Belliard finished with two hits and two runs, and Lastings Milledge had two hits and two RBIs.
"That was a good glimpse of what our offense can do," manager Manny Acta said. "And we didn't even have everyone there today."
The Nationals are confident with the lineup they're using.
"We were explosive once we got Wily Mo last year," said Zimmerman, referring to an August trade. "We could be real special. From top to bottom there is a lot of potential here."
Houston's Lance Berkman hit an RBI double in his first start of the spring.
New Astros closer Jose Valverde struggled in his second outing, allowing two runs, two hits and two walks in one inning. He didn't display the same control as he did on Friday, when he retired three consecutive batters against Cleveland.
"Sometimes you throw it good, sometimes you throw it bad," Valverde said. "Today I didn't throw it good. I wanted to throw strikes, but the split-finger wasn't working and the first few got away from me."
Shawn Chacon tossed two innings in his first start since signing with the Astros on Feb. 20 to compete for a spot in the rotation. Chacon pitched well, outside of Zimmerman's homer.
"I threw one bad pitch and Zimmerman hit it out," said Chacon, who got five groundball outs and one strikeout. "Other than that, I was very happy with my stuff.
"I was trying to take command of my fastball, pound the ball down in the strike zone, and mix in quite a few changeups," he said.
Zimmerman's homer came on a hanging curveball.
"It wasn't bad pitch selection, just poor execution on my part," Chacon said.
The 30-year-old right-hander split time as a starter and reliever with Pittsburgh last season, going 5-4 with a 3.94 ERA. He has a career 4.98 ERA.
He is competing with Chris Sampson and a handful of others for the last spot in the rotation.
The Nationals started Steven Shell, a 24-year-old looking to make the roster despite never appearing in a big league game. Shell allowed one run and three hits in three innings.
Shell spent the past seven seasons in the Los Angeles Angels organization.
Notes: Astros SS Miguel Tejada made his second error of the spring and went 0-for-3. He is 1-for-8 this spring. ... Houston prospect Felipe Paulino pitched two hitless innings, while reliever Doug Brocail allowed two homers in one inning. ... Zimmerman is 4-for-6 this spring.