Canada thistle

Images above: Upper left: Canada thistle flower (Scott Morris, Cornell University). Upper right: Canada thistle mature seedheads (Antonio DiTommaso, Cornell University). Bottom: Canada thistle vegetative tillers (Antonio DiTommaso, Cornell University).

Identification

Other common names:  creeping thistle, small-flowered thistle, perennial thistle, green thistle, field thistle, cursed thistle, corn thistle

Family:  aster family, Asteraceae

Habit:  Prickly perennial herb spreading by deep thickened storage roots.

Description:  Cotyledons of the seedling are stalkless, fleshy, hairless, oval to rounded and 0.2-0.6” (0.5-1.5 cm) long by 0.1-0.2” (0.25-0.5 cm) wide.  The first true leaves of the seedling are egg to lance-shaped, hairy, and have irregularly lobed, spiny edges.  The early leaves form a rosette before bolting.  Seedlings begin to develop horizontal storage roots early, and a plant with only two true leaves may have a perennating storage root up to 6” (15 cm) long.  Vegetative buds develop on the storage root by the third true leaf stage.  Mature plants are 1-5 ft (0.3-1.5 m) tall and branched towards the top.  Stems are hollow, grooved, and sparsely hairy to smooth.  Leaves are 2-6” long, alternate, oblong, and irregularly lobed with sharp spines along the edges.  Upper surfaces of the leaves are hairless, dark green, and waxy, while lower surfaces are pale green and smooth to hairy.  The leaves are stalkless, and clasp the stem at the base. The roots are an extensive system of white, fleshy storage roots that can extend to over 15 ft (4.6 m) horizontally and vertically, and can produce over 200 new buds annually.  Vegetative shoots resemble seedlings, but are larger and lack cotyledons.  Male and female flowers are produced on separate plants in leaf axils and at the end of branches.  Male flowerheads are round and 0.5” (1.3 cm) wide, while female flowerheads are 1” (2.5 cm) wide and flask-shaped.  The flowers of both sexes are purple-pink, with bases enclosed in overlapping triangular or diamond shaped scale-like, spineless green bracts.  A small leaf is present at the base of each flower stalk.  Each female flower head may produce as many as 45 seeds.  The apparent seeds have a thin dry outer coating of fruit tissue.  Seeds are brown, flattened, oblong, and 0.1-0.25” (0.25-0.6 cm) long.  Each seed is weakly attached to a feathery, white brown pappus. 

Similar species:  Many thistles and thistle-like plants have spiny leaves and similar flower heads to Canada thistle.  Nearly all are biennial, however, and have taproots rather than the extensive storage root system of Canada thistle.  Bull thistle [Cirsium vulgare (Savi) Ten.] leaves have hairy upper surfaces and spiny, winged stems, while Canada thistle leaves are hairless above and the stems lack wings.  Sowthistle (Sonchus) species may look similar to Canada thistle.  Sowthistles have yellow flower heads and bleed milky sap when damaged, while Canada thistle has purple flower heads and clear sap.

Management

Because Canada thistle has a deep root system, the only approach for controlling this weed is to exhaust the storage roots.  Food reserves in the roots reach a minimum near the onset of hot weather when the shoots are about 1 ft tall (30 cm) and then increase as energy flows from the shoots to the storage roots (Welton et al. 1929).  Consequently, shoots should be removed for the first time by late spring.  Repeated removal of the shoots before they attain several leaves will exhaust the storage roots within two years and eliminate the weed.  Several studies found a 21-day weeding schedule was optimal (Hodgson 1970, Moore 1975).  Since buds on the roots will continue to sprout well into the fall (Andersson et al. 2013, Brandsӕter et al. 2010, Tørresen et al. 2010), persistence is required.  A dense cover crop of sorghum-sudangrass or a mixture of sorghum-sudangrass with compatible species mowed once or twice during the season reduced Canada thistle shoot density and mass to less than 20% of initial values (Bicksler and Masiunas 2009, Wedryk and Cardina 2012).  Competition from pasture species maintained low Canada thistle populations, but populations increased substantially when released from competition (Edwards et al. 2000).   Accordingly, high-intensity, low-frequency grazing over 2 to 3 years provided better control than high-frequency grazing for a short duration (Tiley 2010).

Long fallow periods may not be cost effective unless thistle pressure is severe, but growing crops that allow repeated cultivation close to the row achieves a similar effect for at least part of the season.  Both spring and fall sown grain crops tend to promote Canada thistle due to the long period in which the shoots can grow without disturbance, but winter wheat is more competitive than spring grains (Anderson 2009).  In a Danish study, cultivation of spring barley with shovels plus hand cutting of shoots in the rows greatly decreased Canada thistle (Graglia et al. 2006).  Because alfalfa is mowed several times per year over a period of several years, this crop is very useful for managing Canada thistle (Moore 1975, Tiley 2010).  Hay that is mowed only once per year is less helpful for managing this weed. 

Control of established Canada thistle stands for one year is usually insufficient for long term control.  Generally, because established plants survive many years, multiple control tactics are required for multiple years (Anderson 2009, Davis et al. 2018, Graglia et al. 2006).  An integrated rotation was proposed for central Pennsylvania including three years of alfalfa followed by a three-year sequence of fall brassicas, early spring vegetables, and a summer vegetable (Nordell and Nordell 2009).  On a Maryland organic farm, a program including two years of repeated summer cultivations followed by dense plantings of winter barley for haylage reduced a heavily infested field of Canada thistle by 76%, allowing transition to alfalfa followed by successful establishment of row crops with minimum Canada thistle populations after five years (Teasdale and Maravell unpublished data). 

The root reserves are sufficient to push the shoot through any amount of loose mulch, and Canada thistle growth will benefit from the soil moisture conserved by the mulching materials.  A tough synthetic tarp, however, can prevent shoots from reaching light.  Transfer of energy into the dying shoots reduces vigor of the storage roots.

Preventive measures that reduce seed dispersal are an important part of any control strategy.  Canada thistle growing along field margins, fence rows, or drainage ditches can easily spread into fields unless eliminated.  Because Canada thistle is often a problem in grain crops, thistle stalks, including mature seeds, are often present in baled grain straw.  Similarly, manure that includes grain straw bedding is often contaminated with Canada thistle seeds. 

Ecology

Origin and distribution:  Canada thistle probably originated in southeastern Europe and the eastern Mediterranean.  It has spread throughout Europe, North Africa, through central Asia to Japan, and across the northern U.S.A. and southern Canada.  It is also naturalized in South Africa, New Zealand and southeastern Australia.  (Moore 1975, Tiley 2010)

Seed weight:  0.96 mg (Moore 1975), 1.08 mg (Terpstra 1986), 0.95-1.44 mg (Tiley 2010), 1.30 mg (Gaba et al. 2019), 1.58 mg (Stevens 1932), 1.70 mg (Benvenuti et al. 2001).

Dormancy and germination:  Most seeds are not dormant when shed from the parent plant (Moore 1975), however some investigators report the need for chilling and after-ripening to achieve full germination (Tiley 2010).  Seeds germinate best at warm daytime temperatures (77-86 °F or 25-30 °C) (Moore 1975), but are inhibited from germination by hot (104 °F or 40 °C) temperatures (Tiley 2010).  Light, day/night fluctuation in temperature, and to a lesser extent, nitrate, all increase germination (Bostock 1978, Tiley 2010).

Seed longevity:  A few seeds may persist in the soil for 17 to 21 years (Burnside et al. 1996, Toole and Brown 1946), but most disappear within the first few years (Chepil 1946).  Seeds persist better when buried over 1 ft (30 cm) than when near the soil surface (Tiley 2010, Toole and Brown 1946). 

Season of emergence:  Seedlings emerge primarily in the spring or fall (Chepil 1946).  Shoots from rootstocks begin emerging in mid-spring and emerge continuously until frost (Doll 2002, Moore 1975, Tiley 2010). 

Emergence depth:  Optimum depth for seedling emergence is 0-0.4” (0-1 cm) but seedlings can emerge from as deep as 2.4” (6 cm) (Benvenuti et al. 2001, Moore 1975, Tiley 2010).  Shoots can emerge from roots several feet deep, but the optimum depth for shoot emergence and plant canopy spread is 4” (10 cm), compared to shallower or deeper depths (Sciegienka et al. 2011).

Photosynthetic pathway:  C3 (Tiley 2010)

Sensitivity to frost:  Shoots tolerate light frost (as in spring) but are killed by hard frost (as in fall) (Moore 1975, Tiley 2010).  The root system persists over the winter (Moore 1975).  Seasonally higher concentrations of sucrose and fructans in roots during fall and winter months allow the roots to adapt to cold temperatures (Wilson et al. 2006).  However, root fragments at the soil surface are killed by 14 to 21 °F (-10 to -6 °C) temperatures (Tiley 2010).

Drought tolerance:  Well developed plants are drought tolerant due to their deep root systems.  Shoot establishment and growth from root fragments are reduced by low soil moisture suggesting that dry conditions can enhance control with tillage practices, and that areas with optimum soil moisture are more prone to proliferation of this weed (Sciegienka et al. 2011, Tiley 2010).  Competition for water between shoots and root buds is a primary cause of bud inhibition under dry conditions (Tiley 2010).

Mycorrhiza:  Canada thistle is mycorrhizal (Harley and Harley 1987) and demonstrates positive growth in the presence of mycorrhiza (Vatovec et al. 2005).

Response to fertility:  Nitrogen increased shoot growth of Canada thistle seedlings, but decreased number of root buds initiated (McIntyre and Hunter 1975).  In another pot experiment, nitrogen increased growth of plants derived from root fragments more than that of seedlings (Hamdoun 1970).  In an Alberta field experiment, nitrogen increased root growth, but not shoot growth (Nadeau and Vanden Born 1990).  In Sweden, soil nitrogen content had little influence on growth or regenerative capacity of Canada thistle roots or shoots (Dock Gustavsson 1997).  In England, with reduced competition, nitrogen and phosphorus increased Canada thistle shoot stands, while potassium and magnesium had no effect (Edwards et al. 2000).  In the presence of pasture species, nitrogen favored the competing pasture grasses and resulted in suppression of Canada thistle (Edwards et al. 2000).  Phosphorus, in contrast, favored Canada thistle both with and without competition.

Soil physical requirements:  Canada thistle tolerates a wide range of soil types.  It does poorly, however, on wet soils, and is particularly vigorous on well drained, fine textured (clay to silt loam) soils suitable for agricultural production.  (Moore 1975, Tiley 2010)

Response to shade:  This species is shade intolerant, but due to its extensive root reserves, shoots can often grow through competing vegetation.  Seedlings will die when light falls below 20% of sunlight (Moore 1975).  Shoots of established plants suffer when shaded by competing taller species (Tiley 2010).

Sensitivity to disturbance:  Canada thistle is highly resistant to cutting, hoeing and even to deep tillage (Tiley 2010).  The root system commonly extends to depths of 6.6 ft (2 m) or more (Moore 1975, Tiley 2010) and new shoots quickly reappear after the previous shoots have been removed.  Carbohydrate reserves in the root system reach a minimum at about the time of flower bud formation, and the plant is most sensitive to removal of the shoots at that time (McAllister and Hadarlie 1985, Moore 1975).  Plants arising from root fragments were most sensitive to disturbance when about 8 leaves have formed (Dock Gustavsson 1997, Nkurunziza and Streibig 2011), although recent research has found that minimum underground biomass is reached as early as 1-2 leaves with the suggestion that early disturbance may be preferable for optimum control (Verwijst et al. 2018).  Plants arising from deeply buried root fragments (8” or 20 cm deep) have weaker regrowth potential than plants derived from shallow root fragments (2” or 5 cm deep) (Dock Gustavsson 1997).  Root fragments larger than 0.1” (3 mm) or 0.05 oz (1.4 g) fresh weight will usually produce new plants, while smaller fragments may if they are near the soil surface (Bostock and Benton 1983, Moore 1975).  A single cultivation event increases and disperses the population of propagating rootstock, thereby increasing the abundance of this species (Edwards et al. 2000), so multiple cultivations (Hodgson 1970) or integration with additional suppression tactics (Edwards et al. 2000) are required for control (Anderson 2009).  Shoot emergence from root fragments decreases in early fall, but production of underground shoots continues suggesting that fall tillage could still be effective for reducing reserves (Andersson et al. 2013, Liew et al. 2012).  

Repeated clipping at soil level reduced shoot biomass by 18%, but root biomass by 72%, suggesting that plants compensated for loss of shoot growth at the expense of root development (Cripps et al. 2020).  Genetic variation in tolerance responses to clipping by 36 genotypes of Canada thistle from New Zealand indicate potential for selection of populations tolerant to this control practice (Cripps et al. 2020).  Seeds are unable to mature and become viable when stems are cut in full bloom (Gill 1938, Tiley 2010).

Time from emergence to reproduction:  Plants flower in response to long daylengths of 14 to 16 hours, generally between late June and August.  Thus, plants emerging from rootstocks in spring take about 8-10 weeks to flower.  (Doll 2002, Moore 1975, Tiley 2010)

Pollination:  Canada thistle is pollinated by insects, especially honey bees.  Since male and female flowers occur on separate individuals, the species typically cannot self-pollinate.  Consequently, many infestations are single clones that cannot produce seeds.  (Moore 1975, Tiley 2010)

Reproduction:  Canada thistle reproduces both vegetatively and by seeds (Moore 1975, Tiley 2010).  Seed production varies from nothing to 5,000 seeds/shoot depending largely on the thoroughness of pollination.  This in turn depends on the proximity of male and female plants (Moore 1975, Tiley 2010).  Seeds mainly allow the species to disperse between locations and does little to maintain local populations (Tiley 2010).  Seedlings become perennial from 6 to 10 weeks after emergence, and can thereafter reproduce from sprouts of deep and rapidly spreading roots (Moore 1975, Tiley 2010).  By the end of summer, a single plant produced 26 aboveground shoots, 154 underground shoots, and 364 ft (111 m) of roots in Alberta (Nadeau and Vanden Born 1989).  Plots initiated with individual root fragments expanded to cover a 161 ft2 (15 m2) area and roots penetrated to 4.6 ft (1.4 m) depth in one year, and covered 538 ft2 (50 m2) to a depth of 7.2 ft (2.2 m) in two years (Nadeau and Vanden Born 1990).  Roots have approximately 1-2 root buds per 4” (10 cm) of root length throughout the year.  Root fragments from a 10-year old stand of Canada thistle had the capacity to produce a similar number of shoots per unit of root, regardless of their location along the 6 ft (1.8 m) rooting depth of the stand (Nadeau and Vanden Born 1989).  Development of underground shoot length is greatest in late fall and winter following death of aerial shoots as the plant becomes primed for shoot emergence when soil warms in spring (McAllister and Hadarlie 1985). 

Dispersal:  The seeds have a special feathery structure (the pappus) that aids in wind dispersal (Moore 1975).  The efficiency of wind dispersal is low but it ensures introduction to new sites and maintains genetic diversity (Tiley 2010).  Canada thistle is common in field borders (Sosnoskie et al. 2007) which undoubtedly serves as a source of field contamination both by seeds and spreading roots.  The seeds also commonly disperse in contaminated crop seed, hay and straw used for livestock or for mulch and compost in gardens (Tiley 2010).  Seeds can blow into irrigation or drainage waters and then be transported long distances while maintaining 52-65% viability (Kelley and Bruns 1975, Wilson 1980).

Common natural enemies:  Goldfinches and other birds eat the seeds.  Numerous nematodes, insects, and fungi have been identified on Canada thistle (Tiley 2010); a selection of these organisms are discussed below.  Four species have been successfully established as biocontrol agents on this weed, including Hadroplontus litura, Urophora cardui, Larinus carlinae, and Rhinocyllus conicus, but their impact is typically limited overall (Winston et al. 2017).  Larvae of painted lady butterfly (Cynthia cardui) can defoliate the plant.  Larvae of Canada thistle midge (Dasypeura gibsoi) eat the seeds and larvae of Orellia rficauda attack the flowering heads.  None of these organisms is sufficient to control Canada thistle alone (Moore 1975).  The weevil Ceutorhynchus litura has been released in Canada as a biocontrol agent.  Larvae mine the leaf veins, stems and root collar and can greatly reduce a population, but the insect is difficult to get established (Moore 1975).  The gall fly Urophora cardui deposits eggs and produces galls in terminal shoots of Canada thistle, but has had little impact on thistle populations despite successful colonization after release in eastern Canada (Peschken et al. 1982).  The shoot-boring weevil, Apion onopordi, infested and reproduced in Canada thistle shoots, but had little effect on thistle performance (Friedli and Bacher 2001).  Inoculation with the rust pathogen, Puccinia puntiformis, had no effect on growth of Canada thistle, but in combination with cutting, reduced fertile flower production (Kluth et al. 2003).  The fungus, Phoma macrostoma, is locally systemic in stems, but not roots, of Canada thistle in England and Canada and causes a bleaching disease (Evans et al. 2013).  A particularly active strain of this fungus has been registered as a bioherbicide in Canada.  Pseudomonas syringae pv. tagetis also can cause dramatic bleaching symptoms (Tiley 2010).

Palatability:  Canada thistle is unpalatable to humans.  Nutritional content of Canada thistle is relatively high, but it has a low palatability for livestock (Marten et al. 1987, Tiley 2010).

References:

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