I have a "pop up" greenhouse - basically a clear 10 x 10 igloo like plastic tent. Love it. But what I do in it is not winter sowing. It is spring sowing of annuals.In pots, in trays, in old bottoms of milk jugs - everything without tops because I water everything every day. I put it up on Easter and take it down on Memorial day. Easy peasy.

I think of hoop houses in the same way as this pop up - unheated, no rain/snow inside structure. Great for getting heat to heat loving annuals in the spring, but for perennials - -WSing in jugs OUTSIDE works best for me.

Why? I can do it in January/Febuary, when there is not much else to do. I can't put up my pop up then, snow would cave it in. The weather (snow/rain) takes care of my jugs of perennial seeds completely - I make them up, place them outisde then forget them till the spring weather hits. No watering. Perennials for my zone seem to like this method.

On the "repotting" the seedlings and letting them grow on.... I did this a lot my first few years, unconvinced that those tiny babies could "take it" in the beds. I don't anymore, with a few exceptions, because it takes time and potting mix I don't have to spare and I find small seedlings grow much more quickly in the beds. The only seedlings I consistantly pot up are heurchura, as they are so so tiny. I let them grow on in pots and plant in the fall. This year, after years of trying to get Monks hood to sprout by WSing, I finally got some tiny sprouts. I potted these up to to grow on. Every else - over 100 milk jugs of perennial and hardy annuals - straight into the beds.

Maybe someone from your zone will have advise based on their experience.....

Nancy

It doesn't rain or snow in the hoop house and there is also no breeze. You're going to have to water and keep an end open to get in the fresh air.