By Celestine Duncan and Melissa Brown Munson
Introduction and Biology
Canada thistle (Cirsium arvense L) is a deep-rooted perennial weed that infests natural areas, pastures, rights-of-ways, seasonal wetlands, and cropland. It is considered the most widespread of all thistle species (Morishita 1999) with near-global distribution between 37° and 58 to 59° N latitude in the northern hemisphere (Moore 1975), and latitudes greater than 37° S in the southern hemisphere, exclusive of Antarctica (Amor and Harris 1974).
Canada thistle is probably native to southeastern Europe and the eastern Mediterranean (Moore 1975; Erickson 1983). Since its introduction into North America in the early 1600s (Dewey 1901), it spread throughout temperate regions. Canada thistle was first introduced to New France and New England probably as a contaminant in hay, crop seeds, and ship ballast (Hansen 1918; Erickson 1983). The weed had become sufficiently troublesome by 1795 that Vermont enacted control legislation and New York followed in 1831 (Moore 1975). Canada thistle growth is limited or stopped when temperatures exceed 85 degrees for extended periods (Lym and Zollinger 1995). Rapid spread of Canada thistle is facilitated by production of abundant, wind and water disseminated fruitlets (Hope 1927; Moore 1975).
Canada thistle is best adapted to open sunny sites on well-drained, deep, fine-textured soils (Holm et al. 1977; Moore 1975). The weed forms clumps or patches from an extensive creeping root system. Plants can grow up to about four feet in height and have small flower heads that are either male or female (dioecious), with patches of a single sex often occurring. Canada thistle produces flowers from mid- June through September. Cross-pollination is required for successful seed development. Each light brown, elongate, seed-like fruitlet is under 0.15 inches (4 mm) long and topped with a dense, white, feathery pappus that serves as a parachute for wind dispersal. Each flower head contains between 83 and 90 seeds and a single plant averages about 1,500 seeds per plant (Moore 1975; Sather 1987). Although 90 percent of all seeds germinate within one year of dispersal, buried seeds can remain viable for as long as 21 years. Once dispersed, each single-seeded fruitlet is able to establish either a male or female clone through vegetative propagation of its root system.
Several varieties of Canada thistle have been identified (albidflorum, mite, vestitum, integrifolium, horridum, and arvense (Barkley 1986; Moore and Frankton 1974). These varieties are classified primarily by leaf shape, size, and abundance of leaf spines. In addition, many ecotypes of Canada thistle differ in growth, phenology, photo-periodism, and susceptibility to herbicides and biological agents (Hodgson 1964a, 1964b, 1968a, 1968b, 1970; Moore 1975; Hunter and Smith 1972; Turner et al. 1981). Flower color can range from purple to light lavender or even white. Stem color also can differ from green to lavender.
Canada thistle root system may extend horizontally as much as 18 feet in one season, but individual roots live only for about 2 years (Rogers 1928; Hayden 1934). Although short-lived, roots are able to regenerate entire plants from very small fragments (Prentiss 1889). In addition to the extensive root system and high seed production, Canada thistle also produces phytotoxins that inhibit growth of other plants. Extracts of roots and foliage from Canada thistle inhibit germination of its own seed and clover, and reduced growth of seedlings of seven other species (Bendall 1975). It also has an allelopathic effect on crops such as sugar beet, wheat, alfalfa and corn (Wilson 1981; Helgeson and Konzak 1950).
Environmental and economic impacts of Canada thistle are described in Lym and Duncan (2005) and briefly summarized as follows. Canada thistle can displace desirable grasses and forbs and form dense monocultures limiting forage and livestock production on rangeland (Sheley et al. 1999; Morishita 1999). Canada thistle can also affect wildlife habitat. For example, reduction in Canada thistle density with herbicide treatments in riparian areas increased species richness important to waterfowl (Krueger-Mangold et al. 2002). Canada thistle infestations can change structure and species composition of natural areas and reduce plant and animal diversity (Hutchison 1992). Infestations of Canada thistle may contribute to the elimination of endangered and/or endemic plant species (Cheater 1992). Canada thistle has the potential to increase fire frequency and perhaps severity as a result of its abundant and readily ignited litter (Hogenbirk and Wein 1995). The economic impact of Canada thistle in natural areas is undocumented; however, infestations on these sites may invade adjoining cropland causing economic damage. Canada thistle causes yield losses in small grains and other seed crops (O’Sullivan et al. 1982 and 1985; Moyer et al. 1991).
Integrating various management methods is the most effective means of controlling Canada thistle. On tillable lands, this includes a combination of cultivation, herbicides and seeding competitive desirable plants (Diamond et al. 1988; Trumble and Kok 1982). In natural areas with limited infestations, pulling, cutting and spot application of herbicides could be used. In native prairies, a spring burn followed by cutting or spot application of Milestone® herbicide (Hitzeman personal comm.) or glyphosate during late bud or early bloom can provide control of Canada thistle (Sather 1987). A combination of mowing, grazing, and herbicide application has shown promise for reducing Canada thistle populations in pasture and rangeland conditions (Beck et al. 1993; Schreiber 1967; and Welton et al. 1929). Canada thistle control with herbicides may be improved when preceded by two or three mowing events (Beck 1998; Sebastian and Beck 1994). Wilson (1994) reported that glyphosate, followed by rototilling and seeding perennial grasses reduced Canada thistle density 75 percent at 9 months after seeding. The combination of 2,4-D with mowing or dragging provided better suppression of Canada thistle and higher grass yield than either application of 2,4-D, dragging, or mowing alone (Cole et al. 1992).
Herbicides used for controlling Canada thistle on lands grazed by livestock and wildlife include aminopyralid (Milestone® herbicide), aminopyralid + 2,4-D (GrazonNext® HL herbicide), aminopyralid + metsulfuron methyl (Opensight® herbicide), 2,4-D, clopyralid (Transline® herbicide), picloram (Tordon® 22K herbicide), metsulfuron (Escort and others), chlorsulfuron (Telar), glyphosate (Accord® XRT II and others), and dicamba (Banvel and others). These herbicides are most effective when combined with cultural and/or mechanical control (Beck 1998).
MILESTONE® herbicide at 5 to 7 fluid ounces per acre (fl oz/A) provides good to excellent (80 to 95+%) control of Canada thistle when applied at the early bud to bloom stage and in fall (Deneke et al. 2012). In ten studies conducted across the Great Plains Region, Milestone provided Canada thistle control comparable to Tordon 22K and better control than Transline, dicamba alone, and dicamba + 2,4-D amine (Enloe et al. 2007). Milestone provided similar Canada thistle control than older standard herbicide treatments at lower use rates and is more selective on desirable forbs (Almquist and Lym 2010; Lym 2005; Carter and Lym 2017; Mikkelson and Lym 2013) (Figure 1). Mowing Canada thistle seven days before and two weeks following application with Milestone at 5 fl oz/A reduced control compared to non-mowed plots (Burch et al 2006).
FIGURE 1. CANADA THISTLE CONTROL ONE YEAR AFTER TREATMENT WITH MILESTONE® HERBICIDE APPLIED AT 5 OR 7 FLUID OUNCES PER ACRE (FL OZ/A) IN MAY (rosette-bolt), JULY (early bud), AUGUST (bloom), SEPTEMBER (fall), OR OCTOBER (fall-post frost) IN CLARK COUNTY, SOUTH DAKOTA. DATA BARS WITH THE SAME LETTERS ARE NOT SIGNIFICANTLY DIFFERENT (Denke et al. 2012).
OPENSIGHT® herbicide combines aminopyralid and metsulfuron-methyl in a dry, water dispersible granule formulation. The combination of the two active herbicide ingredients provides excellent Canada thistle control (>85%) and expands the weed control spectrum over Milestone alone (Dow AgroSciences LLC internal data). Application rate for Canada thistle control is 2.5 to 3.3 ounces of product per acre (oz product/A). The maximum label use rate for Opensight is 3.3 oz product/A, which contains 1.7 oz acid equivalent (ae) of aminopyralid (equal to 7 fl oz Milestone) and 0.36 oz active ingredient of metsulfuron-methyl (0.5 oz product such as Escort).
TRANSLINE® herbicide at 2/3 to 1 pint per acre (pt/A) provides good control of Canada thistle. The lower rate of 2/3 pints per acre (pt/A) applied when Canada thistle is in the bolt to early bud growth stage provides greater than 70 percent control one year after treatment. Application rates of 1 pt/A are recommended at bud growth stage or on fall regrowth (Morishita 1999; Beck 1998; Beck and Sebastian 1988; Bixler et al. 1991). Effectiveness of Transline plus 2,4-D to control Canada thistle may be improved when preceded by mowing two or three times (Beck 1998). Beck (1998) suggests that mowing should be initiated when Canada thistle is 12 to 15 inches tall, and mowing repeated at about one month intervals.
The herbicide 2,4-D does not control Canada thistle roots and has a short residual providing only season-long suppression of Canada thistle. Applications of 2,4-D at 1.5 to 2 pounds of acid equivalent (ae) per acre (1.5 to 2 quarts/A of a formulation with 4 lbs ae/gallon) should be made before Canada thistle reaches bud growth stage to provide the best suppression. 2,4-D applied at bud stage at rates of 0.5 lb ae/A and above has been shown to stop seed production (Donald 1990). Beck (1998) applied 2,4-D in combination with other herbicides as a split-season application to improve Canada thistle control.
DICAMBA (Banvel and others) at 1 pint to 1 gallon per acre (0.5 to 4 lbs ae/A) will suppress or control Canada thistle; however, results can be inconsistent especially with rates less than 2 quarts per acre. Although dicamba can be applied any time of the growing season, control is greatest when applied at late-bolt or early-bud growth stage, or to fall regrowth (Lym and Zollinger 1995; Beck 1998).
METSULFURON-METHYL (Escort and others) at 1 oz product/A or chlorsulfuron (Telar and others) at 1 to 1.3 oz product/A will suppress Canada thistle when applied at the bud to bloom stage or on fall rosettes (Sebastian and Beck 1991). These herbicides do not provide effective for long-term Canada thistle control, but can be tank-mixed with other herbicides to broaden spectrum of weed control (Morishita 1999).
GLYPHOSATE (Accord® XRT II and others) at 2 to 3 pounds acid equivalent per acre (Accord XRT II at 2 to 3 quarts/A) may be applied to Canada thistle when plants are beyond the bud growth stage and also in fall. Glyphosate is a non-selective herbicide (will kill both grasses and broadleaf plants) so wick- or wiper-type applicators should be used to avoid eliminating surrounding desirable vegetation unless reseeding the infested site is planned.
Use of competitive (smother) crops that develop earlier in the season than Canada thistle and form dense cover to shade developing thistle shoots is part of a strategy for managing this plant (Hodgson 1968a). In cropland, alfalfa suppresses Canada thistle since cutting several times a season depletes carbohydrate food reserves in Canada thistle roots. A twice yearly mowing of an alfalfa field in Montana reduced stem counts 14 percent after one year and essentially eliminated Canada thistle after four years (Hodgson 1968a). Establishing desirable perennial grasses and forbs can reduce the competitive ability of Canada thistle in prairie. Canada thistle cover that exceeds 20 percent in prairie grasslands should be treated to reduce impact to desirable forbs and grasses (Sleugh et al. 2015).
Cultivation is limited to cropland or pastureland that can be cultivated. Tilling to a depth of 5 to 6 inches in the spring when shoots appear followed by regular shallow cultivation every 21 days throughout the growing season until first hard frost provides effective Canada thistle control (Hodgson 1968b). Regular cultivation is necessary since each root segment can produce new shoots about 10 days after tillage (Hodgson 1968b) and roots segments can survive about 100 days in soil.
In pastures, mowing can impact Canada thistle reproduction and spread. A minimum of one mowing at the early bud stage will reduce or eliminate seed production for that year, but repeated mowing during the season may have a more long-term effect on the plant (Haber 1997; Beck 1998). Hutchinson (1992) found that pulling or hand-cutting light infestations several times a year was effective in suppressing Canada thistle in natural areas. However, mowing seven times during a growing season was ineffective in controlling the weed in Canada (Diamond et al. 1987). Repeated mowing in prairie grasslands may have negative impacts on the desirable plant community (Sleugh 2015).
Herbicides in combination with mowing can provide effective control of Canada thistle (Sebastian and Beck 1994). In irrigated grass hay meadows, fall herbicide treatments that follow mowing can be an effective management system because more Canada thistle foliage is present after cutting to intercept herbicide (Beck 1998). Additionally, root nutrient reserves decrease after mowing. Mowing in spring and again in mid-season followed by an application of 2,4-D plus dicamba, or Transline provided good control of Canada thistle (Diamond et al. 1988). Perennial grasses in combination with Transline applications were more effective than mowing alone in suppressing Canada thistle (Wilson 1994).
Canada thistle response to fire is variable and control depends on soil moisture, other site conditions, fire intensity, and season of the burn. Late spring burns in May and June may suppress Canada thistle (Hutchison 1992). Although a significant reduction in Canada thistle was reported in a native North Dakota mixed grass prairie after a single burn event (Smith 1985); others report that Canada thistle density may initially increase after fire (Travnicek et al 2005). Continued use of prescribed fire may decrease Canada thistle over time (Travnicek et al. 2005) but may also decrease plant diversity (Wenjin et al. 2013). Fire in combination with an application of Milestone effectively controlled Canada thistle and shifted the dominance of desirable perennial grasses (Gramig and Ganguli 2015).
Sheep have been reported to graze and trample Canada thistle that was treated with salt (Detmers 1927, Cox 1913). Goats, sheep, and cattle can damage Canada thistle with repeated grazing to prevent flowering (Univ. Idaho 2016). Goats are the preferred grazing animal, followed by sheep and cattle. Sheep and cattle prefer to graze Canada thistle when it is young before spines develop. Grazing is most effective when repeated during the season and for multiple seasons to prevent seed production and to deplete root reserves. High intensity low frequency rotational cattle grazing reduced Canada thistle shoot density, biomass and flowering. Two intense defoliations annually over two to three years nearly eliminated Canada thistle stems (DeBruijn 2006). Livestock management is critical since overgrazing creates disturbance and is a contributing factor to thistle establishment.
Biological control is the use of native or foreign insects, pathogens or other living organisms to attack weeds. As a weed management method, biological control offers another tool to compliment conventional methods. Canada thistle is a problematic weed even in its native range in Europe, in spite of the fact that a large number of insects are found on the plant and within various organs. Zwölfer (1965) reported 86 species of insects on Canada thistle, with half of the species found within the plant (stems, buds, roots). In Canada, Maw (1976) compiled a list of 84 insects that are believed to feed on Canada thistle.
Several biological agents have been evaluated for control of Canada thistle in the United States. These include insects, fungi and nematodes (Moore 1975; Maw 1976; Donald 1994). The USDA Animal Plant Health Inspection Service is reluctant to permit movement of foreign insects that attack Canada thistle due to non-target effects on native thistles (Merenz personal comm.). Native insects such as larvae of the painted lady butterfly (Vanessa cardui) will defoliate Canada thistle (Moore 1975); however, control is intermittent (Lym and Zollinger 1995). The insect generally is found in southern states such as Arizona and New Mexico and will build up populations large enough to migrate north only once every 8 to 11 years.
Two insects are currently available on Canada thistle. Hadroplantus (Ceutorhynchus) litura, the Canada thistle stem weevil, effectively exposes Canada thistle plants to opportunistic pathogens, but the agents themselves are not particularly effective (Merenz personal comm.). This agent is established in Idaho and is readily available in many states in the early spring. The stem-boring weevil was introduced to Canada in 1965 (Hein and Wilson 2004) and the United States in 1972 (Rees 1990). Females lay eggs underneath the Canada thistle leaves in early spring. Larvae bore into the main leaf vein, then down into the plant's crown area reducing plant vigor. Combining the weevil with cultural techniques or herbicides may improve control. Collier and others (2007) reported that combining weevil attack with clopyralid (Transline) led to the most consistent and greatest suppression of root biomass of Canada thistle.
Urophora cardui, the Canada thistle stem gall fly, deposits eggs in the stems of Canada thistle plants and the larvae tunnel into the stems causing galls to form (Merenz pers. comm.). Stems above the galls often do not produce flowers. This agent is established in Idaho and in neighboring states. The fly does best in moist, disturbed areas that are semi-shaded. Galls can be collected in the fall, winter, or early spring.
Rust pathogens, such as Puccinia punctiformis, have been studied for their potential use to control Canada thistle (Bailiss and Wilson 1967; French and Lightfield 1990). This organism is effective in reducing flowering and vegetative reproduction in Canada thistle. It is found in British Columbia, Ontario, and Quebec in Canada (Harris 1996), but only rarely found in the Prairie Provinces.
There has been considerable research conducted with herbicides, mechanical, manual, and cultural management techniques on Canada thistle. Continued research on integrating various management methods on Canada thistle will help ensure success of sustainable long-term management strategies.
This comprehensive literature review was originally prepared in 2001 and trademark updated in 2019.
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The most comprehensive reviews of Canada thistle control are found in Moore (1975), Holm et al. (1977), Evans (1984) and the Element Stewardship Abstract on Canada thistle prepared by Sather (1987). The following compilation of references provides an updated bibliography of much of the primary work related to Canada thistle, its biology and control.
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Bailiss KW and IM Wilson. 1967. Growth hormones and the creeping thistle rust. Annals of Botany 31: 195-211.
Barkley, T.M, ed. 1986. The Flora of the Great Plains. Lawrence, KS: University Press Kansas. Pp. 909-910.
Beck KG, JR Sebastian and RG Lym. 1993. An integrated Canada thistle management system combining mowing with fall-applied herbicides. In Proceedings of the Western Society of Weed Science 46: 102-104.
Beck KG. 1998. Canada thistle. Colorado State University Cooperative Extension Bulletin 3.108.
Beck KG and JR Sebastian. 1988. Canada thistle control with chlorflurenol, dicamba, and clopyralid in a Colorado pasture. Western Society of Weed Science Research Progress Report.
Bendall GM. 1975. The allelopathic activity of Californian thistle (Cirsium arvense (L.) Scop.) in Tasmania. Weed Research 15: 77-81.
Bixler LL, AW Cooley, VF Carrithers, and RG Lym. 1991. Canada thistle control at two stages of plant growth with clopyralid. In Proceedings of the Western Society of Weed Science 44-47.
Burch PL, RG Wilson, ES Hagood, RA Masters, and VF Carrithers. 2006. Canada thistle management with Milestone herbicide. Weed Science Soc. of Am. 46th Conf. Proceedings. New York, New York.
Carter TR and RG Lym. 2017. Canada thistle (Cirsium arvense) affects herbage production in the Northern Great Plains. Invasive Plant Science and Management 10 (4): 332-339. Cheater M. 1992. Alien invasion. Nature Conservancy 42(5):24-29.
Collier TR, SF Enloe, JK Sciegienkac, FD Menalled. 2007. Combined impacts of Ceutorhynchus litura and herbicide treatments for Canada thistle suppression. Biological Control 43:2: 231–236
Cole DE, DJ Richardons, K Stromme. 1992. Chemical and non-chemical control of Canada thistle in grass. Res. Rept. Expert Comm. Weeds West Can. 39:2:1002-1003.
Cox HR. 1913. Controlling Canada thistle. U.S. Dept. of Agriculture, Farmer's Bulletin 545.
De Bruijn SL. 2006. Biological control of Canada thistle in temperate pastures using high density rotational cattle grazing. Biological Control 36:305-315.
Deneke D, M Moechnig, D Vos, J Alms. 2012. Optimal herbicide application timing for Canada Thistle Control. In Proceedings, Western Society of Weed Science. p. 22-23.
Detmers F. 1927. Canada thistle (Cirsium arvense Tourn), field thistle, creeping thistle. Ohio Experiment Station Bulletin 414: 1-45.
Dewey HL. 1901. Canada thistle. U.S. Department of Agriculture Bureau of Botany. Circular 27. 14 pp.
Diamond JF, MG Sampson, AK Watson. 1987. Integrated management of Canada thistle with clopyralid and dicamba. Res. Rep. Expert Comm. Weeds East. Can 31:1: 6-7.
Diamond JF, MG Sampson, AK Watson. 1988. Integrated management of Canada thistle in pastures in Eastern Canada. Abstr. Weed Sci. Soc. Am.:28:50-51.
Donald WW. 1990. Management and control of Canada thistle (Cirsium arvense). Rev. Weed Sci. 5: 193-250.
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Eloe SF, RG Lym, R Wilson, P Westra, S Nissen, G Beck, M Moechnig, V Peterson, RA Masters, M Halstvedt. 2007. Canada thistle (Cirsium arvense) control with aminopyralid in range, pasture, and noncrop areas. Weed Technology 21:890-894.
Erickson LC. 1983. A review of early introductions of field (Canada) thistle (Cirsium arvense L. Scop.) To North America and its present distribution. West. Soc. Weed Sci. Proc. 36:200-204.
French RC. and AR Lightfield. 1990. Induction of systemic infection in Canada thistle (Cirsium arvense) by teliospores of Puccinia punctiformis. Phytopathology 80: 872-877.
Gramig GG and AC Ganguli. 2015. Managing Canada Thistle (Cirsium arvense) in a Constructed Grassland with Aminopyralid and Prescribed Fire. Invasive Plant Science and Management 8(2):243-249.
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Hansen AA. 1918. Canada thistle and methods of eradication. USDA Farmers Bulletin 1002.
Harris P. 1996. Status of introduced and main indigenous organisms on weeds targeted for biocontrol in Canada. Agriculture and Agri-Food Canada, Lethbridge Research Centre, Alberta.
Hayden A. 1934. Distribution and reproduction of Canada thistle in Iowa. American Journal of Botany 21: 355-373.
Hein GL and RG Wilson. 2004. Impact of Ceutorhynchus litura Feeding on Root Carbohydrate Levels in Canada Thistle (Cirsium arvense). Weed Science 52:4. pp. 628-633
Helgeson FA and R Konzak. 1950. Phytotoxic effects of aqueous extracts of field bindweed and Canada thistle, preliminary report. North Dakota Agricultural Experiment Station Bulletin 12: 71-76.
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Hodgson JM 1964b. Growth habits of Canada thistle. In Proceedings Idaho Noxious Weed Control Conference, July 27, 1964, Preston, Idaho. pp. 21-25.
Hodgson JM 1968a. The nature, ecology, and control of Canada thistle. U.S. Department of Agriculture Technical Bulletin 1386. 32 pp.
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Hodgson JM 1970. The response of Canada thistle ecotypes to 2,4-D Amitrole and intensive cultivation. Weed Science 18:253-255.
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Holm LG, DL Plucknett, JV Pancho, JP Herberger. 1977. Cirsium arvense (L.) Scop. In, The World's Worst Weeds: distribution and biology. pp 217-224. Published by the University Press of Hawaii for the East-West Food Institute. Honolulu.
Hope A. 1927. The dissemination of weed seeds by irrigation water in Alberta. Science and Agriculture 7: 268- 270.
Hunter JH and LW Smith. 1972. Environmental and herbicide effects on Canada thistle ecotypes (Cirsium arvense). Weed Science 20: 163-167.
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Lym RG and R Zollinger. 1995. Perennial and biennial thistle control. NDSU Extension Service Bulletin W-799.
Lym RG. 2005. Control of invasive weeds with aminopyralid in North Dakota. In. Proceedings, Western Society of Weed Science, Vancouver, B.C. Canada. p60.
Lym RG and C Duncan. 2005. Canada thistle (Cirsium arvense (L.) Scop. In Duncan, C.A. and J.K. Clark, eds. Invasive Plants of Range and Wildlands and their Environmental, Economic and Societal Impacts. Weed Science Society of America, Allen Press. pp 69-83.
Maw MG. 1976. An annotated list of insects associated with Canada thistle (Cirsium arvense) in Canada. Canadian Entomologist 108: 235-244.
Merenz, Richard. Personal communication. USDA APHIS, Helena, MT. Feb. 2016.
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O’Sullivan PA, GM Weiss, VC Kossatz. 1985. Indices of competition for estimating rapeseed yield loss due to Canada thistle. Canadian Journal of Plant Science 65:145-149.
Prentiss AN. 1889. On root propagation of Canada thistle. Cornell University Agricultural Experiment Station Bulletin 15: 190-192.
Rees NE. 1990. Establishment, dispersal and influence of Ceutorhynchus litura on Canada thistle (Cirsium arvense) in the Gallatin valley of Montana. Weed Science 38: 198-200.
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Rogers CF 1928. Canada thistle and Russian knapweed and their control. Colorado Agricultural Experiment Station Bulletin 434. 44 p.
Sather N. 1987. Cirsium arvense. Element Stewardship Abstract prepared for the Midwest Regional Office, The Nature Conservancy, Minneapolis, MN.
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Smith KA. 1985. Canada thistle response to prescribed burning. Restoration and Management Notes. 3: Note 94.
Travnicek AJ, RG Lym, C Prosser. 2005. Fall-Prescribed Burn and Spring-Applied Herbicide Effects on Canada Thistle Control and Soil Seedbank in a Northern Mixed-Grass Prairie. Rangeland Ecology & Management 58:413-422.
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University of Idaho Rangeland Ecology and Management – Targeted Grazing Canada Thistle. Accessed Feb. 2016. Online http://www.webpages.uidaho.edu/rx-grazing/Forbs/Canada_Thistle.htm
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Zwölfer H. 1965. Preliminary list of phytophagous insects attacking wild Cynareae (Compositae) in Europe. Commonwealth Institute for Biological Control Technical Bulletin No. 6. pp. 81-154.