Determining non-invasiveness in ornamental plants to build green lists

1. Ornamental horticulture is the most important pathway for plant invasions world-wide. Large numbers of non-native plants are used as ornamentals, and current policies fail to effectively address the continued escape of species from cultivation. Legislative measures are often limited to the listing of few high-risk species banned from sale or planting. This approach, however, may give the impression that non-listed species are not considered to be a risk and are therefore safe to use. The continued widespread use of species not yet identified as having the risk to become invasive may create a problem in the future. 2. This article argues that knowledge gained from studies designed to identify invasive traits of species and factors contributing to their invasion success can also be used to identify species with a low invasion risk that can be recommended to the horticultural trade, landscaping sector and gardeners for safer use. 3. The criteria identified for non-invading species to be listed in a green list include sufficiently long residence time, high propagule pressure, no records of invasive occurrences elsewhere and an indication of some robustness to climate change. An example is provided for a random sample of ornamental species used in Britain. 4. Synthesis and application. There is a pressing need to prevent further plant invasions resulting from ornamental horticulture. A green list of non-native ornamental species that have been assessed as having a low risk of escaping cultivation will contribute to the prevention of plant invasions. Species not yet assessed which may have the potential to become invasive in the future can then be more easily avoided. This list would be particularly useful in large-scale plantings and landscaping projects. Currently, policies often focus on (i) species that are already introduced and recognized as invasive and (ii) the prevention of potential further invasions from new introductions. This study suggests that attention to non-invasive species already present in the country and widely used in horticulture can help to advance policies for dealing with invasive ornamental species.