Building a greenhouse in Burlington ON. Once covered, the inside temperature often reaches 25-30 degrees C on sunny days. This warm, fragrant air comes into the house via a fan, as an effective source of heat through March, April, May, September and October. The owner is able to sit out in the greenhouse from March to December. Cherry tomatoes last until Christmas; kale, lettuce, green onions, chard, chives, thyme, and rosemary last year round.