Mutation is the source of all heritable diversity, the essential material of evolution and breeding. While mutation rates are often regarded as constant, variability in mutation rates has been observed at nearly every level-varying across mutation types, genome locations, gene functions, epigenomic contexts, environmental conditions, genotypes, and species. This mutation rate variation arises from differential rates of DNA damage, repair, and transposable element activation and insertion that together produce what is measured by DNA mutation rates. We review historical and recent investigations into the causes and consequences of mutation rate variability in plants by focusing on the mechanisms shaping this variation. Emerging mechanistic models point to the evolvability of mutation rate variation across genomes via mechanisms that target DNA repair, shaping the diversification of plants at phenotypic and genomic scales.