Invasive plant species are one of the leading threats to the structure and function of native ecosystems. Increased attention has been given to the mechanisms that drive successful invasions, including those influencing belowground interactions between invasive and native plant species. To determine how the prominent invasive shrub Lonicera maackii (Rupr.) Herder affects a co-occurring native plant, we examined how a root/rhizosphere extract from L. maackii affected growth and mycorrhizal infection of Impatiens capensis Meerb. using epifluorescence microscopy of live roots. We also examined how the presence of a live soil microbial community influenced growth and mycorrhizal infection of I. capensis and how it the altered the effect of L. maackii extracts on these variables. In live soils, exposure to L. maackii extracts reduced mycorrhizal infection, reduced height, and slightly reduced shoot mass. In sterilized soils, exposure to L. maackii extracts slightly increased mycorrhizal infection, reduced height, and generally increased shoot, root, and total biomass. Soil sterilization substantially reduced mycorrhizal infection but increased growth of plants by 1.5–2 times over plants in corresponding treatments in live soils. The overall effect of the root/rhizosphere extract was dependent on the presence of a live microbial community in the soil, having generally positive effects on biomass in sterilized soils but negative effects in live soils. Despite strongly reducing mycorrhizal infection, removal of soil microbes by sterilization benefitted plant growth perhaps by removal of pathogenic microbes and/or microbes capable of metabolizing allelochemicals of L. maackii into more toxic forms.