The timing you mention is typical. As a season extender, I think we don't get an early start in an unheated greenhouse but we do have a longer harvest period. I will start heating my palram about March 20th because our overnight lows will be up to about 26f so maybe nasty expensive to heat at night, even using a natural gas heater but it will only be needed for a short period. Right now, I can't even see tbe bright green led lighting because of the snow! One thing a greenhouse is great for is that any sunny day, it is a pleasant place to putter. Even in tne dead of winter. Grab those days to get free vitamen D and good vibes!