Italian thistle, slender thistle, shore thistle, Italian plumeless thistle
Closely related California natives
Closely related California non-natives:
Carduus acanthoides, C. nutans, C. tenuiflorus
CalEPPC List B,CDFA A
Carla Bossard,Richard Lichti
HOW DO I RECOGNIZE IT? 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
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).
WHERE WOULD I FIND IT?
California, Italian thistle
infests areas below 3,000 feet (1,000 m) throughout most of the state except for
Basin and northern
Desert. It is common in
chaparral and oak savanna inthe inner Coast Ranges from
SolanoCounty 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).
WHERE DID IT COME FROM AND HOW IS IT SPREAD?
to the Mediterranean,
Italian thistle is now widespread in temperate zones and a major pest in
It was accidentally introduced into United
(Batra et al. 1981) and California
(Goeden 1974) in the 1930s. Robbins (1940) reports it as early as 1912 near
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
WHAT PROBLEMS DOES IT CAUSE?
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
HOW DOES IT GROW AND REPRODUCE?
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
years (Evans et al. 1979, Parsons 1992).
generally occurs in autumn with the first substantial rains. In
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
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
HOW CAN I GET RID OF IT?
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.
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
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
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,
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).
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
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