Some plant species tend to spread easily across continents and to islands, either because people bring them deliberately or because the plants’ seeds hitchhike attached to human travelers and their stuff. We wanted to find out why some plant species spread more easily than others. Once a plant is introduced (on purpose or accidentally) to a new area, what allows it to establish and spread? To answer these questions, we studied the spread of legumes, a family of plants that includes peas and beans, as well as the mutualistic bacteria that live amongst some of their roots. Although these bacteria help some legumes grow in their native ranges, we found that the species of legumes that form mutualistic relationships with soil bacteria are less likely to colonize new areas than species of legumes without mutualistic bacteria.