Division Chief Michael Jones said the fire was reported around 12:50 p.m. when the property owner noticed grass burning behind him.
The property owner was doing weed abatement and had already cut high grass around a barn, Jones said.
The blade of the mower might have struck a rock, sparking the blaze, Jones said.
The fire near Calistoga Road in unincorporated Sonoma County near the Santa Rosa city limit occurred in a mutual threat zone bringing a response from the Santa Rosa and Rincon Valley fire departments and Cal Fire, Jones said.
The 47 firefighters who responded contained the fire in 42 minutes, Jones said. A Cal Fire helicopter made 104 water drops to slow the spread of the fire so crews advancing with hose lines could suppress both flanks of the fire, Jones said.
The fire could have spread to the top on Rincon Ridge where high- tension electric wires and some structures are located in open land, Jones said.
Jones said the fire demonstrates the need to cut grass early in the morning when there is still moisture present and not during the heat of the afternoon.
Firefighters also recommend 30 feet of firebreak and 100 feet of defensible space around structures.