The Dirt | Weed of the Month October 2026 | Canada thistle (Cirsium arvense)
Our October Weed of the Month is Canada thistle (Cirsium arvense). Canada thistle is a perennial thistle species with deep, creeping roots. It is readily identified by its distinctly crinkly leaves with sharp spines on the margins. The purple/pink plume flowers typical of many thistle species bloom from June-October. This species is present in every county in the state of Oregon and invades a variety of habitats.
Since this species is so widespread, it is extremely difficult to eradicate once established in large populations. This species presents a real threat to agricultural/cropland systems once established. Management strategies for removing Canada thistle include manual removal (hand pulling/digging), mechanical removal (use of tools such as string trimmers to cut stems), and chemical removal (herbicide use). A combination of management strategies might be necessary depending on the size of the infestation. Regardless of management strategy, it is likely to take several growing seasons of repeated treatments to fully eradicate this species.
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