Thorold firefighters on the scene at a grass fire in the area of Pine and Ormond streets.

On the south corner of Beaverdams and Ormond, Thorold residents watched what they called “the annual bush fire.”

In the open field, flames — at times reaching 50 feet in the air — scorched five to six acres of grass on on Tuesday.

Larry Coplen, the city’s fire chief, said his department found two fires at the same time when they arrived in the area. The first fire was knocked down quickly, but the second fire was more extensive, he said.

Although it was not immediately clear what started the fire, Coplen said it could have been intentional or caused by sparks from motorbikes.

Coplen said the field has been catching fire for the entire 25 years that he has been here.

There wasn’t that much damage, Coplen said, except for the grass that burns every couple years. Now that it is burnt it won’t burn for another couple years.

“We’ve seen big ones,” said Bonnie Handley, who was born and raised in Thorold.

She added that this fire is early in the season. “It’s usually not until June or July.”

Jennifer Davidson said her son said, “Mommy, the grass is on fire” around 3 p.m.

The crews managed to put out the flames within an hour.

An estimated 40 firefighters from three Thorold fire stations tended to the flames in shifts because the extreme temperatures. One firefighter was treated at the scene for symptoms of heat exhaustion.

Coplen said there are no prevention methods in place despite the fact that the fire has been happening in the same area for at least 25 years.

“There is nothing you can do to keep people out of here,” he said.

He said that the police department and the fire department share the responsibility of finding out the cause of the fire.

“We’ll never find out exactly,” Coplen said.