I use cardboard milk/juice cartons to grow dahlia tubers or other plants before transplanting them in the garden.
I have never used them for WS, but I don't see why not, since people have used gallon pots and other opaque containers for WS. They use clear plastic to cover the container rims.

I also WS in newspaper pots. They are as opaque as milk cartons. Give it a try. I am for recycling as you are. :-D

I use 2-litre bottles myself, and have used milk cartons sometimes but the disadvantage is the ease of covering them. Depending on the weather, you may want to open your containers during the day, and re-close them at night. It's just easier if you have an easily re-closable container. I used the milk cartons for annuals which I usually plant in early spring and don't have much of a need for opening and re-closing. The opaqueness of the sides of the containers doesn't present a problem.

I've used them successfully, as well. While it was somewhat tedius, I simply cut windows in at least two of the sides, put in a saran wrap "window" (held in by duct tape), and it worked fine. Held up all season. I did make a mental note that with at least 2 bigger windows in the sides, growth was better than if I put only two smaller windows in the 'slant' part of the carton at the top.
I did the same thing with ice cream boxes- big window in the top.

I use a lot of the milk/OJ cartons and have had good germination in them. I cut mine down to 5-6 inches and put them in a produce bag or bread wrapper secured with a twist tie at the top. I put drainage holes in the bottom of the plastic bag (and carton) and transpiration holes in the top.

Wow, thanks so much to everyone for sharing your experiences with different containers! I hadn't even thought of cutting windows in the cardboard milk carton sides...that is a great idea. And it would be great to have another use for the plastic bags the bread comes in...I'm one of those save-and-use-everything people, and I do actually wash and save our bread bags and zip-lock bags.

I think for my first year of winter sowing I'll try a combination of containers and see what works out the best. I have a lot of those plastic 2-liter bottles and various plastic juice bottles, so will try those too. Yesterday I realized I can probably winter sow in those plastic boxes that our baked goods are packaged in from the supermarket...

Thanks again to everyone for your follow-up posts! I'm so excited to get started...

Hello Topie! you will love w/s! it's so much fun!
I started in 2001/2002 season with Cool Whip and cottage cheese containers as well as 2 litre bottles...the bottles did not do well for me- we did have a WET winter, and I did not have enough drain holes in the bottoms..

the last few years I use Solo Cups and the strofoam cups that are listed as 'recycleable'- they are easy to punch holes in, and I reuse them too! I don't need as many plants now, because of w/s I have my little city cottage garden stuffed, so the cups do fine for me. I put plastic wrap on the top, held with pony tail rubber bands and punch holes in it for moisture/air

the main thing I have found about containers: you can use most anything if you can get at least 3 inches of soil in it..

I have tons of seed to share- if you want to send me a bubble env- and 2 stamps- then you can experiment and really have fun!

Linda