More than half of the world's most invasive plant species spread into new habitats from botanic gardens, an analysis of historic "alien" escapes has concluded.

Although most cases analysed happened between the 1800s and the mid-1900s, there are reports of more recent releases which merit a tightening up of biosecurity, researchers warn.
Plant species can escape from specialist gardens through waterways, wind dispersal and animal transportation. Once in the wild they can invade and take over native habitats.
Most accounts of escapes have been anecdotal, so to assemble a broader picture, Philip Hulme at Lincoln University in Christchurch, New Zealand started with 34 plants that had made it on to a list of the world's 100 worst invasive species, collated by the International Union for Conservation of Nature. (...)

The article:
http://www.newscientist.com/article/mg20928043.700-botanic-gardens-blamed-for-spreading-plant-invaders.html