Invasive species can have detrimental effects on the health of local ecosystems. Lupinus polyphyllus is an herb native to western and northeast North America, but the species has become invasive worldwide. In northeastern Ohio, United States, the species is nonnative, but not invasive and not spreading rapidly. Because physical distance is not a barrier, there are other reasons behind the inability of the species to become locally invasive. Here, the net effect of the local soil biota on the range expansion of the species was tested to explore 2 alternative, non-mutually exclusive, hypotheses. First, if belowground enemies limit this species range, the net effect of the soil biota would be negative. Alternatively, soil mutualists might have a relatively greater net effect on plant fitness. A greenhouse experiment was conducted with 3 populations of seeds from the invasive range across 2 experimental treatments: a general fungicide (ZeroTol®) treatment and a water-control treatment. Fungicide treatment reduced total biomass in 2 out of 3 populations, consistent with limitations by the abundance of belowground mutualists. Fungicide treatment also changed root structure by reducing the number of nodules, root length, and diameter, while increasing root tissue density. Although the fungicide treatment hindered the growth of lupines overall, the changes in root structure indicate that the treated individuals were able to partially compensate by shifting to more of an outsourcing method of resource acquisition. The results suggest that in addition to belowground mutualistic interactions, phenotypic plasticity and intraspecific genetic variation may also contribute to the success of L. polyphyllus in its nonnative ranges.