Bullthistle [Cirsium vulgare (Savi) Ten.][CIRVU][CalEPPC: B] [CDFA list: C] 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 of Distribution

Wavyleaf thistle [Cirsium undulatum (Nutt.) Spreng.][CIRUN][CDFA list: B] Photographs Map of Distribution

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GENERAL 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.

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.

MATURE PLANT:Leaves variable, +/- sessile, toothed to lobed, sometimes with lobes toothed.

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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.

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FRUITS 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 when wet.

POSTSENESCENCE 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.

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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.

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MANAGEMENT FAVORING/DISCOURAGING SURVIVAL: Heavy grazing and disturbances that create bare soil patches facilitate seedling establishment and survival.

bullthistle: Cultivation, 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.

SIMILAR 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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