A field experiment was established to examine the effect of nutrient addition on the growth and competitive abilities of Parmelia caperata, P. saxatilis and Xanthoria parietina. Thallus fragments were attached to roofing slates in a split-plot experimental design in monospecific and multispecific mixtures and sprayed with nutrient solution at application frequencies representing a range of nutrient enrichments. Growth rates were measured as change in thallus diameter and the strength of competition determined by counting the number of thallus overlaps. Nutrient application significantly affected thallus growth with medium application rates promoting more thallus growth than controls (distilled water) or higher application rates in all three species. Nutrient addition at medium applications significantly increased the number of thallus overlaps made by all three species compared with controls, suggesting that competition became more intense. Although symmetrical competition was observed between all three species it was noted that growth and competition of thalli in two-species mixtures could not be used to predict the result in three-species mixtures. These results suggest that elevated nutrient levels can affect lichen growth rates and competition between thalli, leading to alterations in lichen community structure.