Common Name: Italian plumeless thistle Scientific Name: Carduus pycnocephalus L. Code: CAPY2 Group: Dicot Family: Asteraceae Growth Habit: Forb/herb Duration: Annual What does it look like? Italian thistle ranges in height from a few inches to a few yards tall. The leaves are white-wooly below, hairless-green above, and deeply cut into two to five pairs of spiny lobes. The stems usually have spiny wings. Italian thistle flower heads are thimble-sized and clustered in groups of two to five. They are smaller and fewer than Canada and Bull thistles and vary from pink to purple (NPS 2003).

This thistle reproduces by seed only and can produce 20,000 seeds in one season in two forms: brown seeds and silver seeds. Brown seeds fall from the plant to the ground at the end of the season. The silver seeds spread by wind and can travel 75-325 feet from the parent plant in substantial wind gusts. Italian thistle flowers from mid-September to December. Germination usually occurs in autumn with the first substantial rain (Bossard et al. 2000).

Where does it grow?

Native to the Mediterranean, southern Europe, and North Africa, Italian thistle is now widespread in temperate zones in many different countries. It is documented in California as early as 1912 and is known to grow in Mendocino County.

This thistle thrives in a warm, dry climate and can be in meadows, pastures, ranges, roadsides, and disturbed sites with bare soil. It spreads rapidly in rangelands dominated by annual grasses. Drought and any type of disturbance of vegetative cover also leads to an increase in Italian thistle (Bossard et al. 2000).

Is it in our watershed?

Italian thistle has not been documented in the Mattole watershed at this time.

Why is it a problem?

This thistle is a dominant exotic that takes over and prevents growth of natural forage in meadows and pastures. The blanketing effect of the rosettes can severely reduce the establishment of other plants. It is not desired by livestock due to its harsh spines, which also discourages grazing on neighboring forage plants (Bossard et al. 2000).

How do you get rid of it?

Manual Removal: Hand pulling can be successful at eradicating small patches, but the root must be cut at least four inches below the ground so it doesn’t resprout. Be sure to do this well before seed has set.

Mechanical Removal: Mowing or cutting is rarely successful because plants simply grow back and produce seed. Cutting before seed production may eradicate the species but it must be diligent until the seedbank is depleted, which can be up to ten years.

Biological Control: There are no approved biological species for controlling Italian thistle in California. Only three species are host-specific: Psylloidas chalcomera, Rhinocellus conious, and Ceutorhynchuys trimaculatus. These have caused sufficient injury to decrease Italian thistle populations, but there is concern that these species may prey on several native thistles that are on the endangered species list in California.

Several species of rust fungus 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) (Bossard, 2000).

Grazing: Grazing management using sheep or goats, has showed some promise in controlling Italian thistle populations in Australia. Infested areas were 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 were then heavily grazed at twice normal stocking rate for three weeks (Bendall, 1973) (Bossard, 2000).

Author:
Unity Peterson, Invasive Plant Program Coordinator, Mattole Restoration Council, Petrolia, CA.

Photograph:
Saint Mary's College of California, 1995

References:
Bossard, CC., J.M. Randall and M. Hoshovsky. (eds.) 2000. Invasive Plants of California’s Wildlands. University of California Press, Berkeley, CA. Online version: http://groups.ucanr.org/ceppc/Invasive_Plants_of_California's_Wildlands/

NPS 2003. Invasive Non-Native Plants. Sequoia and Kings Canyon National Parks. Natinal Park Service. http://www.nps.gov/seki/snrm/nnp/html/badcapy.htm