Rowand's sixth inning solo shot put the Giants ahead for good at 3-2 and Andres Torres got two hits, drove in two runs and crossed the plate twice as San Francisco swept a three-game series from the Astros at AT&T Park.
Zito improved to 6-1, handling Houston over seven innings, allowing three runs on six hits while walking only only one and fanning four.
Brian Wilson notched his ninth save of the season for the Giants by pitching a scoreless ninth inning.
Brett Myers (2-3) was saddled with the loss for Houston, surrendering four runs on five hits in eight innings of work.
Carlos Lee homered and got two hits in the Astros' loss.