Canada Sucks at Growing its own Food

Canada, while having food shortages internally, imports everything we need in excess. We have lots of livestock, canola, potatoes and grains, but fresh vegetables are rarely available in many cities because the climate isn't quite right. Tomatoes come from California, bananas from Mexico, and so on. Solutions as seen in dry, sunny places, using greenhouses to evaporate seawater, purifying it and growing plants in the moist environment of the greenhouse. In cooler climates in non-agriculturally-productive Canada, this isn't quite an option, but we have plenty of fresh water, and greenhouses provide enough heat to grow certain plants year round. Even placing greenhouses on green roofs on residential and corporate towers would allow for more local food year-round.