Cultivated origins and introduction history of invasive pampas grass (Cortaderia selloana) in California

International trade in horticultural plants is a major pathway of introduction of invasive species. Pampas grass (Cortaderia selloana) is an invasive species of horticultural origin that is native to South America but cultivated as an ornamental in regions with Mediterranean and temperate climates worldwide. To gain insight into the cultivated sources and demographic history of invasive populations in California, we analyzed microsatellite marker variation in cultivated and invasive pampas grass. We sampled 275 cultivated plants from diverse sources, including cultivars, selections, botanical gardens/arboreta, retail nurseries, and landscape plantings. In addition, 33 invasive populations were sampled from four geographical regions in California. A model-based Bayesian clustering analysis identified seven distinct gene pools in cultivated pampas grass. Two of the seven gene pools were sources of 78% of the genomes of the 698 invasive plants sampled. The two gene pools were well represented in landscape plantings in California but not among cultivars and selections in general, indicating unique introductions of these gene pools into cultivation in California. Extensive admixture in invasive individuals, despite strong genetic differentiation among invasive populations, points to landscape plantings as sources of admixed genotypes rather than gene flow between invasive populations. Analysis of population structure revealed a pattern of genetic variation expected from geographical spread of invasive populations driven by multiple introductions from multiple genetically distinct sources. The results indicate dispersal from landscape plantings as the process leading to invasive spread of pampas grass in California.