<title>Abstract</title>Reproductive isolation plays an important role in population differentiation and speciation, thus enhancing biodiversity in wild plants. Hybridisation sometimes involves rapid reproductive isolation between parents and their hybrids through the novel traits of hybrids derived from a new combination of genomes. Here, we report how a hybrids’ new phenotype contributes to rapid reproductive isolation between two ecotypes of <italic>Imperata cylindrica</italic>. The two ecotypes differ in their flowering phenology and habitats. An analysis with genetic markers revealed that hybrid populations consisted of only F₁ individuals. Both parental ecotypes flowered in spring, but F₁s flowered in fall. This drastic shift in flowering phenology prevented backcrossing parental ecotypes to F₁. F₁s flowered in fall and dispersed seeds in winter. The germination percentage of seeds set on F₁ was extremely low in their habitats, and seedlings did not survive due to the low temperatures in winter, resulting in the absence of a F₂ generation. In conclusion, flowering phenology mismatch promotes reproductive isolation between parents and F₁, resulting in a hybrid population consisting of only F₁s.