Background and Aims: There is a widely used crude method to estimate the age of hedgerows (Hooper’s rule) based on species richness. The aim of this study was to try and establish a similar field method for estimating the age of grasslands based on the accumulation of macro-somatic mutations. Methods A countrywide survey was carried out by the British public to investigate the relationship between grassland age and the number of creeping buttercup plants Ranunculus repens with extra petals. In addition the relationship between grassland age and R. repens pollen viability was also investigated. Key Results Each plant with flowers with additional petals in a sample of 100 was found to equate to approximately seven years. A higher significant correlation was observed between pollen viability and population age, however, this is not amenable to providing field estimates. Conclusions: The age of British grasslands can be easily and reliably estimated in the field by counting the number flowers with additional petals in R. repens in meadows up to 200 years old. An attempt to estimate the heritability of extra petals suggests that the phenotype results from the slow accumulation of somatic mutations in a species which primarily reproduces vegetatively.