I also have a greenhouse (a "flowerhouse" pop up one 10' x 10')which I use for my heat loving annuals (start Easter, take down Memorial day) but I Winter Sow perennials outside in Jugs and plastic salad box containers. Why?
- WSing starts in winter. I don't want to be out there watering. Jugs get rain/snow water through their spouts - no work.
-When Easter comes, I want to start my annuals. If the greenhouse was full of perennials, there wouldn't be room. the perennials do SO well in the containers. It's amazing. I wouldn't want to mess with success.

I don't know a thing about foam vs. promix. I know an old neighbor used those foam blocks for soft wood cuttings from bushes and trees. I guess I've assumed they are expensive. I've never seen them mentioned here (WSing) as a way to start seeds. I'm sure your are right that it's less messy - - but I LIKE playing in the dirt!

Tanyuu, you could certainly do what you propose experimentally, and see how it worked for you, but it wouldn't be winter sowing as 'defined' by the methods typically used on this forum.

How chilly does your greenhouse get - to work for the moist chill needed by many perennials, trees, shrubs to break seed dormancy it should be 40F or cooler.

Sowing exposed to the elements eliminates the need to tend or water seedlings over winter. Once moistened and water breaches the seed coat, a seed will die if allowed to dry out again.

We sow in regular mature-plant container mix/potting soil, not even seed starting soil for most of us. Did you plan to pot up into potting soil after germination if your ground is frozen and you can't plant? Simply put, we are mimicking mother nature - but using potting soil for best drainage and aeration, fluctuating temps (which can be beneficial to kick-starting germination), natural light, water/snow melt provided for us.

The season begins on the solstice. I do have a few things I'll sow in late summer and Fall, those things that have seeds that are only briefly viable or will develop a deep dormancy that can take many months to germinate if seeds are allowed to dry will be sown as soon as ripe. Another group are the seeds that need a warm moist period before a chill before they will germinate - those I will Fall sow.
However, the bulk of seeds are sown first day of winter (and that can translate to as soon after the holiday season as we have time for :))

Here is a link that might be useful: FAQs

Thank you for your responses! I forgot to factor in the wetness aspect.

As for the foam, I mostly ask because I'm experimenting with adding the leftover bits to my potting soil, or reusing some of the foam cubes that didn't sprout seeds.

I do have one question I might have missed: I have a durable shelving unit that can be outside. One FAQ touched on this lightly, leaving me to think it'd be okay to just place them in shelves to save space, but will that hurt any seedlings placed on lower shelves?

I've been using shelving units for years. I have 6 set up in a long line in a narrow corridor of the yard I have claimed just for WSing. I think it is a great way to keep hundreds of jugs/containers tidy. I use to worry that plants on the bottom shelves would receive too little light and too much water (these shelves are "grid" so water goes through) but I have not had any problems. I just make sure I do move things "up" as the seedlings mature so thatthey get more light.

Uncovered shelving?

I have a couple of three-tiered, wheeled wire plant carts that I will put sown pots in on my deck, the shelves are grids there too - and they work just fine. And no green pot bottom marks on my deck at the end of season.

I'm not sure the bottom tier is any wetter than the top tier - everything is wet here in winter :) I just have to remember to keep them aimed narrow exposure to the wind or roll them up against the house in a storm so they don't go over.

Thanks! I'll have to see how things go this winter- it'll be in a corner, so that'll protect things a bit.

I don't understand what you mean about growing in foam..can you post a picture of what you are using to start your seeds?

Pippri21:

The link below is one brand I use; I'm able to get it cheaply through a local store, and it makes it easy to plant up to pots. I generally use it because I don't have to worry about introducing those darn fungus gnats inside the house, and I still get to play with dirt later.

I'll probably use the foam for the plants I'll grow inside, and the dirt for the plants I do outside.

Here is a link that might be useful: Oasis Growing Foam