Summary Invariant petal number is a characteristic of most flowers and is generally robust to genetic and environmental variation. We took advantage of the natural variation found in Cardamine hirsuta petal number to investigate the genetic basis of this trait in a case where robustness was lost during evolution. We used quantitative trait locus (QTL) analysis to characterize the genetic architecture of petal number. Αverage petal number showed transgressive variation from zero to four petals in five C. hirsuta mapping populations, and this variation was highly heritable. We detected 15 QTL at which allelic variation affected petal number. The effects of these QTL were relatively small in comparison with alleles induced by mutagenesis, suggesting that natural selection may act to maintain petal number within its variable range below four. Petal number showed a temporal trend during plant ageing, as did sepal trichome number, and multi‐trait QTL analysis revealed that these age‐dependent traits share a common genetic basis. Our results demonstrate that petal number is determined by many genes of small effect, some of which are age‐dependent, and suggests a mechanism of trait evolution via the release of cryptic variation.