Winter sowing

I planted 6 seed varieties on March 2nd as an experiment in winter sowing. I got the idea from Skippy's Vegetable Garden (actually, I get a lot of ideas and tips from her, very experienced gardener based in Belmont, MA). Basically you cut open a plastic container of some sort and poke some holes in the bottom for drainage. I used clear plastic milk containers rescued from the neighbor's recycling bins. Put a few inches of soil in, plant your guys, and then tape the top back on. I also poked holes in the top, but this isn't really necessary because you leave the caps off which allows for transpiration of the seedlings. Then these little buggers are all cozied up and ready to go outside, even though it would be too early to plant directly in the ground. (I surrounded mine with old leaf debris that had been piling up in the back parking lot [soon-to-be gorgeous courtyard!] for what seems like years. Seriously, I bet there was at least 3 years worth of leaf pile-up on top of that asphalt. We dug a bit and found an inch of black soil and tons of worms, loving it in the gunk.)
The individual containers create tiny greenhouses for the seedlings, trapping in heat and moisture. They key is to use plants that might enjoy the cooler spring weather or are fairly hardy. We definitely had below-freezing weather after I put these outside, but look how happy they are!!

radishes

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beets

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wild flower mix
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dino kale, maybe?

The red onions and carrots have yet to sprout. Not sure if they like the cooler weather outside, although the bottles seem to be doing a great job of keeping the moisture and heat in. I have only had to water them once since planting. They drain pretty well, too, having survived the monstrous rain pour last week. Has anyone else tried outdoor winter sowing? Successes/failures?