In the Yukon and I don't heat and usually plant the green house end of May or early June depending on what kind of spring we're having. This past year we had frost until the beginning of June and the first one at the end of August. The greenhouse keeps the frost off the plants - except for the beds beside the plastic- for quite awhile. As of today (Sept 18) it's still frost free for most plants. and the peppers are doing well.
My g'house is about 30'X 40" with a dual liner that is kept separated by a couple of air blowers.
This unit has motorized sides as it gets stinking hot in there some days (like 100F+)
This year we grew peppers (bell and jalapenos) carrots, parsnips, beans, peas, beets, tomatoes, zuccini, herbs, onions, pumpkins. A few years ago I successfully grew cantaloupe as well.
The picture is from about a month ago.