vulgare (Savi) Ten.][CIRVU][CalEPPC: B] Photographs Biocontrol
Canada thistle [Cirsium
arvense (L.) Scop.][CIRAR][CalEPPC: B][CDFA list: B] Photographs
Map of Distribution
Yellowspine thistle [Cirsium
ochrocentrum A. Gray][CIROH][CDFA list: A] Photographs Map
Wavyleaf thistle [Cirsium
undulatum (Nutt.) Spreng.][CIRUN][CDFA list: A] Photographs
Map of Distribution
DESCRIPTION:Erect prickly plants with
purple, pink, or white flower heads that consist only of disk flowers.
Refer to the table below for a quick comparison of some important characteristics.
Also see Comparison of Spiny-leaved thistles.
~ 1 mm
phyllary midrib white & glandular;
corolla lobes mostly
7-10 mm long
SEEDLINGS:Cotyledons oval to oblong, fused at the base, thick, dull,
glabrous or slightly granular. First leaves alternate, elliptic to oblanceolate,
tapered at the base into a winged stalk, ~ 2-4 times longer than cotyledons.
Margin teeth terminate with a weak prickle. Subsequent few to several leaves
typically resemble first leaves except increasingly larger.
PLANT:Leaves variable, +/- sessile, toothed
to lobed, sometimes with lobes toothed.
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and UNDERGROUND STRUCTURES:ROOTS and UNDERGROUND
FLOWERS:Flower heads consist of several overlapping rows of spine-tipped
phyllaries and numerous disk flowers interspersed with bristles on the receptacle.
Insect- and self-pollinated.
and SEEDS:Achenes ovate to elliptic, slightly compressed, , smooth,
glossy, with a basal attachment scar and a short beak (+/- 0.5 mm long) surrounded
by a collar at the apex. Pappus bristles plumose and deciduous,
forming a ring at the base and falling as a unit. Surface of achenes mucilagenous
CHARACTERISTICS:Flower stems typically senesce
in fall, often with the onset of frosty nights. Old flower stems with flower
head remnants usually persist for an extended period. Dead bullthistle
stems may remain standing for 1-2 years.
HABITAT: Open disturbed sites, roadsides, fields, pastures, hillsides,
rangeland, forest openings. Thistles typically do not tolerate deep shade
or constantly wet soils.
PROPAGATION/PHENOLOGY:Most seeds fall near the parent plants or disperse short distances
with wind. Some seeds disperse to greater distances with human activities,
water, soil movement, and as seed or hay contaminants. Birds and small mammals
can consume and disperse some seeds. Seed dormancy at maturity is variable,
depending on environmental conditions and biotype. Soil disturbance facilitates
seed germination and seedling establishment. Seedlings typically emerge from
soil depths to ~ 5 cm. Rosette foliage may be killed by a hard freeze and
re-grow from roots in spring.
FAVORING/DISCOURAGING SURVIVAL: Heavy grazing
and disturbances that create bare soil patches facilitate seedling establishment
mowing, or hand-pulling just before flowering can control infestations. Cut
flower heads can still develop viable seed.
perennial thistles: Repeated
cultivation, mowing, or hand-cutting reduces and can eventually eliminate
infestations. However, occasional cultivation may increase infestations by
dispersing root fragments. Plantings that create dense shade may reduce infestations.
Planting agricultural fields to alfalfa and mowing at least twice a year can
significantly reduce and eventually eliminate Canada thistle.
SPECIES:There are numerous native thistle
species in California, some of which are rare or endangered. Bullthistle
is easily distinguished by having stiff, sharp-pointed, papillae-based
hairs on the upper leaf surfaces, and Canada thistle is the only
thistle with unisexual flower heads. However, several native species
are difficult to distinguish from yellowspine and wavyleaf thistles,
and plants should be positively identified before an eradication plan
is implemented. Only yellowspine and wavyleaf thistles have
all of the following characteristics: perennial with creeping
roots, phyllaries appressed to erect with abruptly outward-curving
to reflexed yellow spines and entire margins, white outer phyllary
midribs with a +/- sticky glandular area, and corollas
29-50 mm long.
Cultural Control: In irrigated conditions, perennial grasses
and alfalfa can compete effectively with Canada thistle. However, alfalfa is
an effective competitor only after it is well established and will perform poorly
as a seedling in dense Canada thistle infestations.
Mechanical Control: Mowing may provide some control of Canada thistle but does
not frequently result in complete kill. Multiple cuttings per year (approximately
every 7-21 days over the growing season) for up to four years may greatly reduce
populations. However, this may be difficult to do in most natural areas. Infrequent
mowing is ineffective and not recommended as a control strategy. Mowing a minimum
of 2-3 times per year may prevent seed production. The optimal timing is when
plants reach the early bud to early flowering stages. To prevent seed production,
mowing must be done by the time flowers begin to open. Viable seed may be produced
eight days after flowers open. Low mowing also removes apical dominance and
may initially increase stem densities from new shoots emerging from dormant
root buds. Mowing may be more effective when combined with herbicide applications.
Repeated tillage may be an effective strategy to exhaust root reserves, but
is not generally an option in natural areas. Tillage can also spread root fragments
to new areas, resulting in new infestations. Deep tillage is generally more
effective than shallow tillage for effective control.
Biological Control: Several organisms have been intentionally introduced into
the United States for Canada thistle control. However, no single agent has been
successful at dramatically reducing infestations. Two agents have been released
in California, Ceutorhyncus litura and Urophora cardui. However, establishment
of both has been very poor with little impact. Neither insect is currently available.
The female weevil Ceutorhyncus litura lays eggs underneath the Canada thistle
leaves in early spring. Larvae bore into the main leaf vein, then down into
the plant's crown area. If the population is high enough, plant death can occur.
However, infested plants are generally stressed and less vigorous. Ceutorhyncus
alone will not effectively control Canada thistle. It must be combined with
other methods to be successful. Combine the weevil with cultural techniques
that allow for maximum desirable plant competition. Research to combine Ceutorhyncus
with herbicides or mowing has not been conducted. The stem gall fly females
(Urophora cardui) lay eggs on apical meristems of developing shoots. Larvae
burrow into shoots. Their feeding triggers huge galls to form that stress the
plant, occasionally resulting in death of the shoot. Galls that form near the
terminal meristems keep the weed from flowering and reduce seed set. In addition
to these insects, a seedhead weevil, Rhinocyllus conicus, has been accidentally
introduced into California, but has had little impact on Canada thistle populations.
Prescribed Fire: Canada thistle may respond both positively and negatively to
burning, depending upon the timing of the burn and the competitive nature of
the surrounding plant community. While burning will not control the belowground
portion of the plant, dormant season burning may remove the dense canopy layer
and release nutrients, which may favor other species and result in increased
competition with Canada thistle. However burns timed to kill actively growing
thistle plants may also negatively impact the surrounding community. With adequate
moisture, Canada thistle will rapidly respond positively following the burn
with little long term negative impacts.
Chemical Control: It is important to know that several ecotypes of Canada thistle
occur that differ in their susceptibility to herbicide treatment. However, the
most effective treatments include glyphosate, clopyralid chlorsulfuron, and
dicamba. The rate, timing, and effectiveness of these treatments may vary. Always
consult the label for recommended rates and timings.
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