The fire was reported at 11:35 a.m. in the area of Alhambra Avenue and Macalvey Drive, a dispatcher said.
Firefighters called for a second alarm at 11:50 a.m., fire district spokeswoman Emily Hopkins said.
Carpenter said the flames were not too close to the houses, but that the wind picked up shortly after the fire broke out and firefighters were afraid it would blow the flames toward the houses and a nearby eucalyptus grove.
Firefighters from Cal Fire and the East Bay Regional Park District were on standby to help, but county firefighters were able to contain the blaze on their own, Carpenter said.
No homes were damaged, Carpenter said.
A fire investigator was at the scene this afternoon working to determine what caused the fire, Carpenter said.