A formulation is provided for application to a host plant to reduce, inhibit or impair one or more of growth and development of the host plant. A method of inhibiting growth plant growth and development is also provided as a means of controlling weedy species. The method comprises: selecting a suitable gene for growth suppression in a target plant; identifying an at least one target site accessible to base pairing in the suitable gene; identifying an at least one divergent site in the at least one target site; designing a construct complementary to the at least one divergent site; adding an at least one RNAi inducer to the construct; and delivering the construct to the target plant.