Canada thistle (Cirsium arvense)

Also Known As: Creeping thistle, California thistle
Plant Family: Sunflower (Asteraceae)

Identification Guide: *Print version
Phenophase Field Guide: *Print version

2009 Live Map - View

Did you Know? Although a native species in most of the temperate regions in Europe, Canada thistle threatens many natural plant communities in the US. This highly invasive thistle prevents the coexistence of other plant species through shading, competition for soil resources, and possibly through the release of chemical toxins poisonous to other plants. Like all other thistles it is a biennial, so it puts out a basal rosette of leaves the first year, flowers the second year, then dies. So if you can pull the basal rosettes in the first year you can control its spread. Note that there many native thistle species, so make sure you do not accidentally pull native species! Natives generally have a large stem and often have whitish hairs on bottom sides of leaves or on the stem.

Identification Hints: Canada thistle is distinguished from other thistles by its deep running perennial rootstocks, dense clonal growth, more slender stems, spiny lobed leaves, and small compact flower heads. Other weedy thistles have winged stems and large flower clusters.

Phenological Observations of Interest: First Flower, End of Flowering, First Ripe Fruit, All Leaves Withered

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2009 Live Map for Canada thistle

Below you can view the latest 100 observations that have been reported in 2009 for Canada thistle (If the map below is empty then there have not been any 2009 observations reported.)

Use the navigation buttons on the left to zoom in/out and pan around. Click on each place marker to get detail information about that observation.

= First Flower
= End of Flowering
= First Ripe Fruit
= All Leaves Withered

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