|Exotic plants are alluring sometimes due to their beauty|
Exotic and exotic invasive plant species are problems primarily because the lack of competition, predators, control and the fact they are very persistent in the ecosystem once escaped.
Many exotic invasive species have become introduced into an environment by first being planted in a landscape.
The difference in using an exotic plant species in the ground level landscape and the same exotic plant species on a green roof is exponential in nature.
The area of adjacent influence for the exotic green roof plant is much larger than for the same plant on the ground. Moreover, the warmth of a green roof system may allow for those species subject to cold damage to survive where they would normally die back, further expanding their range.
Additionally, wind influence on exotic species becomes amplified when those species are part of a green roof project. Wind can carry seeds, leaves or vegetative matter capable of rooting and spreading. Blown from a roof, the plant's vegetation can spread across a much broader radius than from the ground level.
Stormwater too becomes a vector for exotic green roof plants, potentially carrying stems or leaves that may re-root downstream vast distances.
There are lots of reasons to use native plant species on a green roof.
As a plant designer I don't always stick with natives, using food, other (hopefully well-behaved plants) landscape plants and flowers in addition to many natives.
Importantly, we must all be cognizant of those plant species we are using. Green roofs have the potential for becoming an important source of exotic plant species if we as an industry are not careful.