A trimmer head for a rotary trimmer, includes an upper housing and a lower housing secured to the upper housing, wherein either the upper housing or the lower housing has a passageway. The trimmer head also includes a cutter assembly maintained in the passageway, wherein the cutter assembly has a knife, and wherein trimmer line inserted into the passageway is cut by the knife. A related method of cutting a trimmer line using the cutter assembly incorporated into a trimmer head is also disclosed.