Scotch thistle [Onopordum
acanthium L. ssp. acanthium][ONRAC] [CDFA list: A] Photographs
Map of Distribution Biocontrol
Illyrian thistle [Onopordum
illyricum L.][Bayer code: none][CDFA list: A] Photographs
Map of Distribution
Taurian thistle [Onopordum
tauricum Willd.][Bayer code: none][CDFA list: A] Photographs
Map of Distribution
SYNONYMS:scotch thistle: cotton thistle, woolly thistle, winged
thistle, jackass thistle, heraldic thistle
DESCRIPTION:Vigorous biennial, or
short-lived perennials with coarse, spiny leaves and conspicuous
spiny-winged stems. See Comparison
of spiny-leaved thistles. Plants typically germinate in fall after
the first rains and exist as rosettes throughout the first year until flowering
stems develop during the second spring/summer season. Severe infestations
can form tall, dense, impenetrable stands, especially in fertile soils. To
date, biological control agents have been unsuccessful in the United States.
Introduced from Europe and the Mediterranean region.
SEEDLINGS:Cotyledons oval to oblong, gradually tapered at the base, fleshy,
~1.5-2 cm long. Leaves elliptic to oblanceolate and irregularly spiny-toothed.
PLANT:Stems wings, spiny, continuous,
conspicuous. Leaves alternate, spiny.
and UNDERGROUND STRUCTURES:Taproots stout.
FLOWERS:Heads spheric to hemispheric, mostly solitary but sometimes
in clusters (cymes) of 2-7. Heads consist of numerous spiny-tipped phyllaries
in many overlapping rows and numerous disk flowers. Receptacles fleshy,
deeply pitted, with pits bordered by membranous extensions of tissue,
and lacking bristles (chaff). Phyllary spines less than 5 mm long.
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and SEEDS:Achenes narrowly obovate, more
or less 4- or 5-angled, glabrous, and mottled brown to blackish. Surface roughened
with wavy transverse ridges. Pappus bristles equal, numerous, minutely barbed,
fused into a ring at the base which separates as a unit.
CHARACTERISTICS:Stems can persist into the
next season with spiny phyllaries and receptacles attached.
HABITAT:Natural areas, disturbed sites, roadsides, fields, and especially
sites with fertile soils.
DISTRIBUTION:Infestations uncommon, but plants can be locally abundant at
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PROPAGATION/PHENOLOGY:Reproduces only by seeds. Most seeds germinate in fall
after the first rains, but some seeds can germinate year round under favorable
moisture and temperature conditions. Buried seed of Scotch and Illyrian
thistle can remain viable in the soil seed bank for at least 7 years and
possibly to 20 years or more. Yearly seed production and seed dormancy are
highly variable depending on environmental conditions.
is discouraged in well-managed perennial grass pastures where gaps are minimized
from fall through spring. Unlike sheep and cattle, goats are to known to forage
on Illyrian thistle rosettes and flowering stems. Survival is encouraged
by overgrazing in grasslands and by low intensity burning of fields that often
stimulates a flush of seed germination from the upper soil seed bank.
SPECIES:Onopordum thistles are distinguishable
from other genera of thistles with spiny stern-wings and/or leaves by having
receptacles that lack bristly chaff and have deep pits surrounded
by membranous extensions.
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Prevention: While Scotch
thistle is more widely distributed across California, Illyrian and Taurian
thistles are confined to only a few areas. Therefore, intensive control efforts
may be more fruitful for Illyrian and Taurian thistles. Scotch thistle, however,
should not be disregarded. This plant may reach heights of nearly 3 m and
form impenetrable stands. All three thistles reduce productivity and strongly
compete with native plants for resources. Previous research has demonstrated
certain requirements for Scotch thistle seed germination and may have some
implications for management. For example, a water soluble inhibitor in the
achene coat must be leached prior to germination. In addition, seed germination
is much higher when seed ~ soil contact is maximized. Finally, seeds exhibit
a germination response to light. These characteristics suggest certain practices
may help in scotch thistle management. Since emergence may be greater in wet
years, populations should be monitored more frequently for late season germination.
Livestock should also be removed from infested areas to reduce seed dispersal
and physical trampling of seed into the ground. Finally, any factors that
increase light intensity at the soil surface, i.e., overgrazing or soil disturbance
should be minimized. A dense canopy of competitive, perennial grasses may
be the most effective practice for preventing Onopordum spp. establishment.
Mechanical: Small infestations
should be physically removed or cut a few inches below the soil surface. Mowing
by early flowering will reduce seed production, but may require repeated treatment
because populations typically exhibit a wide range of developmental stages
among individual plants. Slashing should be done prior to flowering since
seed may mature in the capitula (seed head) after cutting. Plants should not
be mowed following seed set, as this increases chances for dispersal.
Biological: There are no
biological agents which have been specifically released for scotch thistle
control in the United States. However, a specific strain of Rhinocyllus conicus
(thistle head weevil) that attacks Italian thistle (Carduus pycnocephalus)
has also been shown to feed on scotch thistle. Establishment of this strain
in the Pacific Northwest has been unsuccessful. A thistle crown weevil (Trichosirocalus
horridus) that feeds on musk, bull, plumeless, Italian, and Canada thistles
will also feed on Scotch thistle. In Australia, this insect has been shown
to kill scotch thistle rosettes. A thistle flea beetle from Italy (Psylliodes
chalcomera) will attack Illyrian thistle, but has not been approved for release
in the United States. There are several other potential insects being examined
in Australia, but plans for their use in the United States are uncertain.
Studies in Australia indicated that goats prevented seed production in Illyrian
thistle, by completely removing the flower heads. However, sheep alone or
goats and sheep together did not completely prevent seed production. Goats
were also effective in digesting seed as < 1% of the ingested seed was
found in the feces, and none was viable.
Chemical: One of the primary
difficulties in chemical control of these thistles is their ability to germinate
nearly year round. From fall to spring, a range of plant sizes can be found
which may result in variable chemical control. These herbicides are all very
effective on seedlings and young rosettes, but control becomes more variable
with increasing plant age. Onopordum spp. seeds may persist for several years
in the soil. Buried seed may persist for up to twenty years, and reinfestation
is likely without yearly management. Therefore several years of retreatment
may be necessary. Dicamba and 2,4-D will inure or kill other broadleaf plants
including legumes. Clopyralid is more selective for controlling plants in
the Asteraceae family but will also injure or kill legumes. Table 1 gives
specific herbicide rates for Onopordum spp. management.
Table 1. Herbicides used for Onopordum
Integrated management: Seedbank
longevity is a major factor in managing these thistles. Reestablishing competitive
perennial grasses and monitoring infested areas on a yearly basis is critical.
Herbicides can successfully be used for reducing thistle populations and giving
grasses a competitive advantage. However, they cannot be used as a stand alone
solution. These techniques must be linked with good grazing practices in rangeland
areas. Otherwise, the thistles will recolonize and rapidly replenish the seed
bank to pre-control levels.