Classification of alien plant species can be found in Petr Pysek, David M. Richardson, Marcel Rejmanek, Grady L. Webster, Mark Williamson & Jan Kirschner. 2004. Alien plants in checklists and floras: towards better communication between taxonomists and ecologists. Taxon 53: 131-143. [Corresponding author: Petr Pysek - [EMAIL PROTECTED]
Short summary of this classification was posted in BEN (Botanical Electronic News # 324): http://www.ou.edu/cas/botany-micro/ben/ben324.html Adolf Ceska, Victoria, British Columbia, Canada
