Negative plant-soil feedbacks can be viewed as Janzen-Connell effects and influence plant population dynamics in grasslands. However, even though plant-soil feedbacks are often referred to as a mechanism for Janzen-Connell effects, for grassland species this is based on pot experiments and these effects have rarely been examined in the field. We examined the spatial distribution of a monocarpic perennial Jacobaea vulgaris to infer whether there is a distance- and/or density dependent effect and if the pattern is soil-mediated. Replicated plots were constructed to investigate J. vulgaris populations at two sites. Rosettes and flowering plants were marked, and their coordinates were recorded within each plot. For three plots, plants were tracked repeatedly during a single season to examine temporal distribution patterns. We then examined distance- and density dependent effects with spatial point pattern analysis. We also collected soil underneath flowering plants and 0.5-meter away from each plant. Seed germination, survival and growth of seedlings were traced in these soils with bioassays. Further, we measured biomass of J. vulgaris grown in soil from patches with high densities of J. vulgaris and in soil from outside of these patches. The density of rosettes was generally lower than expected from null models at close distances from flowering plants. The degree of clustering decreased from rosettes to flowering plants indicating density dependent self-thinning. Both the distance-based decay in rosette density and life-stage-dependent spacing became stronger over time in the plots where repeated measures were taken. Seed germination was higher in soil further away than in soil underneath flowering plants. However, seedling mortality and biomass did not differ in soils from different distances, and plants produced similar biomass in soil from pairwise patches. Our study provides spatial-based evidence for Janzen-Connell effects of J. vulgaris, and suggests plant-soil feedbacks play a minor role in mediating Janzen-Connell effects.