What's that PNW Weed: Invasive Canada Thistle
- Katie Burnham
- Sep 12, 2024
- 3 min read
General Description of Canada Thistle
When I was a little kid, one of my very favorite books was Winnie the Pooh. I loved the imaginative storytelling of A.A. Milne and how Pooh's friends in the Hundred Acre Woods came to life with such vivid pictures in my mind as I read each page. While everyone remembers that Pooh had a sweet spot for honey (ok, more like an obsession), does anyone else remember that Eeyore's favorite snack was thistles?
I remember asking my mom what a thistle was and I wasn't able to place it--as thistles weren't common where I grew up (at least I don't remember them being). But now, living in the PNW, I see thistles everywhere and those bright purple flowers always bring back the memory of Eeyore's favorite snack.
Here in Western Washington, Canada thistle is ubiquitous along roadsides and in fields--thorny leaves and fluorescent purple flowers lighting up an otherwise bleak landscape. Unfortunately, like many of the weeds I talk about here, Canada thistle is also incredibly invasive and has been listed on Washington's Noxious Weed list since 1988.
Canada thistle (Cisium arvense) is in the Asteraceae family and originates from Europe and Asia.
Canada thistle is an herbaceous perennial (it dies back to its roots in the winter) that can grow between 2 to 5 feet tall. It has deep green, alternating leaves with spiny prickers at the tips. Stems are also dark green, slender, grooved, and branch at the top forming multiple flower heads.
Flower petals are typically purple, but can range in colors from pink to white and bloom from June-October. Canada thistle flowerheads can be clustered and grow between 1/2-3/4 inch in diameter. Flowerheads typically dry on the stem, becoming brown and slightly shriveled.
On average, one plant can produce between 1,500 and 5,300 seeds.
Why Canada Thistle is an Invasive Weed
Canada thistle originates from Europe and Asia, yet has naturalized in Washington State. It aggressively spreads through rhyzomes with clonal growth in a circular pattern. In just one growing season, a single plant can spread 10-12 feet! This rapid growth out-competes native vegetations and can alter ecosystems.
Additionally, Canada thistle's wind-dispersed seeds can invade far-reaching areas and seeds can remain viable up to 20 years in the soil.
Like other invasive plants, Canada thistle can thrive in just about any environment: pastures, agricultural fields, prairies, dunes, stream edges, meadows, along roadsides, and in garden beds.
How to Manage Canada Thistle
Mechanical Removal: As Canada thistle infestations tend to be enmasse, mechanical removal of individual plants may prove futile. In agricultural settings, frequent tilling (every 7-28 days) for several years has been effective. Alternatively, in a home landscape, frequent mowing will weaken branches and prevent the plant from flowering. Plants may eventually die from constant mowing.
Planting Control: In agricultural settings, Canada thistle has been outcompeted by planting crops such as grasses and alfalfa. In garden settings, planting fast-growing perennials close together will out out-compete Canada thistle, as it cannot tolerate shade.
Recap of Canada Thistle
Canada thistle (Cisium arvense) is a noxious, invasive weed originating from Europe and Asia. It can grow up to 5 feet tall and is best recognized by prickly, green leaves and fluffy purple flowers. Canada thistle spreads through clonal rhizomatic growth and wind-blown seeds and will grow in nearly any sunny environment. The best form of control is tilling, mowing, and preventing the spread of seeds and rhizomes.
If you see Canada thistle growing on your property, do what you can to prevent spreading, remove when feasible, and utilize other (native or ornamental) plants to shade it out.
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