Zardes finished a Marcelo Sarvas feed almost midway through the first half, and it stood up in a tight, intense battle -- the best of their three meetings this season -- that fit nicely into series lore.
The Galaxy had a chance to ice the game late, when Sarvas won a penalty kick in the seventh minute of second-half stoppage, tripped trying to dribble past Nick Rimando. But the RSL goalkeeper dived to his right to deny the Brazilian's spot kick.
The Galaxy (6-3-6) extended their unbeaten streak to seven games. RSL (7-4-7) remain second in the West, seven points behind Seattle, which is home Sunday night against Portland.
Real Salt Lake used the same lineup, including Rimando and Kyle Beckerman, as in in their Fourth of July victory over New England. The Galaxy, playing the first of three games in eight days, made just one change from their July 4 draw with Portland, inserting Baggio Husidic into midfield in place of Stefan Ishizaki. Defender Omar Gonzalez, recuperating from an ankle injury suffered in the United States' World Cup loss to Belgium, was unavailable for LA.
The sides matched formations -- both in 4-4-2s with diamond midfields -- and RSL held more possession and took more shots but created fewer genuine scoring opportunities. They forced the Galaxy to attack from the flanks, which they were happy to do.
LA went ahead from a play originating out wide in the 20th minute, after Javier Morales was stripped of the ball to the left of LA's box and was left lying prone. Sarvas carried it up the right wing, and, when he was even with the top of RSL's box, cut the ball back, in front of the retreating Beckerman.
Landon Donovan let the ball roll by for an onrushing Zardes, who fended off a challenge from Joao Plata and, with four defenders around him, lashed a shot from 20 yards that struck the left post and bounded across the goalmouth and into the netting inside the right post.
The goal was Zardes' fourth of the MLS season and sixth in all competitions this year, and it ended Rimando's bid to pull even with MLS career shutout leader Kevin Hartman in front of a hometown crowd.
Zardes nearly struck again two minutes later, but he couldn't quite latch on to a Robbie Keane ball into the box from the left flank, which was cleared before Juninho could could collect the spill.
RSL was more proactive in the second half, with four good chances to equalize. Beckerman's one-bounce header from a Morales free kick in the 59th minute failed to trouble Jaime Penedo, who then parried a sharp Plata shot from Tony Beltran's low feed in the 62nd. Beckerman fired wide after a giveaway in the Galaxy by Ishizaki, a late substitute, in the 83rd, and Ned Grabavoy was wide from a through ball in the 89th.
The visitors wanted a penalty kick when Juninho appeared to kick Chris Wingert in the face while clearing a corner kick from LA's box in the 85th, but referee Silviu Petrescu wasn't moved. Several minutes later, the Galaxy had a penalty of their own, but Rimando came up with the save to keep the deficit at one.
Real Salt Lake opens a three-game homestand next Saturday against Vancouver.