A win and two losses for teams above it in the standings was good news for Los Angeles.
Zardes' strike came in the 20th minute. Marcelo Sarvas' cross was cut back deep and Landon Donovan smartly let it pass by to Zardes, who held off a defender and picked his spot from 18 yeards.
He almost scored a second soon after, but shot wide from 4 yards out with the goal at his mercy.
LA had one penalty denied in stoppage time and Nick Rimando saved one that was given. The RSL shot-stopper got away with clipping the heels of Donovan on a breakaway. He then tripped up Sarvas on an almost identical chance, but denied him from the spot.
LA (6-3-6) is in sixth place, but closes the gap on second-place RSL (7-4-7) and fifth-place Vancouver, which lost to Chivas USA. The Galaxy also have the luxury of at least two games in hand on every team above them.