Bullthistle [Cirsium 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 of Distribution

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

SYNONYMS:

• bullthistle: spear thistle, plume thistle, bur thistle, roadside thistle, bank thistle, bird thistle, blue thistle, black thistle, button thistle, common thistle, Fuller’s thistle, Carduus lanceolatus L., Carduus vulgaris Savi, Cirsium lanceolatum (L.) Scop., Cirsium lanceolatum (L.) Scop. var. hypoleucum DC., Cirsium abyssinicum Sch.Bip.ex A.Rich, Cnicus lanceolatus (L.) Willd., Ascalea lanceolata (L.) Hill, and others
• Canada thistle: creeping thistle, corn thistle, Carduus arvensis(L.) Robson, Cnicus arvensis (L.) Hoffm., Serratula arvensis L., Cirsium incanum (Gmel.) Fish, Cirsium ochrolepideum Juz., Cirsium setosum (Willd.) Bess. ex Bieb., Breea arvensis (L.) Less., and others
• yellowspine thistle: Cnicus ochrocentrus (Gray) Gray
• wavyleaf thistle: wavy-leaved thistle, gray thistle; Carduus undulatus Nutt.; Cnicus undulatus (Nutt.) Gray

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.

• bullthistle: Coarse biennial, annual, or short-lived perennial to ~ 2 m tall, with stiff-hairy foliage and conspicuous prickly-winged stems. Bullthistle is common throughout temperate and mediterranean climate regions of the world. Regional biotypes vary primarily in life cycle patterns and seed dormancy and survival. Introduced from Eurasia. The bullthistle gall fly (Urophora stylata) was released in the Pacific Northwest as a biocontrol agent. It is not established in California at publication time.
• Canada thistle: Clump- or patch-forming perennial to 1 m tall, with extensive creeping roots and small unisexual flower heads. Plants are male or female (dioecious), and dense patches of a single sex often occur. Canada thistle is widespread in cool temperate regions of Eurasia and North America. In the U.S. it is listed as a noxious weed in 28 states. Some taxonomists recognize 4 varieties or biotypes that differ in growth habit, phenology, seed germination, and leaf characteristics. Introduced from Europe. The Canada thistle stem weevil (Ceutorhynchus litura), bud weevil (Larinus planus), and thistle stem gall fly (Urophora cardui) have been released as biocontrol agents. At publication time, only the thistle stem fly is established in California.
• yellowspine thistle, wavyleaf thistle: Clump- or patch-forming perennials to 1 m tall. Yellowspine thistle often spreads vigorously from deep creeping roots.
• Wavyleaf thistle typically forms compact clumps from short, thick, weakly creeping roots. These species hybridize, and plants with intermediate characteristics may occur where their ranges overlap. Introduced from the Central U.S.

  	roots	stems	upper leaf surfaces	head diameter	phyllary spine length	other
  bull thistle	taproot	prickly-winged cobwebby	stiff-bristly	2-4 cm	1-5 mm +/- slender	flower heads with 1-few bract-like leaves immediately below
  Canada thistle	creeping network	not winged +/- glabrous	+/- glabrous	0.5-2 cm	~ 1 mm very slender	unisexual flower heads
  yellowspine thistle	taproot & creeping laterals	not winged densely woolly	lightly woolly	2-4 cm	5-12 mm stout	phyllary midrib white & +/- glandular; corollas lobes mostly 9-14 mm long
  wavyleaf thistle	taproot & short creeping or tuber-like laterals	not winged densely woolly	lightly woolly	2-5 cm	2-5 mm slender	phyllary midrib white & glandular; corolla lobes mostly 7-10 mm long

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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.

• bullthistle: Cotyledons ~7-20 mm long, 3-7 mm wide, paler below. First leaf margins unevenly toothed. Leaf surfaces, especially upper, covered with long, stiff, papillae-based hairs. Lower surfaces +/- granular, initially cobwebby. Rosettes often reach a diameter ~ 60 cm the first spring.
• Canada thistle: Cotyledons ~ 5-14 mm long, 3-6 mm wide, lower surface midvein shiny. First leaf margins +/- wavy to unevenly toothed. Surfaces covered with stiff hairs. Lower surfaces +/- covered with soft cobwebby hairs. Seedlings initially develop a deep taproot. Creeping roots develop in ~ 2-4 months. Seedlings sometimes initiate stems early and have poorly developed rosettes.
• yellowspine thistle, wavyleaf thistle: No description available.

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

• bullthistle: Rosette leaves elliptic to oblanceolate, ~ 10-40 cm long. Margins +/- deeply coarse-lobed and toothed, with main prickles ~ 5-15 mm long. Stem leaves smaller, more deeply lobed and spinier than rosette leaves. Upper surfaces of all leaves green, evenly covered with stiff, sharp-pointed, papillae-based hairs ~ 1 mm long, sometimes sparsely cobwebby. Lower surfaces +/- covered with cobwebby hairs. Leaf bases extend nearly all the way down stem internodes as conspicuous prickly wings. Stems coarse, usually single, branched, loosely covered with white cobwebby hairs, +/- glandular.
• Canada thistle: Rosette leaves few or lacking. All leaves oblong to lanceolate, ~5-20 cm long. Margins nearly entire to shallowly lobed and toothed. Main prickles ~3-6 mm long. Upper surfaces +/- glabrous, green. Lower surfaces sometimes sparsely woolly. Leaf bases sometimes extend briefly down stems as inconspicuous prickly wings to ~ 1 cm long. Stems slender, +/- glabrous, leafy, several to numerous from creeping roots.
• yellowspine thistle: Rosette leaves elliptic to oblanceolate, ~ 10-25 cm long, mostly less than 3 cm wide. Margins deeply coarse-lobed and toothed. Lobes often narrow, ruffled, crowded, and stiffly spreading. Main prickles ~3-12 mm long, yellowish. Stem leaves smaller and spinier, with prickles 5-15 mm long. Leaf upper surfaces +/- gray, lightly covered with white woolly hairs. Stems and lower leaf surfaces densely covered with white woolly hairs. Leaf bases extend down stems as prickly wings to ~2 cm long.
• wavyleaf thistle: Foliage resembles that of yellowspine thistle, except rosette leaves are often more than 3 cm wide and slightly longer (~15-30 cm long), with margins +/- shallowly coarse-lobed and toothed and main prickles ~2-5 mm long. Stem leaf prickles 2-10 mm long. Leaf bases extend down stems as inconspicuous prickly wings to ~1 cm long.
  

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ROOTS and UNDERGROUND STRUCTURES:ROOTS and UNDERGROUND STRUCTURES:

• bullthistle: Taproots thick, fleshy, to ~70 cm deep, often branched into several arms.
• Canada thistle: Root systems consist of an extensive network of vertical and creeping horizontal roots. Most roots occur in the top 45 cm of soil, but vertical roots 2-3 m deep are common. Horizontal roots may spread several meters in all directions. Individual roots survive up to ~ 2 years. Roots are brittle and fragment easily. Some roots survive in frozen soil.
• yellowspine thistle: Taproot thick, deep, typically with many vigorously creeping lateral roots that produce new shoots from soil depths to 1.2 m. Small rosettes from creeping roots are often conspicuous in late summer.
• wavyleaf thistle: Taproot thick, deep, usually branched into weakly creeping, often tuber-like lateral roots that can produce new shoots. Plants often develop into compact clonal clumps, but are typically not widely spreading.

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.

• bullthistle: June-October. Heads 1-few, hemispheric to bell-shaped, +/- loosely covered with cobwebby hairs, and with at least 1 bract-like leaf just below. Involucres (phyllaries as units) 2.5-4 cm long, 2-4 cm diameter. Phyllaries lanceolate to linear, spreading to reflexed. Spines 1-5 mm long, +/- yellowish. Corollas purple, ~ 25-35 mm long, lobes mostly 5-8 mm long.
• Canada thistle: June-October. Heads unisexual, numerous, often clustered, +/- cylindrical or ovoid to bell-shaped. Involucres 1-2(2.5) cm long, 0.5-2 cm diameter, often purplish, glabrous or with white woolly hair. Outer phyllaries ovate, close-laying (appressed) with +/- spreading tips. Spines ~ 1 mm long, fine, +/- dark. Corollas pink, purple, or white, male ~ 10-14 mm long with a shorter pappus, female ~ 14-20 mm long with a longer pappus. Corolla lobes 2-3 mm long.
• yellowspine thistle: April-August. Heads 1-few, +/- ovoid to bell-shaped, +/- loosely covered with white woolly hairs, later becoming +/- glabrous. Involucres ~ 2.5-4 cm long, 2-4 cm diameter. Outer phyllaries ovate to lanceolate, appressed to stiffly erect, with conspicuous white, often glandular midribs and stout yellow spines ~ 5-12 mm long that are spreading to abruptly curved to downwards (reflexed). Corollas white, pink, or pale purple, ~ 25-40 mm long, lobes mostly 9-14 mm long.
• wavyleaf thistle: June-October. Heads resemble those of yellowspine thistle, except involucres are sometimes larger (to 5 cm long, 5 cm diameter), and outer phyllary spines are slender and ~ 2-5 mm long. Corollas white, pink, or pale to bright purple, ~ 25-50 mm long, lobes mostly 7-10 mm long.

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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.

• bullthistle: Achenes 3-5 mm long, ~ 1 mm wide, gray or tan, sometimes with darker longitudinal striations. Pappus bristles ~ 15-30 mm long, +/- white.
• Canada thistle: Achenes 2-4 mm long, ~ 1 mm wide, tan. Pappus bristles ~12-20 mm long, tan.
• yellowspine thistle: Achenes 5-8 mm long, ~ 3 mm wide, pale orangish-brown, +/- with fine red-brown striations, +/- slimy (mucilaginous) when wet. Pappus bristles ~ 25-30 mm long, tan.
• wavyleaf thistle: Achenes 4-7 mm long, ~ 2.5 mm wide, pale straw-colored to tan, +/- mucilaginous when wet. Pappus bristles ~ 25-40 mm long, white to pale tan.
  

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.

• bullthistle: Also agronomic crops, orchards, recently logged and newly planted forestry sites. Grows best on heavy fertile soils.
• Canada thistle: Also agronomic and vegetable crops, stream banks, gardens. Tolerates a wide range of soil types, but grows best in moist soils.
• yellowspine thistle, wavyleaf thistle: Often inhabit shallow sandy soils.

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DISTRIBUTION:

• bullthistle: Common throughout California, except deserts. To 2300 m (7550 ft). Widespread in North America and throughout much of the world.
• Canada thistle: Scattered throughout California, except southern Sierra Nevada, Sonoran and eastern Mojave deserts, and Channel Islands. To 1800 m (5900 ft). Widespread in North America.
• yellowspine thistle: Uncommon, populations scattered. Cascade Range (ne & ce Siskiyou Co.), Modoc Plateau (Modoc, Lassen cos.), eastern Sierra Nevada (se Plumas Co.), southcentral Transverse Ranges (sc Ventura Co.), Santa Rosa Island; to Central U.S. To 1700 m (5500 ft).
• wavyleaf thistle: Uncommon, populations scattered. Cascade Range (ne Siskiyou, se Shasta cos.), Modoc Plateau (e Modoc, c & ne Lassen, se Plumas, e Sierra cos.), San Francisco Bay region (sc Contra Costa Co.), southern South Coast Ranges (cw San Luis Obispo Co.), South Coast (se Los Angeles Co.), Santa Catalina Island, Peninsular Ranges (sw San Diego Co.); to Central U.S. To 1600 m (5200 ft).

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.

• bullthistle: Reproduces by seed. Plants exist as rosettes until flowering stems develop at maturity. Seeds germinate in fall after the first rains or in spring. Fluctuating temperatures and moisture stimulate germination. Germination occurs under a wide temperature range (5-40º C) and with or without light. Compared to other thistles, bullthistle seeds can germinate under low moisture conditions (to –0.75 MPa). First year rosettes usually persist through summer, but may die back during a dry summer and re-grow in fall. Rosettes (typically second year) require a cold period (vernalization) and the presence of sufficient soil nitrogen to initiate growth of flower stems. Thus, plants on very poor soil may take more than 2 seasons to mature. Seed and flower head production is highly variable, depending on environmental conditions. Seeds per flower head range from less than 100 to more than 400, with an average of ~ 200. Plants in grazed pastures often produce more seed than plants in adjacent ungrazed areas. Most seeds either germinate within the first year or die, but seeds buried ~ 15 cm or deeper may survive to ~ 3 years or more.
• Canada thistle: Reproduces vegetatively from creeping roots and by seed. Root reserves are lowest when flowering begins in early summer. New roots and shoot buds develop in winter, and shoots emerge in spring. Subterranean shoots can develop roots and buds at any node. Root and subterranean shoot fragments ~ 1 cm long or more can develop into a new plant. Female plants only produce viable seed if male plants are within pollinator range, ~ 400 m or less in one study. Abundant seed typically develops only when the sexes are less than ~ 50 m apart. If pollinated, viable seeds develop in ~ 8-10 days after flowering. Male flowers occasionally have functional ovaries and a few seeds may develop. Most seeds germinate within 3 years of maturing, but deeply buried seeds can survive 10 years or more. Seeds mostly germinate in mid- to late spring. Rosettes typically do not flower the first season. Plants only flower when day length is 14 hours or more depending on biotype.
• yellowspine thistle, wavyleaf thistle: Reproduces by seed and vegetatively from creeping roots. The biology of these species is poorly understood.

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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.

CONTROL METHODS:

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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