In 1927, Werner Heisenberg proposed that there are fundamental limitations to the study of subatomic particles, as the act of measuring them affects their behavior. Here we show that experimenter-induced uncertainty also applies in plant ecology, with potentially dramatic consequences for field biologists. We tested whether the simple act of visiting marked plants once per week for eight weeks influenced the intensity of herbivory experienced by six plant species in an old field community. Half of the plants were touched once per week to simulate taking morphological measures, while the other half were left undisturbed (neither visited nor touched). After eight weeks, visitation resulted in (1) decreased leaf damage by insects on one species, (2) increased leaf damage on a second species, (3) a marginally significant increase in survival for a third species, and (4) no effect on the remaining three species. These results serve as an important reminder that seemingly benign experimental methods may themselves dramatically affect the performance of experimental subjects. Our results raise concern about studies that use repeated visitation of focal plants either to compare rates of herbivory among species or to investigate some factor that can either directly or indirectly be influenced by the rate of herbivory (e.g., seed production, competition, etc.). Since the six species in our study responded differently to visitation, visitation effects must be accounted for in the design of future field experiments.