That is exactly how I start my onions in flats. Works great for me! the onions seem to do much better than in all starting mix. Never tried it with other plants cant see why it would not work just as well with them. Cant hurt as I see it.

Trudi said she doesn't use any specialized soil, she uses potting mix, so I don't know why it wouldn't work.

Realizing it's Spring, are you planning on sowing now using the Winter Sowing method - outside in containers?
If so, there is no need to buy seed starting mix at all, just use a good quality potting mix.

Save your seed starter mix for things you are sowing indoors under lights.

I don't even use potting soil mix; I use Miracle Grow garden soil that I fluff well before putting in containers. Its cheap at Home Depot when it goes on sale at $1 off as it just has in my area.

"If so, there is no need to buy seed starting mix at all, just use a good quality potting mix."

What decides whether you need potting mix or seed starting mix? Just want to understand.

I don't like seed starting mix and I never use it anymore for anything. I wintersow seeds as well as starting them inside under lights and I use regular ProMix potting mix for everything with good results.

Karen

Seed starting mixes are lightweight, finely textured, and sterile to discourage the pathogens that can wreak havoc on seedlings indoors where there is limited air movement and fairly steady temperatures, no ups and downs.

Nothing is sterile outdoors, and there is no need for it to be - wind, sun, rain, fluctuating temps take care of those issues.

The thing is, it's only sterile when opened. If you don't use it all immediately, I'd consider leftovers contaminated. I do sterilize my potting mix before using for indoor seed starting, but not for WSing.

I don't like it because it (seed mix)is so finely textured, and seems to compact and compress so much.

Karen

Karen, I agree and use the Cornell peat lite blend (mixed by a local nursery, I dont do it) outdoors and in, but for someone not completely experienced with indoor sowing, fan, watering techniques rather than have them reaching for fungicides I'll usually simplify and suggest the sterile products to begin with.

I use regular potting mix for my inside sowing as well and have never sterilized it. No problem with damping off. Many people use a fan to move air about but I don't do that either as the one year I did the mix dried out so quickly the seedlings needed to be watered several times a day or dried up. Should have known that if it works don't mess with it. Maybe there is enough airflow in this old house. To thicken tomato stems I lightly brush the plant each time they are watered and turned.

Jagg,
some people use seed-starting soil when they sow indoors because it is sterile.
But sowing outdoors in containers, seeds grow more like they would in nature - if they just dropped off the plant and hit the earth.
I use potting mix for WS'ing. Also potting mix often comes with a bit of fertilizer which can be useful, seed-starting mix- to me there is just nothing there to get the seeds going.
Try asking a local nursery to sell you their own potting soil. That's what I do and it is cheaper than buying it in a bag, and better.

I don't have the space to do any growing indoors, so I pretty much only wintersow and springsow at the moment. I have never used anything other than potting mix, and to me the appeal of wintersowing in the first place was that it is something I can do pretty inexpensively. I don't use milk jugs anymore only because I just haven't anywhere to store them throughout the year - the first year I did this my laundry room was over run with them. It's been easier for me to buy clear plastic cups and either use one over another as a lid or put them in zip lock baggies with ventilation holes. I re-use what doesn't desintegrate over the season. I use whatever is least expensive as far as potting mix but I try to get something that doesn't clump...this year I found something for $3.50 at Family Dollar, it's like Pro Potting Mix or something like that and I've done about 50 cups and still have 1/2 a bag left or more.

I tend to buy cheaper topsoil and mix it with vermiculite for drainage. So far so good this is my third year Winter sowing. I used some miracle grow and later my mix the first year and didn't see any difference in the resulting sprouting/growth of the plants.

Last year and this I kept track of how many containers I filled from a $30 compressed bale of professional growers mix. Last year it was 200 gallon-size milk jugs from a 3.8 cu. ft. bale which came out to 15 cents per container. I'm guessing it would take a whole lotta potting mix to match that altho' I haven't done a side-by-side comparison.

Like quilt_mommy I don't have space inside to start plants other than a small tray table in front of my south-facing bedroom window. I can't be bothered with that since WS is so much better all around for growing everything, including trees and shrubs. I still have the 4-5 unopened bags of seed starting mix I bought 3 years ago before stumbling onto the winter sowing method.

That said, I realize not everyone has a nursery supply business close by as I do where I can buy the compressed bales, have them loaded into my car and set them inside my garage until they're needed. I've got 1/4 left of bale #3 this year + a dandy array of WS containers lining both sides of my breezeway.

I'm even cheaper than y'all. I'm re-using the potting mix from last year's outdoor containers for wintersowing. I bought mine by the bale. It's still fluffy and chunky, so I figure it should be fine. If I get random sprouts from last summer's petunias and four o'clocks, I'll consider it a bonus. For indoor seedlings I'm using fresh seed starter, to prevent a bug infestation in my warm apartment. I start eggplant and peppers indoors under lights, because in my climate I need early varieties *and* a headstart to get them to fruit before frost.
Edie

I do rather like edie_h above and reuse a lot of last years stuff. I dilute it up a bit with the cheapest fresh stuff, mix and sieve out the chunks. As mentioned this way one may get random plants from last year. My only successful ornamental grasses from seed came up the second year from the reused potting soil.

I do reuse my buckets full of soil that grow annual veg during the year.