Plant communities can involve native species only, exotic species only or both. Making this decision is most straightforward in rural situations where native species predominate and are the overwhelming determinant of landscape character. Conservation of native biodiversity is a cornerstone in the management of these landscapes, and outside of gardens it is generally less appropriate to use exotic species. In such situations there is increasing pressure to use local genotypes rather than just native species per se, although the scientific and philosophical justification for this view are sometimes debatable (Wilkinson 2001; Sackville-Hamilton 2001).
The native-only presumption becomes weaker as sites become increasingly urban, although where sites border onto areas of high conservation significance, native plant material may continue to be most appropriate. In urban situations where exotic species are often widely cultivated in gardens and public landscapes, and context is far more eclectic, it becomes very much a question of free choice, shaped more by the aspiration of local people and designers. Exotic species that spread aggressively by seed or vegetative means should, however, be avoided, especially in situations adjacent to seminatural corridors, such as waterways, woodland, etc.
The fear of exotic species escaping and impacting negatively on native species has become much stronger as the biodiversity movement has developed. Whilst this is a rational concern, with naturalised species posing a significant local threat in some regions, it is simply scientifically incorrect to stigmatise all exotic species as invasive and all natives as non-invasive. It is understandable that some conservation biologists should support such dogma, but this view is untenable for designers and managers working in the cultural landscape of urban places. An excellent review of the native-exotic plant debate can be found in Kendle and Rose (2000). The idea of producing mixed communities of native and exotic species in urban landscapes is anathema to many conservation perspectives because it is seen as devaluing the spirit of nativeness. This is clearly no more than a philosophical-political notion rooted in romanticism and needs to be seen as such.