Growing agaricus bisporus, or white, crimini and portabella mushrooms takes several weeks. First the mushroom farmer must prepare the compost or substrate. The substrate is the food source for which mushrooms grow. This substrate is pasteurized at 160F/71C to get rid of any bad bacteria.

Unlike other plants that grow from seeds, mushrooms grow from microscopic particles called spores. Spores are produced under the cap of the mushroom in the gills. Spawn is made by inoculating a piece of sterile grain with mushroom spores. Farmers buy spawn from sterile laboratories where they are produced. The spawn is spread on trays full of substrate in rooms that are climate controlled to promote growth

The root system, which looks like white fuzz, is called mycelium. It allows the mushroom spores to receive nutrients in the substrate. The substrate is covered with a layer of peat moss. Within three weeks small mushrooms appear through the peat moss. One week after the mushrooms start pinning, they are ready to be harvested with exception to the Portabella. Portabella mushrooms are actually full grown crimini mushrooms. They have been left on the bed for approximately 4-5 days longer to allow them to grow to their gigantic size.

Harvesting the mushrooms is done by hand. Mushrooms are processed, packaged and refrigerated quickly before being shipped to restaurants or food stores within 24 hours. The production to market takes about three months. Since the growth does not depend on the weather, Canadian mushroom farms produce freshly grown mushrooms all year.