Low-Input seed starting
The first thing you need to remember is to think ahead, and bring in the compost before three feet of snow and ice lands on top of it. That was my big discovery two years ago, and like so many big discoveries was a. unpleasant and b. completely obvious - in retrospect. Living in a linear society, it can be difficult to get cyclical.
You see, I knew you could start seeds in lightly sifted compost - in fact, I'd seen Rodale Institute tests that showed that some varieties seeds did best in finished compost. So, the year before, I'd gone out in February, dug up some compost, let it defrost, and then sifted it through an old screen and used it, with lovely results. All those living organic bacteria made a very happy arrangement, and the seeds I started that way did far better than those I did in organic potting mix.
Because that winter was an extremely mild one, it didn't occur to me that normally, getting compost out of the pile in February in upstate NY was going to be trouble. If I'd thought about it for 2 seconds, I would have realized I had to do it in October, but I didn't, and thus, the trouble. Every year since, I've taken 10 minutes to shovel and not regretted it!
Which is all just a reminder of how seed starting, as most of us do it, is a heavily energy intensive process. It can involve lights, heating mats, plastic containers, lots of purchased seed starting mixes and various liquid substances that make your house smell vaguely of rotting fish for a week after you use them. All of these substances have to be transported to you. How do you get along without all those things, either if you have to or if you want to? How do you use less of them, at least?
Well, the first thing you'd do is rethink how much you need to start inside. I do a lot of transplanting, from preference - I enjoy it, and I think it saves me time on weeding later and works better with my mulching techniques. But there's no reason why I have to do so much. There are a few crops that need an advance start, but even a few cherry tomatoes will self seed and make a late crop. Now I want tomatoes earlier than that, and different varieties, but I could make do, and start fewer inside. Some crops I get a head start on, like broccoli, could easily be entirely direct seeded. Many perennial crops can be seeded in the late summer, when it is warm, and held over the winter, then planted out the second year, which takes advantage of natural warmth to help germinate seeds. So if I needed to reduce my inputs, the first place to begin would be with reducing the number of advanced starts and rethinking some timing issues.
The next strategy I'd probably use is starting things a little later - my house is quite cool in February, and since space by the stove is always limited, I'd probably need to cut down on the things that simply don't germinate well without some bottom heat in cold temps - peppers, eggplant and basil being some of the biggies. But a smaller number of these could be germinated by the stove, and waiting until late March would mean later harvests (late August, September), but would require less supplemental heating. In March, the windowsills are warmer, because there's more sun and the outdoor temps are higher - not high, mind you, but higher.
Another alternative would be a hotbed. This is a coldframe with a thick layer ( a foot or so) of uncomposted horse or pig manure, covered with a layer of soil, and a cover on top and insulators on the side. The manure, decomposing, heats the soil and creates a great environmental for little seedlings, using wholly natural processes.
In a perfect world, I'll have a greenhouse, and if you are going to use plastic for anything, 10 mil plastic for season extension is probably a better use of time and resources than a thousand little plastic trays and pots. There are a lot of inexpensive greenhouse models out there that can extend a season a little ways - enough to make it viable for those of us in cold climates to do a lot more with less.
What about those plastic flats? Can I get rid of those? Well, they really aren't hard even for a klutz like me to knock together with scrap wood, and we aren't going to run out of old food cans and yogurt pots for a long, long time - ask your neighbors to save them.. With some holes in the bottom, they make find pots for seed starting. Same with old plastic containers. I don't think we'll see a shortage for some time. Milk carton bottoms work well, so do a host of other creative solutions.
What about seed starting material? Well, you can plant things in straight, *finished* (that is, no longer heating up) compost, but that does use your compost apace, and you do have to plan ahead in cold places. Now the next part is controversial. If you use soil, the conventional wisdom is that you are supposed to sterilize it by baking the soil at 250 degrees. That's supposed to spare you damping off disease. My own feeling is that this is stinky, unpleasant, a waste of energy and kinda nuts. That is, I think killing all the good soil bacteria so that you can get rid of a single bad bacteria is a bad idea.
If you use light dirt (you can mix some sand or compost in to lighten it up - I find 1-1 compost and dirt to be nice, I don't bake it, I dig it in the fall (remember, cold weather people have to plan ahead), and bring it in. I mix in the compost, and let it sit. I've never had any more problems with damping off disease using real dirt than I have with potting mix - damping off tends to be a problem when I overwater, not linked to the soil I use.
Two other great options - a soil block maker and a newspaper pot maker. The former uses a compost-sand-dirt mix to start seeds without any framing, the latter uses old newspapers to make biodegradable pots to transplant directly into soil.
What about light? Well, windowsills are still a good idea, particularly if you make a reflector from tinfoil and cardboard and put it behind the plants to maximize light access. But if you haven't sunny windows, you'll have to use hotbeds and cold frames - that is, plants will have to go straight from germination outside, in a protected way. An easy cold frame is an old window and some hay bales, but you can get more complicated and build some structures. I've also seen (but not tried) pop up greenhouses, that can be set over a row in the garden. The big problem with starting out in the cold isn't the cold, but the heat, I find - a bright sunny day can fry your seedlings even when it is quite cold. So either keep a close eye on the temps and open them up a little, or acquire an automatic opener - these are powered by temperature changes and don't need any energy, but they are pricey.
How about fertilizing? Compost and manure teas will do it - they do get a little ripe smelling in the house, but no worse than fish emulsion or most kelp-fish mixes. And if you use compost as a large portion of your seed medium, you won't need much fertilizer, another plus. Or use diluted urine - dilute your own pee 1 part urine to 7 parts water, and use that to fertilize. It is a great natural fertilizer, and very low risk - just stop using it a week before harvest.
Thomas Jefferson reportedly planted juniper seeds in a bed by feeding the seeds to his chickens and confining them where he wanted the seeds to grow. Despite the cold, they did rather well, encased in chicken manure. So there's always that, too - mimicking nature works rather well.
Seed starting doesn't have to be expensive and use a lot of plastic and purchased inputs - it can be simple, cheap, recycled and environmentally friendly, just like the rest of your garden.