Urtica dioica is a sub-dioecious plant species, i.e. males and females coexist with monoecious individuals. Under standard conditions, seed sex ratio (SSR, fraction of males) was found to vary significantly among seed samples collected from female plants originating from the same population (0.05–0.76). As a first step, we investigated the extent to which SSR and sex expression of male, female, and monoecious individuals is influenced by external factors. We performed experiments to analyse: (1) whether the environment of a parental plant affects the sex ratio (SR) of its offspring, (2) whether SSR can be affected by environmental conditions before flowering, and (3) whether sex expression of male, female and monoecious plants that have already flowered can be modified by environmental conditions or by application of phyto-hormones. Within the range of our experimental design, SSR was not influenced by external factors, and gender in male and female plants was stable. However, sex expression in monoecious plants was found to be labile: flower sex ratio (FSR, fraction of male flowers) differed considerably between clones from the same individual within treatments, and increased toward 100% maleness under benign conditions. These results provide strong evidence that monoecious individuals are inconstant males, which alter FSR according to environmental circumstances. In contrast, we consider sex expression in male and female individuals to be solely genetically based. The observed variation in SSR between maternal parents cannot be explained by sex-by-environment interactions.