Population genetic and ecological data may help to control invasive plants, which are considered a major threat to natural habitats. In contrast to expected bottleneck events, genetic diversity of such invasive populations may be high due to extensive propagule pressure or admixture. The ecological impact of invasive species has been broadly evaluated in the field; however, long-term studies on the fate of invasive plants are scarce. We analysed genetic diversity and structure in invasive Spiraea tomentosa populations in eastern Germany and western Poland using Amplified Fragment Length Polymorphism. Potential hybridization between co-occurring diploid Sp. tomentosa and tetraploid Sp. douglasii was investigated using Flow Cytometry. The genetic analyses were complemented by data from a 13-year vegetation study in an area invaded by these Spiraea species. We found no evidence for hybridization between Spiraea species. In populations of Sp. tomentosa both genetic diversity (He = 0.26) and genetic structure (ΦPT = 0.27) were high and comparable to other outcrossing woody plants. Low levels of clonality, presence of seedlings and new patches in sites that had been colonized over the last 13 years imply that populations spread via sexual reproduction. In all habitat types, native species diversity declined following Sp. tomentosa invasion. However, detailed aerial mapping of a forest reserve with ongoing succession revealed that Spiraea spp. populations have declined over a 10-year period. Despite its potential for dispersal and negative effects on native plant communities, invasive Spiraea populations may be controlled by increasing canopy cover in forest habitats.