Source: California Invasive Plant Council

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Invasive Plants of California's Wildland

Carduus pycnocephalus
Scientific name   Carduus pycnocephalus
Additional name information:   L.
Common name   Italian thistle, slender thistle, shore thistle, Italian plumeless thistle
Closely related California natives   0
Closely related California non-natives:   Carduus acanthoides, C. nutans, C. tenuiflorus
Listed   CalEPPC List B,CDFA A
By:   Carla Bossard,Richard Lichti

Distinctive features:  

The annual Italian thistle (Carduus pycnocephalus) varies in height from ankle to head high. Its leaves are white-woolly below, hairless-green above, and deeply cut into two to five pairs of spiny lobes. The terminal lobe spine grows longer and more rigid than the other spines. Stems are slightly winged. Flower heads are covered with densely matted, cobwebby hairs. The thimble-sized, rose to pink to purple flowers are clustered in groups of two to five. The flowerheads are smaller and fewer than those of bull thistle or Canada thistle, and Italian thistle has narrow bracts under its heads with many tiny, firm, forward-pointing hairs on them (Roche 1992). The closely related slender thistle, C. tenuiflorus, differs in having stems with continuous wings, leaves with twelve to twenty lobes, and five to twenty heads per cluster.

Asteraceae. Winter annual. Stems: 8 in-6.6 ft (2-20 dm), glabrous or slightly woolly, narrowly spine-winged. Leaves: basal 4-6 in (10-15 cm), 4-10 lobed, cauline +/- tomentose. Inflorescence: heads 2-5 per cluster, sessile or short peduncled, involucre 0.5-0.8 in (1-2 cm) diameter, cylindric to elliptic, phyllary bases loosely tomentose, margins not scarious, tips ascending, linear-lanceolate, spiny, scabrous. Flowers: corollas 0.4-0.6 in (1-1.4 cm), pink to purple, tube 0.2-0.3 in (5-8 mm), throat 0.1 in (2-3 mm ) lobes 0.2 in (4-5 mm). Fruit: 0.3 in (4-6 mm) golden to brown, veins 20, pappus 0.4-0.6 in (1-1.5 cm) (Hickman 1993).


In California, Italian thistle infests areas below 3,000 feet (1,000 m) throughout most of the state except for the Great Basin and northern Mojave Desert. It is common in chaparral and oak savanna inthe inner Coast Ranges from Solano County north. It also occurs in meadows, pastures, and ranges, on roadsides, and in disturbed wildland areas. It is partial to warm, dry mediterranean climate areas, basalt soils, soils of naturally high fertility or soils with a relatively high pH > 6.5 (Bendall 1975). It commonly colonizes disturbed habitats with less intense interspecific competition (Parsons 1977).


Native to the Mediterranean, southern Europe, and North Africa to Pakistan, Italian thistle is now widespread in temperate zones and a major pest in Australia, New Zealand, South America, and South Africa. It was accidentally introduced into United States (Batra et al. 1981) and California (Goeden 1974) in the 1930s. Robbins (1940) reports it as early as 1912 near Fort Bragg in Mendocino County. It is spread by seeds on wind, vehicles, and animals. The seeds are mucilaginous, which aids in dispersal (Goeden 1974, Evans et al. 1979). Seeds can disperse by wind an average of seventy-five feet (23 m) from the parent plant and can travel more than 325 feet (108 m) in strong winds. Ants may also play a role in dispersing seed (Uphof 1942). This species can also spread through seed-contaminated hay and soil from infested quarries.


Italian thistle dominates sites and excludes native species, crowding out forage plants in meadows and pastures. The blanketing effect of overwintering rosettes can severely reduce the establishment of other plants, as the leaves of the rosette can become erect in dense stands (Parsons 1973). Most animals avoid grazing on it because of its spines. The spines also discourage grazing on neighboring forage species (Parsons 1992). Italian thistle is troublesome in meadows and grasslands, along roadways, in firebreaks, and in utility and railway rights-of-way where control is uneconomical or impractical (Batra et al. 1981). It grows well in oak savanna and can carry grass fires to tree canopies.


Italian thistle reproduces by seed only; it has no means of vegetative reproduction. An annual, it flowers from mid-September to December, and plants die early the following summer. Italian thistle is known to hybridize with Carduus tenuiflorus (Pitcher and Russo 1988), although Olivieri (1985) reported low levels of successful seed set in these hybrids. Italian thistle is bisexual, self-compatible, and pollinated by many different insects (Bendall 1975, Evans et al. 1979, Olivieri et al. 1983). A single plant can produce 20,000 seeds in one season (Wheatley and Collett 1981). Seeds are produced in two forms: brown seeds and silver seeds. Brown seeds generally remain in the flowerheads, falling with them to the ground at the end of the season. These seeds can germinate at lower temperatures than the silver seeds. Silver seeds are dispersed by wind and can remain dormant in the soil longer than the brown seeds, up to eight to ten years (Evans et al. 1979, Parsons 1992).


Germination generally occurs in autumn with the first substantial rains. In New Zealand, in a few areas with cold winter temperatures, this species was found to germinate as late as June (Kelly 1988). Seeds can germinate from depths of three inches (8 cm) but usually germinate at less than one inch (0.5–2 cm) (Evans et al. 1979). Partly because of its germination requirements and timing, Italian thistle has been rapidly spreading on rangelands previously dominated by alien annual grasses (Evans et al. 1979). It germinates under temperature and moisture regimes and in seedbed environments that would inhibit germination of the alien annual grass species that currently dominate California grasslands.

(click on photos to view larger image)


Drought favors an increase in Italian thistle (Wheatley and Collett 1981). Any disturbance of vegetative cover encourages establishment of this thistle. Seedlings establish best on bare disturbed soil; in areas with dense groundcover they cannot establish (Harradine 1985). Plants overwinter as rosettes and produce flowering stalks in late spring before summer drought. The rosettes can be so dense that they blanket the soil, inhibiting germination of all other plants. Italian thistle has a branched, slender taproot.


Physical control:  

Manual/mechanical methods: Hand pulling is used at Golden Gate National Recreation Area for small patches, but the root must be severed at least four inches (10 cm) below ground level so the plant does not regrow (Alverez, pers. comm. 1997). Plants should be pulled well before seed is set.


Mowing or slashing is not reliable because plants can regrow and still produce seed. A significant amount of seed can be produced even if thistles are consistently mowed at three inches (8 cm) (Tasmanian Department of Agriculture 1977). Cultivation before seed production may eventually eliminate this thistle, but only if repeated until the seedbank is depleted (up to ten years) (Wheatley and Collett 1981).


Biological control:  

Insects and fungi: Biological control methods offer limited options for containment of Italian thistle. The subject has been extensively researched, but there are no USDA approved biocontrol agents recommended for use in California. Many insects feed on Italian thistle, but the few that effectively control infestations also feed on economically valuable species (Sheppard et al. 1991). Only three insect species, Psylloidas chalcomera, Rhinocellus conious, and Ceutorhynchuys trimaculatus, tested host-specific and caused injury sufficient to decrease reproductive potential of Italian thistle (Goeden 1974). Concern that these insects may prey on several of California’s endangered native thistles in the genus Cirsium has limited the use of these insects for control of Italian thistle.


Several species of rust fungi infest Italian thistle. Puccinnia cardui-pycnocephali is apparently restricted to Italian thistle, although P. cauduorum, P. centaureae, and P. galatica also are found on the plant. Rust fungus reduces growth, especially during the rosette and vegetative phase, but it has insignificant effects on flower or fruit production. Optimal conditions for rust infection and decline of host plants are 18 to 20 degrees C and 90 to 100 percent humidity (Batra et al. 1981, Olivieri 1984, Bruckart 1991).


Grazing: Grazing management showed some promising results in control of Italian thistle populations in Australia. Sheep or goats must be used. Infested areas are closed when thistles start to germinate in autumn and not grazed until plants reach a height of four to six inches (10-15 cm). Areas are then heavily grazed at twice normal stocking rate for three weeks (Bendall 1973).


Chemical control:  

Clopyralid at label-recommended concentrations was effective in controlling Italian thistle in trials in Australia. Clopyralid can be used on advanced or early-flowering plants and causes seed to abort or be sterile (Sindel 1991). Glyphosate (as Roundup®) applied with a rope wick has resulted in good control in New Zealand. Diquat at label-recommended rates kills thistle seedlings but not seedlings of legumes and pasture grasses (Parsons 1992). Picloram, applied in February or March at concentrations of 1/8 to 1/16 lb acid equivalent per acre was recommended for control of Italian thistle in California (Pitcher and Russo 1988). The herbicides 2,4-D ester and MCPA have been used extensively for control of this thistle, but in the rosette stage plants are not as susceptible to these herbicides as during other life stages. Application of 2,4-D ester should be applied when plants are no more than ten inches tall (25 cm) (Wheatley and Collett 1981). Herbicides are likely also to damage desirable native plants and animal species, and consequently are not appropriate for some infested sites, especially those near water. An integrated, long-term plan with persistent follow-up and twice-yearly monitoring is needed to eliminate this thistle. See herbicide table in appendix for California-registered herbicides.