**Carduus pycnocephalus L. (Asteraceae)**

**Italian Thistle**

**Description**. Annual or biennial, herbaceous, from a stout taproot;
stems 15-200 cm tall, erect, glabrous to sparsely tomentose, narrowly
and discontinuously winged, the wings spinose, tomentose, branched above
the lower third, branches erect to ascending. Basal leaves 6-15 cm long,
oblanceolate, deeply 4-10-lobed, the base tapered; cauline leaves
alternate, decurrent, sinuate to pinnately lobed, margins spinose, upper
surfaces loosely tomentose, becoming glabrous, lower surfaces densely
tomentose. Heads discoid (all corollas radial and salverform), 17-22 mm
long, 10-20 mm wide, cylindrical to subcylindrical, sessile to stalked,
solitary or 2-5 in terminal clusters. Outer phyllaries ovate-lanceolate,
loosely tomentose, margins membranous, apices acuminate, terminating in
a straight spine; inner phyllaries narrower, scarious. Corollas 10-14 mm
long, pink to rose-purple, sometimes white. Achenes 4-6 mm long, tan to
brown, sometimes shiny, transversely wrinkled, tubercled above; pappus
10-20 mm long, composed of flat, minutely barbed, white bristles. In
California, flowering from May to July (Clapham et al. 1962, Franco
1976, Keil and Turner 1993, Munz 1959; Robbins et al. 1970, Webb et al.
1988).

**Note:** The closely related species, *C. tenuiflorus* Curtis (Slender
Thistle) differs by stems with continuous wings, leaves 12-20-lobed,
5-20 heads per cluster, and scarious phyllaries that become glabrous at
maturity. Although some authors have suggested that the two species may
hybridize or intergrade (Keil and Turner 1993, Webb et al. 1988),
Olivieri (1985) reported low levels of seed set in putative intermediate
or known hybrids, suggesting that gene flow, if any, was relatively low.

**Geographic distribution**. A native of Mediterranean Europe and North
Africa, it is naturalized in Australia, New Zealand, South Africa,
Europe, and western Asia, where it is considered invasive (Dunn 1976,
Sindel 1991). In the United States it is reported from Alabama,
Arkansas, California, Idaho, Hawaii, Idaho, New York, Oregon, and Texas,
but is apparently well established only in Texas and in California (Dunn
1976, Fernald 1950, Goeden 1974, Wagner et al. 1990).

Naturalized populations occur on San Miguel and Santa Rosa islands
(Junak et al. 1997) and on mainland California from Mendocino County to
San Diego County along the coast, the lower Sacramento River Valley. and
the Sierra Nevada foothills in Butte and Nevada counties (Anonymous
1998, Keil and Turner 1993).

**Ecological distribution**. Italian thistle commonly occurs in
disturbed, often moist habitats, in canyon bottoms, along roadsides,
open grassland, fallow fields, grazed areas, the margins of cultivated
fields, and along irrigation canals (Dunn 1976, Goeden 1974, McClintock
1993, Munz 1959, Robbins et al. 1970, Sindel 1991). In central
California, Italian thistle has been reported as occurring in
grassland-live oak ecotones (Maranon and Bartolome 1993).

**Reproductive and vegetative biology**. Italian thistle is bisexual,
self-compatible, and pollinated by a diverse array of insects (Bendall
1975, Evans et al. 1979, Olivieri et al. 1983). Achenes are either
dispersed widely by wind or remain enclosed in receptacular bracts of
the head (Evans et al. 1979, Olivieri et al. 1983). Seeds remaining in
the heads are generally dull brown in color; those dispersed by wind are
lighter in color and shiny. Fruit dimorphism probably represents a dual
strategy for recruitment *in situ* and by long-distance dispersal,
because dull brown seeds generally fall to the ground partially enclosed
within the dried heads. Both forms of seeds are mucilaginous when
wetted. Although Evans et al (1979) suggest that mucilaginous seeds may
be advantageous for dispersal, studies of other species suggest that
mucilaginous seeds also may confer an advantage during germination and
establishment of seedlings under water-limited conditions (Sagar and
Harper 1960, Sorensen 1986).

Seeds of Italian and slender thistles germinate during conditions
favoring long, cool nights combined with relatively high humidity or
winter precipitation (Groves 1989, Olivieri et al. 1983). Evans et al.
(1979) reported germination rates of 83-96% , especially under
conditions of cool nights (16 hours at 10 degrees C) and warm days (8
hours at 35 degrees C). No differences were found with either fresh or
stored seeds. Seeds buried at depths up to 2 cm germinated more readily
than those on the surface or buried at depths from 5 to 10 cm.

Growth of vegetative plants is promoted by "rich" soils (Groves 1989).
Application of nitrogen and/or slightly acidic pH levels enhance
vegetative growth (Bendall 1975). The occurrence of Italian thistle in
disturbed sites and absence from cultivated fields suggests that it is
not competitive against well established or ungrazed vegetation (Goeden
1974). Experimental studies by Maranon and Bartolome (1993) showed that
Italian thistle is one of several invasive species whose seeds remain
dormant under shaded conditions, and which can germinate and compete
successfully in both light and shade, except when in sites with high
grass (*Bromus diandrus, Avena sativa*) cover.

**Weed status**. Italian thistle is not considered a noxious weed in
agricultural practice, at least at a global level (not listed by Holm et
al. 1977), but is considered a noxious weed by the State Dept. of Food
and Agriculture (Anonymous 1996). Both Italian and slender thistles are
listed as weeds in the United States (Lorenzi and Jeffrey 1987).

**Microbial pathogens**. Several species of rust fungi (*Puccinia
cardui-pycnocephali*, *P. cauduorum*, *P. centaureae*, and *P.
galatica*) are known to infest both *Carduus pycnocephalus* and *C.
tenuiflorus*. *Puccinia cardui-pycnocephali* is apparently restricted to
*Carduus pycnocephalus* (Batra et al. 1981, Oliveri 1984, Bruckart
1991). In general, inoculations of experimental plants showed that the
rust fungus reduces growth, especially at the rosette and vegetative
phase, but has insignificant effects on flower and fruit production.
Based on experimental greenhouse studies, optimal conditions for
infection and decline of host plants are between 18 and 20 degrees C and
90 to 100% relative humidity.

Bruckart (1991) provided evidence of host-plant differentiation among
strains of several *Puccinia* species, with *P. carduorum* found more
frequently than *P. cardui-pycnocephali* on *Carduus pycnochephalus*.
Strains isolated from one species of thistle were most effective on
plants of the same rather than another species of *Carduus*, although
rust fungi isolated from *C. pycnocephalus* were more virulent when
introduced on plants of *C. tenuiflorus*. Olivieri (1984) reported some
evidence of differential susceptibility in both species to infection by
*Puccinia cardui-pycnocephali*. In general, naturalized strains were
more susceptible than native European strains, suggesting that
resistance to fungal infection had been lost during or after the
colonization process.

**Insect pathogens.** Several species of phytophagous beetles
(Curculionidae; weevils) are known to infest *Carduus* (Bolt et al.
1980, Goeden 1974), and have been used as biological agents for control
in California and Australia (Frick 1978, Goeden 1974, 1978, Sindel
1991). However, only a few species appear to be effective as control
agents, by causing severe injury to developing leaf rosettes, developing
flowering heads, and flowers. Julien (1982) reported high levels of
flower abortion in both *Carduus pycnocephalus* and *C. tenuiflorus*,
after infestation with *Rhinocyllus conicus*. *Rhinocyllus conicus* has
been used experimentally in California for control of both *Silybum*
*marianum* (L.) Gaertner (Milk thistle) and *Carduus pycnocephalus*
(Goeden 1974, 1978, Goeden and Ricker 1978, Hawkes et al. 1972).
Although flower abortion (\> 90%) was high in naturalized thistle
populations in both New Zealand and California (Julien 1982, Goeden and
Ricker 1978), populations of Italian thistle did not appear to decline
(Goeden and Ricker 1978). *Ceutorhyncus trimaculatus*, a pest of
artichoke and safflower crops, also has been found to be a potential
predator of flowers and seeds of Italian and slender thistles (Bolt et
al. 1980, Kok et al. 1982). Some success with an integrated pest
management program, using 2,4-D in combination with *Ceutothoryncidius
horridus* (Coleoptera Curculionidae), has been reported by Trumble and
Kok (1980a, 1980b). However, the potential effect of introduced
phytophages on native American thistles remains unknown (Turner 1991).

**Herbicide control**. Related species (*Carduus acanthoides* L.,
plumeless thistle, and *C. nutans* L., musk thistle) are best treated
with 2,4-D for young leaf rosettes or a combination of mowing and 2,4-D
for plants that have bolted (Lorenzi and Jeffery 1987).

**Literature Cited**

Anonymous. 1996. Exotic pest plants of greatest ecological concern in
California as of August 1996. California Exotic Pest Plant Council. 8
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Anonymous. 1998. California county flora database version 2, Santa
Barbara Botanic Garden and USDA National Plants Data Center, Santa
Barbara and New Orleans. URL = plants.usda.gov

Anonymous. 1998. USDA Plants Database. USDA National Plants Data Center,
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