Dewey, I make my own potting soil for larger perennials. Always end up with some weeds. That could be a problem with Winter Sowing (WS?). You could bake it (on a nice day, with the window open, the back door open, everyone out of the house, etc.) I understand that it is possible to sterilize a gallon of soil in the microwave, running at high for 5 minutes - pure hearsay, never done it.

I use equal parts of good garden soil, composted manure, and peat moss in that mix. When I want something a little more special, I'll put in 1/2 part vermiculite and 1/2 part worm castings.

There's a link for organic potting mixes from the National Center for Appropriate Technology below. Scroll all the way down for the recipes.

Good luck,
Steve

Here is a link that might be useful: Potting Mixes

Thanks steve. Those look like useful recipes. I could have posted on the WS forum but I hoped a fellow frug would have some ideas that cost less.

I was in Menards and hit the clearance/damaged goods sections and found some useful items:

Potting soil $1 per 40lb bag
1x6x48"long boards for 49cents each (raised bed sides)
3bags concrete mix (torn) $1ea
and some plants I passed up that were 50%off

That and bag day at a local church sale made my day.

I hope this isn't too late, but do NOT

NOT NOT NOT

Do NOT use the cheap 40-pound bag of topsoil for winter sowing. This is topsoil (not potting soil, and even potting soil should not be used on its own in WS) and great for outdoor use but, WS needs a combination of organic and inorganic soil. The big $1 bags of topsoil or not only weedy, but they also settle and turn to something similar to a brick over the winter in your WS container. It does not have enough air pockets for WS.

Also, do NOT use soil starting mix for WS--it's ONLY inorganic materials and too light and fluffy for outdoors. It does not have the moisture retention needed in WS.

Just right is a soil MIX. As cheap as I am, I do pay for these as it more than pays off in sprouting success rate. I like:

- Miracle-Gro Potting Mix (NOT Moisture Control) (settle with LUKEWARM water)
- Premier Pro-Mix for Potting-Seeding (settle with WARM water)
- Premier Pro-Mix Ultimate Potting Mix (settle with WARM water)

Do NOT buy soil mixes with weed suppressants as they suppress ANY germination, including that of your seeds. You also do NOT need to pay for moisture crystals; they may retain too much moisture in the WS containers.

As suggested, you can mix your own. Any combo of organic and inorganic ingredients is OK (sort of like how everyone has their own chili recipe!). I use:
8 parts potting soil
1 part perlite
1 part vermiculite
4 parts sphagnum peat moss
4 parts compost

Because it's hard to find all these things, and not inexpensive, I do prefer the pre-mixed mixes.

Hope this helps,
Monica