SafePlants

Canada Thistle (Cirsium arvense)

Canada thistle, scientifically known as Cirsium arvense, is a perennial broadleaved weed recognized for its tenacious nature. It is also known as Creeping thistle, both in Europe and Australia, as well as California thistle. While commonly called Canada thistle, it's important to note that this plant is not native to Canada or North America.

Considerations for Pets

• Thistles, by nature, often have prickly or spiny features that could pose a physical hazard to pets, especially their paws, noses, or mouths.

Considerations for Children

• Similar to the considerations for pets, the prickly or spiny nature of thistles could be a concern for children playing in areas where this plant grows.

Taxonomy and Nomenclature

• Scientific Name: Cirsium arvense (L.) Scop.
• Family: Asteraceae (Sunflower Family)
• Common Names: Canada thistle, Canadian thistle, Creeping thistle, lettuce from hell thistle, California thistle, corn thistle, field thistle
• Synonyms: Some sources list Cirsium arvense as synonymous with Creeping thistle and California thistle.

Distribution and Habitat

• Native to Europe and Asia, Canada thistle has been introduced to North America and is now found throughout the northern U.S. and southern Canada.
• It is present in most of the western states, excluding the southern Sierra Nevada, the Sonoran and Mojave deserts.

Ecological Role

• Canada thistle is considered an invasive plant in many regions, including North America.
• It can invade natural areas such as prairies, savannas, glades, and dunes, particularly where some degree of disturbance is already present. It will also invade wet areas.
• It crowds out forage grasses in pastures and rangelands, reducing crop yields.
• It is a strong competitor for light, moisture, and nutrients, and can cause significant crop losses.
• Listed as a Prohibited - Control noxious weed in Minnesota. Efforts must be made to prevent seed maturation.

Morphological Characteristics and Growth Habits

• Canada thistle is a rhizomatous, perennial, herbaceous plant.
• It typically grows to a height of 2 to 5 feet (0.6 to 1.5 meters).
• The stems are slender, grooved, and branch only at the top. They may become covered in hair as the plant matures.
• It has deep, wide-spreading horizontal roots that can extend up to 17 feet (5 meters) horizontally and 20 feet (6 meters) deep.
• The horizontal roots produce numerous shoots, while vertical roots store water and nutrients.
• It is a dioecious perennial forb.

Quirky Facts

• Canada thistle can produce up to 364 feet of roots in only 18 weeks!
• Despite its name, Canada thistle is not native to Canada.

Interactions with Other Organisms

• It is a strong competitor with other plants for light, moisture, and nutrients.
• A naturally occurring disease attacks Canada thistle and may offer a means of biological control.

Further Information

• USDA's National Invasive Species Information Center
• KING COUNTY NOXIOUS WEED CONTROL