Plants may trade off between growth and defense, especially when influenced by factors such as plant-plant interactions. We tested how thistle prickle count is affected by intraspecific competition (same species) and interspecific competition (different species). We manipulated growing conditions and counted prickles on three leaves per plant. Prickle count was significantly lower on leaves grown with interspecific and intraspecific competition than on controls. Our results suggest competition may have an effect on structural defense.