>>2533594>when to start seedlingsthat is entirely dependent on your latitude and topography. if you're getting enough sun, and you can heat the greenhouse, go for it. But think of it like this, the point of a greenhouse is to extend your growing season by letting you start before the last frost, and continue past the first frost. But the absolute dates for those events are location dependent. Depending on your setup (heating, grow lights) you can grow year round if you wish.
i'd post a picture of mine, but it's still under construction, and it looks like it was made by orcs -- as I am not a carpenter. but basically it's 8x12 with suntuff panels on the southern wall and on the single slope roof. EWN sides are 3/4 plywood. there wasn't much sense in spending the money on clear panels for sides that will never see direct sun (not to mention heating/cooling costs that would impact. The exterior is 'finished' (minus trim); on the interior i need to finish the walls and paint. (doing plywood vs drywall because of the humidity)
But to answer your question: I'm in western oregon at ~1100 ft of elevation, we're surrounded on 3 sides by trees and have a butte to our south which blocks out low sun early in the day during fall/winter. I'll be starting my peppers and tomatoes by mid month.