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CIRSIUM arvense

Image of Cirsium arvense

Michael Charters, www.calflora.net

Family

Asteraceae

Botanical Name

CIRSIUM arvense

Plant Common Name

Canada Thistle

General Description

Canada thistle is a fierce and aggressive weed foe. Impossibly extensive, deep root systems make this one of the toughest weeds to eradicate from garden spaces. It's small, purplish, brush-like flower heads produce loads of puffy airborne seeds that float away to colonize new space, and its prickly foliage is painful to the touch. Despite the common name, Canada thistle, this species was introduced to North America from Eurasia. Its remarkable tolerance to many growing conditions means it can be found in landscapes from Greenland and the far northern reaches of Canada down to the South Central United States.

The densely prickly foliage of Canada thistle is medium green and may be slightly hairy or smooth on the surface. Newly colonized plants form neat rosettes of foliage. Extensive slender rhizomes (spreading underground stems) emanate many feet away from a given plant meaning one plant can result in a large, impossible-to-kill colony. The roots can also delve very deep into the ground, and any root remnant left in the soil will send up shoots, meaning whole plants need to be effectively smothered and killed to be eradicated.

In late spring to summer, plants produce tall, branched floral stems that produce loads of purplish, brush-like flower heads. Bees and many other insect pollinators visit the blooms and loads of puffy, white seedheads follow, which float away in the wind. (So it’s key to keep plants from flowering and setting seed.)

This is a weed that tends to grow in open fields, roadsides or disturbed areas where soil has average to good drainage, but ultimately it’s tough and not picky and can be found almost anywhere aside from truly wet soils. Glyphosate herbicides will only peripherally kill back top growth of Canada thistle so don’t waste your time with them. First and foremost, keep an eye out for seedlings and dig and remove them on sight. Deep digging can take care of relatively new plants, but established ones require more aggressive measures. Smothering is a good technique; cover the area with dark, light impermeable mulch cloth or plastic and cook the infested area for a season.

Characteristics

  • AHS Heat Zone

    9 - 1

  • USDA Hardiness Zone

    3 - 9

  • Sunset Zone

    A1, A2, A3, 1a, 1b, 2a, 2b, 3a, 3b, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24

  • Plant Type

    Perennial

  • Sun Exposure

    Full Sun, Partial Sun, Partial Shade

  • Height

    2'-3' / 0.6m - 0.9m

  • Width

    4'-8' / 1.2m - 2.4m

  • Bloom Time

    Late Spring, Early Summer, Summer

  • Native To

    Europe, Asia

Growing Conditions

  • Soil pH

    Acidic, Neutral, Alkaline

  • Soil Drainage

    Average

  • Soil type

    Clay, Loam, Sand

  • Tolerances

    Pollution, Drought, Black Walnut Toxicity, Salt, Soil Compaction

  • Growth Rate

    Very Fast

  • Water Requirements

    Drought Tolerant, Average Water

  • Habit

    Thicket/Colonizing

Ornamental Features

  • Flower Interest

    Insignificant

  • Flower Color

    Purple, Lavender

  • Fruit Color

    Tan, Ivory

  • Foliage Color (Spring)

    Green

  • Foliage Color (Summer)

    Green

  • Foliage Color (Fall)

    Green

  • Fragrant Flowers

    No

  • Fragrant Fruit

    No

  • Fragrant Foliage

    No

  • Bark or Stem Fragrant

    No

  • Flower Petal Number

    Single

  • Repeat Bloomer

    No

  • Showy Fruit

    No

  • Edible Fruit

    No

  • Showy Foliage

    No

  • Foliage Texture

    Medium

  • Evergreen

    No

  • Showy Bark

    No

Special Characteristics

  • Sharp or Has Thorns

    Yes

  • Invasive

    Yes

  • Attracts

    Butterflies

  • Self-Sowing

    Yes