Garlic mustard, the invasive plant of most immediate concern at Effigy Mounds National Monument occupied approximately half of the search units within the park. In 2006, we estimated that the plant occupied somewhere between 36 and 70 acres on the park and between 9 and 17 acres in 2011. Control efforts in 2009, however, could not be conclusively linked with this decrease. We also documented four additional invasive plant species in Effigy Mounds National Monument: common buckthorn, Japanese barberry, multiflora rose, and shrub honeysuckle. The relatively low abundance and high management feasibility of these species suggested that control is possible. The timing of the survey to maximize garlic mustard detection, however, likely overlooked a sizeable portion of invasive plant species that can best be detected during the growing season.