H2O2 is an ubiquitous compound involved in signalling, metabolic control, stress responses and development. The compatibility of leaf tissue levels with these functions has, however, often been questioned. The objective here is to document H2O2 levels and variability under natural conditions, and their underlying causes. Using the FOX method, bulk H2O2 concentrations were analysed in leaf samples from 18 species of herbs and trees throughout the 2006 growing season. Sampling addressing targeted predictions was emphasised in 2007 and 2008. H2O2 levels varied 100-fold through the year, with a main peak in spring. Two hypotheses were examined: (H1) that H2O2 reflects seasonally variable responses to environmental stresses, and (H2) that it reflects metabolism associated with leaf development. Based on poor or inappropriate correlations between H2O2 and indicators of light, temperature or drought stress, support for H1 was minimal. H2 was supported both by seasonal patterns and by targeted analyses of concentration changes throughout leaf development. This study concludes that bulk tissue H2O2 concentrations are poor indicators of stress, and are generally too high to reflect either signalling or metabolic control networks. Instead, the linkage of H2O2 and leaf phenology appears to reflect the roles of H2O2 in cell expansion, lignification and wall cross-linking.