Grow for next year

OK, so this is my grow experiance. Tried hydroponics and failed miserablly :(. So now I'm thinking about growing outside. I live in a pretty isolated area and have a couple a rott's, so security really is'nt an issue. My main problem is the weather. I live in up near canada and we had our last frost this year in the last week of may and it not uncommon to start having colder weather in september even light frost. My plan is to build a decent size greenhouse 8x12 something to that effect. What I would like to know would the greenhouse its self be enough to protect from cold and frost? Or would I have to add heaters? My plan here is to have about 12 plants start them inside around the fist week of april under flourecents and 400watt metal hialide lights going with the natural light cycle outside at the time then move them into the green house june 1st. My problem is going to be about harvest time if it gets cold early the plants are going to be to big to move inside.

The greenhouse will protect from frost or reduce frost long enough for the plants to finish, but will require heat to keep going year round. I would keep them potted in the greenhouse.