural areas, rangelands, and other places. Yellow starthistle is considered one of the most serious rangeland weeds in the northwestern U.S. It has spread rapidly since its introduction into California around 1850. In 1995, it was estimated to infest 10-12 million acres in the state, with the heaviest infestations primarily in the northern and central-western regions. Seeds often contaminate grains, lowering the value and quality of harvests. Yellow starthistle contains an unidentified compound that causes nigropallidal encephalomalacia or chewing disease in horses. The compound only affects horses and permanently damages the area of the brain that controls fine motor movements, including mouth and lip movements. Toxicity effects are cumulative. Horses must consume a 50-150% of an animal's weight in dry-weight plant material over a period of 1 to 3 months to produce symptoms. Because of its bitter taste, horses usually avoid grazing yellow starthistle. However, the disease can occur when horses are allowed to graze infested pastures, especially those that lack adequate amounts of suitable green forage, or are fed contaminated hay over a period of time. Once the toxicity threshold has been reached, symptoms occur rapidly. Symptoms include fatigue, lowered head, an uncontrolled rapid twitching of the lower lip, tongue-flicking, involuntary chewing movements, and an unnatural open position of the mouth. Without intervention, affected horses are unable to eat or drink and eventually die from starvation or dehydration. The impact of yellow starthistle is not always negative. Bees foraging on yellow starthistle flowers produce a flavorful high quality honey. Several biocontrol agents have been introduced from the Mediterranean region to control yellow starthistle. Currently established in California are the yellow starthistle bud weevil (Bangasternus orientalis), hairy weevil (Eustenopus villosus), flower weevil (Larinus curtus), gall fly (Urophora sirunaseva), peacock fly (Chaetorellia australis), and false peacock fly (Chaetorellia succinea). These insects have yet to provide significant reduction in yellow starthistle populations in most areas. Yellow starthistle was introduced from Southern Europe.
Malta starthistle (often called tocalote in California): Is most invasive in the central-western and southwestern regions of California, but is less prevalent than yellow starthistle statewide. Malta starthistle is not known to cause chewing disease in horses and is used medicinally in Spain. A small beetle (Lasioderma haemorrhoidale) unintentionally introduced from the Mediterranean region feeds on Malta starthistle seed heads, but has had little effect in controlling the plant. Malta starthistle was introduced during the 1700's from Southern Europe.
Sicilian starthistle: Appears to be less invasive than yellow or Malta starthistle. Sicilian starthistle was discovered growing in an area near Folsom in 1923. By 1999, the population had only slightly expanded its range. A few smaller populations have been found elsewhere, but these infestations remain small. Introduced from Southwestern Europe.

SEEDLINGS: Cotyledons oblong to spatulate, base wedge-shaped, tip +/- squared, glabrous. First few leaves typically oblanceolate. Subsequent rosette leaves oblanceolate, entire to pinnate-lobed. Terminal lobes largest. Later rosette leaves to 15 cm long. Hair characteristics are visible with 10-14x magnification.
yellow starthistle: Cotyledons 6-9 mm long, 3-5 mm wide. Later rosette leaves typically deeply lobed +/- to midrib, appear ruffled. Surfaces +/- densely covered with fine cottony hairs that +/- stiff thick hairs and leaf surfaces. Lobes mostly acute, with toothed to wavy margins. Terminal lobes +/- triangular to lanceolate. Leaves of rosettes under reduced light levels are larger and more erect.
Malta starthistle: Later rosette leaves entire to deeply lobed +/- to midrib. Lobes +/- rounded. Terminal lobe +/- round. Surfaces evenly covered with stiff thick hairs and resinous dots. Fine cottony hairs sparse on older leaves, +/- dense on new leaves.
Sicilian starthistle: Cotyledons ~ 2-4 cm long, ~ 1-2 cm wide, +/- succulent. First few leaves oblanceolate, ~ 4-8 cm long, ~ 1.5-2.5 cm wide, sparsely pubescent on both surfaces. Later rosette leaves broader and typically more shallowly lobed than those of yellow starthistle. Lateral lobes +/- acute. Terminal lobes +/- ovate, with a broadly acute tip and finely toothed margins. Surfaces sparsely covered with stiff thick hairs. Fine cottony hairs and resinous dots lacking.

MATURE PLANT: 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. Margins smooth, toothed, or wavy. Leaf bases extend down the stems (decurrent) and give stems a winged appearance. Rosette leaves typically withered by flowering time.
yellow starthistle: Largest stem wings typically to ~ 3 mm wide. Lower stem leaves sometimes +/- deeply pinnate-lobed. Foliage grayish- to bluish-green, densely covered with fine white cottony hairs that +/- hide thick stiff hairs and glands.
Malta starthistle: Largest stem wings to ~ 3 mm wide. Foliage +/- grayish-green. Leaves evenly covered with thick stiff hairs and minute resinous dots and +/- sparsely covered with fine white cottony hairs that do not hide stiff hairs and glands. New leaves are often densely covered with fine cottony hairs.
Sicilian starthistle: Stem leaves toothed. Largest stem wings usually ~ 5-6 mm wide. Foliage +/- yellowish-green, sparsely covered with stiff hairs.

ROOTS and UNDERGROUND STRUCTURES:
yellow starthistle: 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.
Malta and Sicilian starthistle: Taproots do not penetrate the soil as deeply as those of yellow starthistle.

FLOWERS: Heads ovoid, spiny, solitary on stem tips, consist of numerous yellow disk flowers. Heads sometimes closely 2-3-clustered in Malta starthistle. Vigorous individuals of Malta and yellow starthistle may develop flower heads in branch axils. Phyllaries palmately spined, with one long central spine and 2 or more pairs of short lateral spines. Insect-pollinated.
yellow starthistle: May-December. Corollas mostly 13-20 mm long. Involucre (phyllaries as a unit) ~ 12-18 mm long. Phyllaries +/- dense to sparsely covered with cottony hairs or with patches at the spine bases. Central spine of main phyllaries 10-25 mm long, stout, yellowish to straw-colored throughout. Lateral spines typically 2-3 pairs at the base of the central spine. Mostly self-incompatible.
Malta starthistle: April-September. Corollas typically 10-12 mm long. Involucre ~ 8-15 mm long. Phyllaries +/- sparsely covered with cobwebby hairs. Central spine of main phyllaries 5-12 mm long, slender, typically purple- to brown-tinged. Lateral spines usually 3-4 pairs, the upper pair on the central spine near the center.
Sicilian starthistle: May-July. Corollas usually 25-35 mm long, paler than those of yellow starthistle. Involucre 12-30 mm long. Phyllaries glabrous. Central spine of main phyllaries 10-25 mm long, stout, yellowish to straw-colored near the tip, blackish to dark brown near the base. Lateral spines typically 3-5 pairs at the base of the central spine.

FRUITS and SEEDS: Achenes (seeds) +/- barrel-shaped, +/- compressed, laterally notched at the base. Pappus bristles slender, stiff, unequal.
yellow starthistle: Produces 2 types of achenes, both glabrous, ~ 2-3 mm long, with broad bases. Outer ring of achenes dull dark brown, often speckled with tan, lack pappus bristles, often remain in heads. Inner achenes glossy, gray or tan to mottled cream-colored and tan, with slender white pappus bristles 2-5 mm long.
Malta starthistle: Achenes ~2-3 mm long, finely pubescent, grayish to +/- tan, usually with slightly darker stripes. Bases deeply notched, narrow, hook-like. Pappus bristles pale tan, 1-3 mm long.
Sicilian starthistle: Achenes ~ 5-8 mm long, glossy dark brown, often faintly streaked tan. Bases deeply notched, broad, hook-like. Pappus bristles dark brown to black, 6-7 mm long.

POSTSENESCENCE CHARACTERISTICS: Stems with old flower heads turn gray-brown and can remain intact for over a year.
yellow starthistle: 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.
Malta starthistle: Plants often senesce earlier than yellow starthistle. Heads retain the central spines, but often shed the loose receptacle and dense fuzzy gray hairs, leaving a shallow bowl of spiny phyllaries.
Sicilian starthistle: Plants often senesce earlier than yellow starthistle. Heads retain the central spines, and a large proportion retain the receptacle and dense fuzzy gray hairs

HABITAT: Open, disturbed sites, grasslands, rangeland, open woodlands, fields, pastures, roadsides, waste places. Yellow and Malta starthistle also occur in cultivated fields.

DISTRIBUTION:
yellow starthistle: Throughout most of California. Common in the Sacramento and northern San Joaquin Valleys, northern Sierra Nevada foothills, Cascade Range, Klamath Ranges, eastern North Coast Ranges, and Central-western region of the state. Uncommon in the desert regions, moist coastal areas, and east of the Sierra Nevada. To East Coast, except some regions of the South and Northeast. Typically to 1800 m (6000 ft), but has been found up to 2600 m (8600 ft). Expanding range in mountainous areas and in the central-western region.
Malta starthistle: Throughout most of California, except Great Basin region. Common in the Central- western and Southwestern regions. Uncommon in desert regions. To Washington, Nevada, Texas; scattered in the Midwest and Eastern U.S. To 2200 m (7200 ft).
Sicilian starthistle: Southern Sacramento Valley (Folsom, ne Sacramento Co.), San Francisco Bay region (Hwy 35 in se San Mateo and nw Santa Clara cos.). Previous small populations located in the adjacent Sierra Nevada foothills (ne El Dorado Co.) and northern San Joaquin Valley (n San Joaquin Co.), may have been eradicated. To 300 m (1000 ft).

PROPAGATION/PHENOLOGY: Reproduce by seed. Seeds fall near the parent plant or are dispersed to short distances with wind and to greater distances with human activities, animals, water, and soil movement. Most seeds germinate after the first fall rains. Plants exist as basal rosettes through winter and early spring until flowering stems develop in late spring or early summer.
yellow starthistle: Seed heads typically contain ~ 30-80 seeds. Seed head production is highly variable (1- 1000 heads per plant) and depends on a variety of factors, including soil moisture and competition. Large plants can produce nearly 75,000 seeds. Some seed is viable 8 days after flower initiation. Most seed does not appear to require an afterripening period. Seed germination is closely correlated with rainfall events. Large flushes of seed germinate after the first fall rains, but smaller germination flushes can occur nearly year round. Non-pappus bearing seeds appear to require higher temperatures to germinate than pappus bearing seeds. Light is required for seed germination. Seed can survive for up to 10 years in the field, depending on environmental conditions, but it appears that few seeds survive beyond 2-3 years in the Central Valley. Shaded conditions reduce flower production and root growth. In some areas, flowering plants may be found year-round in areas not exposed to severe frost. Photoperiod or a period of cool, moist conditions (vernalization) do not appear to influence time of flowering. Yellow starthistle litter appears to inhibit germination of its seeds, probably by blocking light penetration.
Malta starthistle: Seed production is highly variable. Plants can produce 1-60 or more seeds per head and 1-100 heads or more per plant. Wild oat (Avena spp.) litter appears to have allelopathic properties that reduce Malta starthistle seed germination. Young seedlings are especially resistant to the effects of fall drought.
Sicilian starthistle: The biology of this species is largely undocumented. Where species occur together, seedlings and rosettes of Sicilian starthistle often out-compete those of yellow starthistle.

MANAGEMENT FAVORING/DISCOURAGING SURVIVAL:
Monitoring and spot eradication of plants when they are discovered can prevent the spread of starthistles. yellow starthistle: Management techniques, such as grazing, mowing, burning, and cultivation, can prevent seed production and control infestations when employed over a period of 2-3 years or more, depending on the degree of infestation and other factors. These methods must be properly timed to be effective. High-intensity short-duration grazing by sheep, goats, or cattle should be implemented during the period when plants have developed flowering stems, but not spiny heads. Mowing is most effective when plants are cut below the height of the lowest branches and 2-5 % of the total population of seed heads is in bloom. Mowing too early can result in high seed production. Prescribed burns can provide control if implemented after annual plants have dried, but before yellow starthistle seed is produced. Burning at other times may enhance yellow starthistle survival. Repeated shallow cultivation throughout the germination period and prior to seed production can control plants in crop fields. To prevent re-infestation, vigilant monitoring and spot eradication may be required indefinitely. All starthistles are highly susceptible to the herbicide cloppyralid.
Malta and Sicilian starthistle: Although undocumented, cultural strategies used to control yellow starthistle are likely to control these starthistles as well.

SIMILAR SPECIES: Unlike yellow, Malta, and Sicilian starthistle, purple starthistle [Centaurea calcitrapa L.] and Iberian starthistle [Centaurea iberica Spreng.] have purple flowers, upper stem leaves mostly pinnate-divided, and straw-colored spines in the center of the rosettes.