Waterborne spread of introduced plants from a large ornamental garden in northern England, UK

The spread of non-native plants from ornamental gardens is an introduction pathway that has rarely been studied in detail. Here we document the escape of garden plants along a riparian corridor below a large ornamental garden in northern England. Of the 42 naturalised garden escapes found, 14 had originated from the garden itself including two species that were locally invasive (Cardamine bulbifera, Lysichiton americanus). Along with the ubiquitous Impatiens glandulifera, C. bulbifera was the most abundant non-native and had spread by bulbils to form dense stands on sandy riverbanks disturbed by winter floods whereas L. americanus had spread by seed to form dense stands in swampy woodland. All other non-natives were locally distributed. These findings highlight the important role that ornamental gardens play as ‘entry points’ for non-native plants and emphasises the need for gardens to identify, manage and in some cases eradicate potentially invasive species, especially where they have the potential to escape and spread along watercourses.