Canada thistle, Cirsium arvense
Also known as creeping thistle.
Weed identification:
- Growth form is an erect perennial herb initially with a tap root producing far-creeping white or yellowish rhizome-like roots. Roots can expand laterally more than 5 m and 3 m in depth.
- Mature plants have erect stems, channeled and rigid, usually between 30-180 cm tall.
- Seedlings show oval and smooth cotyledons. First true-leaves have spiny margins.
- Leaves are 1-20 cm long and 2-5 cm wide. They are alternate, from oblong to lanceolate. The margins can highly vary from entire to pinnatifid, with crinkled edges with 1-10 mm spines. Upper leaves are sessile.
- Flowers are grouped in capitula, forming corymb-like clusters. Flower heads are 1-2.5 cm long, pinkish in color which varies in its intensity.
Canada thistle, view of buds and flower
Image credit: L. L. Berry, Bugwood.org
Mature Canada thistle plant
Image credit: L. L. Berry, Bugwood.org
Biology and life cycle:
Growth habit: Canada thistle is a persistent perennial weed that reproduces both by seeds and vegetative regrowth from root buds. Its emergence occurs in flushes, primarily influenced by soil moisture. The largest flush of root-derived shoots typically happens in the spring, with a secondary flush often occurring in the fall. Remarkably, regrowth can originate from root fragments as short as an inch, which makes tillage a significant challenge for growers, as it can spread these fragments and exacerbate the infestation.
Emergence dynamics: Seedlings that emerge from seeds tend to grow slowly and are particularly vulnerable to competition, especially for light. This sensitivity makes them less successful in well-established or densely vegetated environments.
Seed production: Canada thistle begins flowering in late spring, triggered by day lengths of 14 to 16 hours. As a dioecious species, individual plants are either male or female, requiring insect-mediated cross-pollination. Each plant is capable of producing between 1,000 and 5,000 seeds. These seeds can be carried over long distances by wind, aided by the pappus (the feathery structure typical of the Asteraceae family). However, the pappus often breaks off, causing many seeds to remain near the parent plant unless dispersed by animals or farming equipment.
Seed viability: Seeds of Canada thistle can remain viable in the soil for up to 22 years, with deep burial further extending their longevity, making eradication particularly difficult without sustained management efforts.
Resistance
No herbicide-resistance cases have been reported in the U.S. In Europe, 2 cases of resistance to auxinic herbicides have been reported a long time ago (1980s), one in pastures and another in cropland.
Seedling, or young rosette, of Canada thistle
Image credit: Phil Westra, Colorado State University, Bugwood.org
Canada thistle plants infesting field corn
Image credit: Phil Westra, Colorado State University, Bugwood.org
Integrated weed management strategies based on Colorado cropping systems:
Effective management requires a multi-faceted approach that integrates biological, mechanical, cultural, and chemical methods, tailored to the specific conditions and severity of the infestation.
Mechanical control
Mechanical methods are widely used to manage Canada thistle. Repeated mowing, particularly at critical growth stages, like the pre-bud phase, can help deplete root reserves and prevent seed production. Deep tillage can disrupt root systems but requires careful implementation as it may also stimulate regrowth if not combined with other methods.
Cultural control
Cultural practices play a crucial role in Canada thistle management by creating conditions that suppress its growth. Planting competitive crops, such as alfalfa or wheat, can shade out thistle seedlings and reduce their ability to establish. Maintaining healthy pastures with dense vegetation also minimizes opportunities for thistle establishment by reducing bare soil where seeds can germinate.
Biological control
Biological control methods have not been very succesfull at field scales. Insects such us the stem-mining weevil (Ceutorhynchus litura) and the Canada thistle gall fly (Urophora cardui) have shown potential in reducing plant vigor by targeting stems or forming galls. The rust fungus Puccinia punctiformis can infect thistle populations, weakening their growth and reproduction.
Chemical control
Herbicides are often a cornerstone of Canada thistle management, especially in larger infestations. Herbicides such as clopyralid and aminopyralid are effective and selective options, with residual activity that provides extended control. Non-selective herbicides like glyphosate are useful for spot treatments or fallow treatments, while 2,4-D can be applied in combination with other herbicides to delay resistance development. Timing is critical for chemical applications. Spraying when plants are at the rosette to early bolting stages in the fall are most effective, based on the plant active translocation to the roots at that period.
References
References
References:
Beck, KK.G. (2013). Canada thistle fact sheet No 3.108. Natural Resources Series. Colorado State University Extension. https://extension.colostate.edu/topic-areas/natural-resources/canada-thistle-3-108/
Jacobs, J., Sciegienka, J., Manelled, F. (2006). Ecology and Management of Canada thistle [Cirsium arvense (L.) Scop.] . USDA – Invasive Species Technical Note No. MT-5.
Tiley, G. E. D. (2010). Biological Flora of the British Isles: Cirsium arvense (L.) Scop. Journal of Ecology, 98(4), 938–983. https://doi.org/10.1111/j.1365-2745.2010.01678.x
USDA (2014) Field Guide for Managing Canada Thistle in the Southwest. (www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5410109.pdf)