Zimmerman (10-8, 3.54 ERA) gave up four runs on eight hits in five and two-thirds innings, striking out seven and walking one.
Zimmerman had to work around early trouble, giving up a two-run homer in the top of the first to Jonathan Lucroy. Ryan Zimmerman got the Nats on the board with a two-out RBI double in the bottom of the first, and then Washington took the lead in the third on a three-run double by Danny Espinosa that made it 5-2 Nationals.
A three-run Rendon homer in the fifth pushed the lead to 9-3.
Matt Garza (6-14, 5.26 ERA) gave up seven runs on eight hits in four and two-thirds innings, striking out seven and walking six.
Washington (62-61) stays five games back of the New York Mets (67-56) in the National League East.