Back when it was first announced that the Argos are coming in, there was a bunch of info about all the plans, incl. the backup pitch being grown. It's mentioned in the following article for example, backup fields being grown in Mount Hope, ON. I think they can even use tarps etc. to grow the sod in bad/cold weather. Sod would be cut extra thick & heavy so it would lie better after installing at BMO.
There were other articles with more info at the time, but here's one: https://www.torontofc.ca/post/2016/0...itch-bmo-field