ICES emphasizes manual control methods, although some species cannot be reasonably controlled without the use of chemical herbicides, particularly perennial species that spread by rhizome. Depicted here is the control of sweet clover, a biennial species. Since it is biennial, the plants die after setting seed the second year. Hence cutting, as here, just prior to seeding is an effective means of controlling seed production and in time the plant population as the seed bank is exhausted. Fortunately many invasive species are annual or biennial species.