LaRoche continued his hot start Tuesday night, hitting a tying double in the eighth inning and a go-ahead single in the ninth to lift the Nationals over the Houston Astros 4-3.
"It's good for him because he's not traditionally a fast starter," manager Matt Williams said. "He really focused on it in spring training and really focused on driving one in. He's going to hit homers with guys on base but that one's big for us as evidenced tonight. If he can do that the homers will come ... but when he can consistently drive runs in like that we're a better team."
Jayson Werth homered as the Nationals won the opener of the interleague series.
"When I'm feeling pretty good, one, I'm taking some walks and two, I'm using the whole field," LaRoche said.
LaRoche is hitting .312 with four home runs, 17 RBIs and 16 walks in 2014.
Werth hit fourth home run in the first inning. Houston scored three times in the third for a 3-2 lead, helped by rookie George Springer's two-run single.
Houston manager Bo Porter was troubled that his team let another one get away late.
"It's very concerning, because our starting pitcher did a very good job," he said. "You feel like you are in position to win the game, you have the right guys matched up, and you just don't get it done."
Werth singled to start the eighth and scored on LaRoche's double to left-center field. Center fielder Dexter Fowler tracked the ball to the wall, but it glanced off his glove as he crashed to the ground.
Washington starter Gio Gonzalez allowed five hits and three runs, striking out a season-high nine in six innings.
Houston's Jarred Cosart gave up five hits, two runs and struck out six in six innings in his first career start against an NL team.
Cosart hit Kevin Frandsen with a pitch to start the third inning and he scored on a double by Lobaton to make it 2-0.
Fowler's two-out single cut Washington's lead to 2-1 and extended his hitting streak to nine games. Jason Castro reached on a double when the ball bounced off the padding on the wall in left field -- Frandsen saved a run when he made a nifty no-look, backhanded stop on the carom.
He said he simply got lucky on the play and shared what he told his teammates when they asked how he did it.
"You stick your glove out and maybe some things go in there," he said with a laugh.
Springer's single on a line drive to left field put Houston ahead.
The Nationals had another good defensive play when Span ran up Tal's Hill in center field and landed on his backside as he caught a fly ball hit by Dominguez for the second out of the fourth inning.
These teams wrap up the two-game series on Wednesday when Houston LHP Brett Oberholtzer opposes RHP Jordan Zimmermann. ... Washington OF Bryce Harper had surgery on his injured left thumb on Tuesday and will be out for an extended period of time. Harper was injured on Friday night against San Diego on a headfirst slide into third base on a triple. ... Houston RHP Scott Feldman, on the 15-day DL retroactive to April 18 with right biceps tendinitis, said he felt good after throwing 35 pitches in a bullpen session on Tuesday. He is eligible to come off the DL on May 3.