A cutting unit for a plant or vegetation cutting device and which comprises a rotary cutting head and at least one cutting filament extending radially from the head, such that upon rotation of the head the filament is extended by centrifugal force and cuts through the plants or vegetation. The filament has at least one localized working zone which includes a cutting edge which is defined by two intersecting faces, and the filament is retained by the cutting head in an angular position wherein the cutting edge constitutes the leading edge that engages the plants during operation of the cutting unit. The filament may have a primary or preferred bending plane, and the filament is held in a configuration wherein the filament is bent essentially in the primary bending plane. Also, the localized working zone may be formed of a material which is different from the material of the remainder of the filament.