A "significant" fire at a flower farm in St. Catharines, Ont., has been contained, but residents in a large area northwest of the massive greenhouse operation are being asked to stay inside, an official says.

No one was injured in the blaze at Pioneer Flower Farms that began late on Friday night, although flames spread to and destroyed about four or five residential buildings that housed migrant workers, according to Jeff McCormick, acting fire chief for St. Catharines Fire and Emergency Management Services.

Smoke, however, remained a concern. Officials from Ontario's ministry of the environment were on the scene Saturday, and a spokesperson said they have been collecting air samples for analysis.

"Preliminary downwind air monitoring results are below emergency screening values," Andrew Buttigieg said in an email. "Local watersheds have been sampled and monitored. Early results show dissolved oxygen and pH readings being good."

Buttigieg said the city is collecting douse water which is being trucked to Port Dalhousie sewage treatment plant as a precaution.

'Like a bomb went off'

The fire was in a structure that is a series of greenhouses and outbuildings and about 650,000 to 700,000 square feet in total.

"It's a significant-sized fire," McCormick said. "Probably, this would be the most significant fire that I have had of my 33-year career."

The fire also spread across the grass and caused a "significant" bush fire, the city of St. Catharines said in a release.

Crews worked all night to bring the fire under control. (David Ritchie)

Firefighters were called to the farm shortly after 11 p.m., and crews from five departments were able to contain the blaze after several hours of effort.

As many as 125 firefighters were battling the blaze at its peak, McCormick said.

"It was like a bomb went off," said Bill Van Vliet, a neighbour who saw the blaze around 2 a.m.

There were flames shooting in the air and "fire tornados," he said.

"I've never seen anything like it," said Van Vliet. "The whole area was just lit. It was really eerie."

Many people from the area, including neighbours, went to the farm to help. At one point, they set up an irrigation pipeline to bring water to one part of the fire.

Pioneer Flower Farms is a family business that started in 1971, according to its website.

Jeff McCormick, acting fire chief for St. Catharines Fire and Emergency Management Services, said he expects crews to be at the scene for some time. (Angelina King/CBC)

From celebration to disaster

John Van Geest, a longtime neighbour and friend, said the farm owners had been celebrating another business's 40th anniversary Friday night when they saw black smoke — and ran from a celebration to a disaster.

"It's devastating to see them lose what they've worked the last 48 years to build up," said Van Geest, who co-owns a greenhouse across the road. The farm offered jobs for many people in the community, as well as several migrant workers, he said.

Van Geest said he's spoken to the owners, who kept talking about how they felt for their employees.

John Van Geest, who co-owns a greenhouse across the road, has known the farm owners for the last 30 years. (Angelina King/CBC)

Residents in the area northwest of the fire were asked to close doors and windows and turn off air conditioning.

Van Vliet said the smoke was strong and choking early Saturday and there were plastics burning — they had to leave because the air quality was so poor.

Structure collapsed before crews arrived

When firefighters arrived on the scene one structure had already collapsed and fallen, making it immediately difficult for trucks to reach the fire.

McCormick said there are no municipal fire hydrants in the area, which he described as the rural end of St. Catharines, and tanker tanks were called from neighbouring departments to set up a water supply.

Thick black smoke billowed from buildings as fire crews on ladders tried to douse the flames. (David Ritchie)

Thick black smoke billowed from buildings as fire crews on ladders tried to douse the flames.

Ontario's Office of the Marshal was on scene Saturday to investigate.

Firefighters from St. Catharines, Thorold, Pelham, Lincoln and Niagara-on-the-Lake were involved in efforts to bring the blaze under control. Early Saturday, firefighters from Fort Erie relieved crews that worked through the night.

As for residents sheltering in place, McCormick could not say how many are affected.

A fire at a flower farm in St. Catharines, Ont., is now contained. (Angelina King/CBC)

Residents told to stay put until further notice

Stephanie Sabourin, spokesperson for the Niagara police, said the shelter-in-place warning applies to a "large area" south of Queen Elizabeth Way, near Seventh Street South.

The wind shifted early Saturday and Sabourin said the shelter-in-place order was adjusted accordingly.

"We are asking residents to stay inside, close their windows, close their doors, turn off their air conditioning and just stay put and to wait for further instruction," said Sabourin, who ​​​ could not say when the shelter-in-place would be lifted.

"There is some concern with the smoke. It's a pretty dynamic situation. We do have resources on the scene examining and monitoring the scene."

Deputy Fire Chief Dave Upper tweeted this photo, saying crews were still working on hot spots Saturday afternoon. (Dave Upper/Twitter)

Cause of fire still unknown

Van Geest ​​​​​said he knows how devastating fire can be, having suffered a much smaller blaze a few years ago. He helped unload items from the house and prevent the spread of the fire last night.

"We all know each other, we all work together in the industry. We look out for each other," he said. "There's ... so little anyone can do to mitigate this disaster."

The fire's cause is not yet known.

People are being asked to stay away from the area.

Pioneer Flower Farms, according to its website, is one of the largest "bulb forcing" farms in North America. It works with bulb stock growers in the Netherlands to produce cut flowers and potted plants.

Fire crews continue to work on the Seventh Street property. While the fire is contained it still remains active with hot spots. <a href="https://t.co/xFBXvNGO9G">https://t.co/xFBXvNGO9G</a> —@St_Catharines