The sedge genus Carex, the most diversified angiosperm genus of the northern temperate zone, is known for its holocentric chromosomes and karyotype variability. Escudero et al. provide the first comprehensive study of population-level patterns of molecular and cytogenetic differentiation in the genus. They demonstrate dispersal and genetic connectivity among populations of the North American Carex scoparia that differ in chromosome numbers, demonstrating that cytogenetically variable sedge species can still cohere genetically. This finding is important to our understanding of what constitutes a species in one of the world’s largest angiosperm genera.