I don't see any big problems with that idea. The only hesitation I might have would be the lack of "sterile" growing medium. Especially if the compost is cold. Then again, Ma Nature starts seeds in non-sterile stuff. I would bet the protection from critters and the consistent moisture provided by WS is more important than the sterile medium. I'd go for it.

Martha

Winter sowing medium doesn't have to be sterile, damp off shouldn't be a problem outdoors. But are you talking about somehow sowing in a compost pile? Because compost in containers is usually unsatisfactory, better saved for where it does the most good - the garden. Drainage (slow) and lack of aeration are issues with both top soil and compost in containers, that's why potting mixes are recommended here. Not a sterile lightweight seed sowing medium, but a potting/container mix.

Sounds like Morz doesn't understand the question and I don't either. Exactly where is the compost and where's the window? Are we talking about a cold frame? Compost bin? Compost pile?

Like Morz said, you don't need sterile mix for wintersowing but nice clean potting mix does have the advantage that it doesn't come with weed seeds and does drain well.

Karen

Thank you for the input, Sorry for not making myself clear, I have a 6 ft long by 4 ft compost two block high area of fresh clean cold compost maybe I could spred some miricle grow bag of soil on top of it and sprinkle seeds on top like clematis,echenaisia, coneflower,lily seeds. and cover with a shower curtain insead with cuts in it for air. and see how we do later I might add tomatoes on a corner by lifting it up and I could place a layer of cement blocks around the edge for keeping curtain down for winter a first for me thanks so much. Live and learn being new at this. Thanks to all and any other ideas apreceated.

OK, now I understand.

That would probably work well. But I'd prop up the plastic shower curtain to keep it off the seeds/seedlings. In your area, it might have to support heavy snow, so props would need to be sturdy. Might as well make it a hoop house. Token is our resident expert on that.

One other thought: Was that compost made by hot composting? Hot enough to kill weed seeds? Will you be able to ID the flowers and distinguish them from weeds? If not, it might be preferable to sow into milk jugs and transplant seelings later.

Karen

Something you might consider there -

Not all of your seeds are going to germinate at the same time (I'm still waiting on clematis seeds sown last year) so you will have some plants requiring different amount of care and circulation/ventilation than others.

I wonder if lifting them to transplant wouldn't create more transplant shock than moving them from containers with restricted root space...or did you plan to grow them on in that space?

Since you are adding a container mix on top your compost, unless you are planning on that being a raised bed from which you don't plan to move seedlings - had you thought about filling containers, sowing seeds, and placing the prepared containers in/on that bed until they've germinated in Spring?

I can tip the contents out of a germinated seed pot and transplant seedling with a whole lot less stress on young plants than lifting a seedling out of the ground and moving it.