Although many prairie grasses and forbs possess a greater tenacity for deep rooting relative to traditional turf, few studies quantify the influence of prairie vegetation on soil structure within urban systems. We measured soil bulk density, field-saturated hydraulic conductivity, cone penetration resistance, and soil organic matter under urban prairie gardens and adjacent turf lawns in residential developments of Madison, Wisconsin. Both site age and the presence of a prairie garden contributed to building soil structure, particularly within 18 cm of the surface. Bulk density at the surface was lower in prairie gardens than lawn, averaged across all sites at 1.24 Mg m^-3 and 1.32 Mg m^-3, respectively (p < 0.10). Older sites (i.e., homes developed prior to the 1970’s) tended to have lower bulk density, greater hydraulic conductivity, and less penetration resistance than newer sites. Prairie gardens exaggerated these changes, as differences in organic matter appear to control these physical changes. For example, regardless of site age or prairie age, prairie gardens had more soil organic matter (4.3%) than lawn (3.8%) and elevated surface hydraulic conductivity (9.7 cm hr^-1) relative to lawn (4.3 cm hr^-1) (p < 0.10). Moreover, the difference in soil organic matter occurred within the macroaggregate size fraction (2000-250 μm), averaging 3.0% and 2.6% beneath prairie gardens and lawn, respectively (p < 0.10). Thus, urban prairie gardens appear to enhance soil structure via accumulation of soil organic matter within the macroaggregate size fraction.