Invasive species such as Canada thistle pose a significant threat to ecosystems. The risk of introducing invasive species has increased with human activities, and the effects of such events have economic and aesthetic impacts. Native to Europe, Canada thistle is now established throughout temperate North America. Although there is documentation of early occurrences to North America, little is known on how it has become established in diverse habitats or how it continues to spread. We examined genetic diversity within and among nearly 1,700 Canada thistle individuals from 85 North American locations with the use of seven microsatellite markers in order to address these questions. PAUP and STRUCTURE programs were used to assess genetic diversity and relationships within and between populations. Populations exhibited greater within-population diversity (> 60%) than expected for a reported clonally reproducing species. Total diversity of sampled locations in North America (0.183) was less than previously reported for European locations (0.715), but the greater mean difference between North American populations (0.264 relative to 0.246 from England) suggests strong founder effects or restriction of gene flow influencing individual populations. Furthermore, analyses identified numerous instances where individuals from geographically distant regions clustered together, indicating long-distance translocation of propogules. However, isolation by distance analysis showed significant correlation between location and population genetic distances (r  =  0.1917, P  =  0.006). Within populations, nearly 92% of individuals sampled harbored unique multilocus genotypes, strongly suggesting that sexual reproduction is common. Within populations, analysis of genetic structure indicated significant admixture of genotypes throughout the invasive range in North America. The recurrent distribution of seed throughout North America has led to a highly diverse gene pool and increased the adaptive success Canada thistle to a wide variety of habitats. Future technologies developed for control of Canada thistle should consider this diversity.

Nomenclature: Canada thistle, Cirsium arvense (L.) Scop. CIRAR.