A research paper in Frontiers in Ecology and the Environment demonstrates how new sources of plants and demand for new plants as climate changes may lead to new invasive plant introductions. Emerging sources of nursery imports include parts of South America, Africa, and northern Europe. Many new introductions are drought-tolerant plants as water restrictions and xeriscaping have increased demand.
The article encourages the use of weed risk assessments, but points out that with new suppliers, a history of invasion by a particular plant might be hard to come by and invasion history is an important factor in determining risk. On a positive note, the authors point out that one nursery promoting drought tolerant plants has mostly increased the number of drought tolerant native plants it offers.
Bradley, B. A. et al. 2012. Global change, global trade, and the next wave of plant invasions. Frontiers in Ecology and the Environment 10(1):20-28.