Predicting how herbivory and neighbor plant interactions combine to affect host plants is critical to explaining variation in herbivores' impact on plant population dynamics. In a field experiment, we asked whether the combined effects of neighbor plants and folivores upon performance of tall thistle (Cirsium altissimum), a monocarpic perennial, can be predicted as the product of their individual effects (i.e. effects of neighbor plants and folivores act independently in suppressing tall thistle performance). Alternately, the combined effects of neighbor plants and folivores might be greater, indicating a synergistic interaction, or less, indicating an antagonistic interaction, than the product of their individual effects. Our experiment involved a neighbor plant clipping treatment and a folivory reduction treatment in a factorial design with manipulations applied to naturally-occurring tall thistle rosettes in restored tallgrass prairie.