There is some farmland in Alberta as far north as High Level, within about 100 km of the NT border. The growing season is short, but somewhat compensated for by the increased daylight in the summertime. I think there could be the possibility for further expansion but maybe Albertans could expound on that.
I take some issue with the map you have in your post. For example, they have Anticosti as part of the St. Lawrence Lowlands, when the island is rather mountainous and infertile. Similarly, they show all the Maritimes as being part of the Appalachian Region. This is true for northern NB, and depending on how you define it, perhaps portions of NS, but large parts of the region are considered coastal lowlands (the Maritime Plain), and are more akin to the coastal plain along the US eastern seaboard.