Relatively few of the many plant species introduced by people have become naturalized, and only a small fraction of them have had a major impact on native species and ecosystems. The risks they pose are driven by environmental disturbance, but whether or not introduced plants cause extinction of native species remains uncertain, nor can we state with certainty which will become established in new areas and persist there without direct or indirect human assistance. It may take decades for some introduced plants to pose a threat to native species whereas others could do so in a few years, but several researchers have argued that until recently the time dimension has received comparatively little attention. The literature shows that the conservational and ecological problems caused by introduced plants do not simply stem from competition with native plants for space, light, nutrients and water. Rather, they involve spatial and temporal combinations of genetics and growth behaviour as well as differential responses to ecological opportunities, how newcomers fit into the biological and environmental structures of the host ecosystem, and the involvements of people, plants and animals. This article argues for long-term, integrated programmes of observation and experimentation in the field and laboratory into the risks that introduced plants pose for native species and ecosystems.