At Carleton Mushroom Farms in Osgoode, just southwest of Ottawa, a giant warehouse farm produces up to 270,000 pounds of fungi each week.

That’s roughly four pounds per square foot at each harvest. Grown in tunnels where temperature, humidity, carbon dioxide and light are tightly controlled, organic cremini and white button mushrooms sprout like tiny ghosts from acres of dark compost, in beds layered one atop another.

This is the life’s work of the Medeiros family. Founded in 1984 by Fernando and Anabela, Carleton Mushrooms is one of the largest producers in Ontario and sells mushrooms from Newfoundland to Manitoba, and as far north as Iqaluit.

Fernando and Anabela met in Toronto while working at Dominion Mushroom. In 1972, one of the managers at Dominion moved to Metcalfe and founded Continental Mushroom. The Medeiros joined him the following year, but seeing no future, Fernando struck out on his own. In 1984, he launched Carleton Mushroom with a couple of partners, whom he bought out in 1988. At the outset, he was producing about 15,000 pounds of mushrooms weekly. The couple’s sons, Mike and Fernando Jr., bought the operation from their parents in 2005 and started a massive expansion.

The expansion didn’t begin with more mushrooms, but with what those mushrooms grow in. The nutrient-rich compost that supports each crop is the foundation of the entire operation — and when supply ran short, it forced a radical rethink. To that end, the Medeiros brothers, in partnership with two other growers — Sharon Mushroom Farm in Newmarket and Champag Organic Mushroom Farm in Montreal — have built a worldwide industry first in the compost business near Prescott. And all because they ran out of compost.

Mike Medeiros is shown top left at Carleton Mushroom in Osgoode, where he and his family run one of Ontario’s largest mushroom operations. The warehouse farm produces up to 270,000 pounds of organic cremini and white button mushrooms each week, while the brothers’ state-of-the-art compost facility ensures consistent, nutrient-rich growing medium for their mushrooms.

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In 2012, the brothers were partners in a compost-making facility just north of Whitby. It was the first indoor facility in North America. But then they expanded their growing operations, building 40 growing rooms in four years. They literally outgrew their compost allocation.

By pure serendipity, two other mushroom growers approached the brothers to partner in the construction of a new compost facility near Prescott. While driving the rural roads, Fernando Jr. found the perfect place: an old farm of 138 acres, blessed with ample power supply and plenty of water, just off Highway 401.

Here, they have built a state-of-the-art, first-of-its-kind, 600,000-square-foot compost facility. As you round the corner, between rows of ready-to-harvest corn stalks, two massive warehouses fill the horizon, and stacks and stacks of giant yellow straw bales occupy the adjoining spaces.

The straw, which comes from Ontario and Manitoba, is the starting point for the rich, dark planting medium that will result at the end of approximately a month of watering, pulverizing, pasteurizing, mixing and seeding. The facility starts with 850 bales weekly, which are fed into giant tunnels, watered with nutrient-rich runoff from previous cycles and left to begin the softening and breaking-down process.

After this, they shred it, and add poultry manure for nitrogen and gypsum to raise the pH. They then pasturize it. Once it cools, they add mycelium.

A state-of-the-art, 600,000-square-foot compost facility near Prescott transforms Ontario and Manitoba straw into nutrient-rich growing medium through a fully mechanized, indoor process that takes about a month and supplies Carleton Mushroom Farms and partner growers. Photos by Hattie Klotz.

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What makes this facility unique is the complete mechanization of the process. “Walking floors” have been installed throughout the cycle, so when hundreds of tons of compost need to be moved from one stage to the next, the bunkers’ floors connect to the hoppers to trundle the compost from one space to the next. In some U.S. facilities, this is still done with a tractor and a front-end loader, one scoop at a time.

The plant took two years to build, using Dutch technology (the Dutch were mandated to move their compost production indoors due to odours), and was overseen entirely by Fernando Jr., who now also runs it day to day. It opened in November 2024. He is a stickler for detail, and the result is a highly automated “farm” that produces consistent, nutrient-rich compost for his brother to grow mushrooms.

The whole facility is spotless and there’s barely any compost odour. That’s because the Medeiros scrub out the ammonia created during the composting process. “Then the air passes through a bio-bed to purify it, and it comes out smelling like pine,” Mike explains. This has kept their neighbours happy.

They empty one bunker daily, and ship the contents to Newmarket, Montreal and Carleton Mushroom in Osgoode. They fill 11 growing rooms weekly, and have 22 rooms producing at any one time. Once a growing room has been harvested twice, it is emptied, sanitized, power-washed and steamed for eight to 12 hours before it is prepared for the next cycle. Some growers harvest a third cycle, but Carleton chooses not to.

“We are 100 per cent organic,” Mike explains, “and the third flush tends to have more disease present, so we just don’t do it.”

Carleton Mushroom private labels for Farm Boy across Ontario, for President’s Choice in the region and almost all of the Giant Tiger stores from Ottawa to the East Coast. They supply Costco stores in Ottawa. They also deliver three times weekly to Montreal for distribution in that region. This breadth of distribution for a fresh product requires consistency. And that’s what the Medeiros brothers are all about. “We want to produce the highest quality, so that consumers know our product lasts,” Mike says. “Consistency is everything.” His brother, Fernando Jr., says the same thing about the compost.

As consumers become more aware of just how important diet is for positive, healthy outcomes, mushrooms are having a moment in the spotlight. Lion’s Mane, Maitake and Reishi have proven brain-boosting properties, while just the common old white button mushroom has surprising curative powers. It is a rich source of ergothioneine and glutathione, essential antioxidants in the human body.

Extensive research by Robert Beelman at Penn State College of Agricultural Sciences has shown a correlation between longevity and lower levels of chronic aging diseases, and the consumption of mushrooms in Japan, Italy and France. Even small amounts of mushrooms consumed daily (approximately four ounces or four mushrooms) can have a significant effect on levels of ergothioneine and glutathione in the body.

Carleton Mushroom is a truly family affair. Six members of the Medeiros clan work in the business, including Mike’s teenage daughter on weekends. And at Christmas, they celebrate their employees like family, too; Mike picks up coffee and breakfast for all the employees working that holiday morning shift.

While this may be business on a huge scale, it’s also very rewarding for Mike. “I enjoy growing and providing healthy meal choices to nourish you and your family,” he says.

Carleton Mushroom Farms
6280 Dalmeny Rd., Osgoode
carletonmushroom.com | @carletonmushroomfarms