Abstract: This rotary filament vegetation trimmer improves the life expectancy of plastic filaments by improved and simplified control of a refrigerated air flow path about the filament of sufficient magnitude to prevent overheating resulting in catastrophic failure from the stresses of twisting as the rotary trimmer head whirls the filament at high rotation speeds. Thus, the head and its filament passageway therethrough are shaped to constitute a centrifugal air pump that draws both air and the filament through the head to position the filament for trimming. The passageway diverges from the shaft into a chamber of significantly increased volume to produce expanding and refrigerated air in the critical region at which the filament curves and induces twisting as the head rotates to tend to heat and cause failure of the plastic filament.