Data sources
Emerging and established sources of nursery plant imports to the US were identified using the US Department of Agriculture�s (USDA�s) global agricultural trade system online database. Nursery product categories assessed included bulbs and roots, trees and shrubs, herbaceous perennials, unrooted vines, and mosses and lichens, but excluded any plant identified by name (eg azaleas). Import dollar value were compiled by country from 1989 (the first year of record) through 2010 (all dollar amounts hereafter are in US$). �Emerging� trade partners were identified as countries with an increasing trend during the period of record, but had an average import value of less than $100 000 per year from 2000�2010. �Established� trade partners had an average import value of greater than $100 000 per year from 2000�2010. This threshold divided the pool of nursery plant trade partners roughly in half. Invasive plants were then determined to have arrived from predominantly emerging or established trade partners by assessing the countries comprising the native ranges for 2608 invasive plants in the US as documented by the USDA.
