Name: Italian plumeless thistle
Scientific Name: Carduus pycnocephalus
Growth Habit: Forb/herb
What does it look
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
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,
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).
Unity Peterson, Invasive Plant Program Coordinator,
Mattole Restoration Council, Petrolia, CA.
Saint Mary's College of California, 1995
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