The Capitals moved to the brink of clinching the best record in the Eastern Conference. They have a four-point lead over second-place Philadelphia. The Flyers have two games remaining and Washington has one.
Semyon Varlamov collected the victory with 31 saves.
After Knuble scored his 24th goal of the season on an early power play, Jason Chimera, Sean Collins and Matt Hendricks scored for Washington. Hendricks gave the Capitals a 4-0 lead 6:37 into the final period.
Washington's Alex Ovechkin picked up his 32nd goal of the season with 12 seconds to play, scoring into an empty net.
Florida remained in last place in the Eastern Conference with its ninth straight loss. Scott Clemmensen gave up four goals on 24 shots for the Panthers.