HOUSTON -- Davey Johnson knew Jordan Zimmermann had plenty left to pitch another inning Thursday night.
But with his team up by three runs and Zimmermann's pitch count nearing 90, the Washington manager didn't see any reason to send his young right-hander back out for the seventh.
"I just didn't want to push him," Johnson said. "He's too valuable."
Zimmermann was pitching on an extra day of rest after experiencing soreness. Johnson said Zimmermann still has some discomfort when he gets loose, so the manager wants to "be real cautious with him."
"He looked like his old self tonight," Johnson said. "He was outstanding ... dominant game. That's him. Very dominant, overpowering."
Morse drove in three runs and extended his hitting streak to a career-best 18 games, the longest active string in the majors after Miami's Jose Reyes was stopped at 26 games earlier in the day. Zimmermann (9-6) pitched six innings of three-hit ball before three relievers finished Washington's seventh shutout of the season.
Zimmermann said he's fine and that he'll be ready to pitch again in five days. He was happy to rebound from his previous outing when he allowed a season-high five runs in five innings.
"I was locating the fastball well and then right when they would foul a few off, I would throw the slider out of the zone and they'd chase it," Zimmermann said.
The NL East-leading Nationals have won six in a row and own baseball's best record at 69-43.
Adam LaRoche drove in a run in the fourth inning before a sacrifice fly by Morse made it 2-0.
Morse's towering shot to left made it 3-0 in the sixth. He sent his second home run the opposite way to the seats in right field in the eighth for his fourth career multihomer game.
"He was attacking the fastball, and I like that in a hitter," Johnson said. "I like attack. I don't like letting fastballs pass that you can get ahold of and drive and that's what he did today and I really liked his approach. It was outstanding."
It was the 11th time this season the Astros were shut out and the first since July 20.
Lucas Harrell (9-8) yielded five hits and two runs with five walks in five innings as Houston dropped its fifth straight.
Zimmermann had retired seven straight, including striking out the side in the fifth, before Marwin Gonzalez singled with one out in the sixth. The right-hander got right back on track after that, fanning Steve Pearce and Justin Maxwell to end the inning.
Roger Bernadina singled to open the fourth before Ryan Zimmerman walked to set up LaRoche's RBI single. Morse's sacrifice fly added a run, but Harrell was able to limit the damage after that thanks in part to Maxwell robbing Jayson Werth of a hit by racing up Tal's Hill in center field to make a great catch.
Harrell needed 32 pitches to get through that inning, however, and was lifted for a pinch hitter in the fifth.
"That fourth inning got him in trouble," Houston manager Brad Mills said. ""He ran out of gas in that fourth inning. He was able to get through the fifth inning but wound up with 100 pitches."
Washington added a run on Zimmerman's sacrifice fly in the ninth inning.
Houston optioned INF Matt Downs and OF J.D. Martinez to Triple-A Oklahoma City after the game. ... Johnson gave rookie Bryce Harper a night off after he went 0 for 11 in the first three games of the series. Johnson said Harper will be back in the lineup Friday. "He's an intense competitor and sometimes, just like all young players, their emotions kind of get in the way of their ability sometimes. He just needed to sit back and relax and take it easy and he'll be fine," Johnson said. ... The Astros acquired INF Tyler Greene from St. Louis for a player to be named or cash. Greene was traveling to Houston on Thursday and will be activated for Friday's game.