Hi George,
I live in south-central Alaska and have been trying to find ways to extend my growing season for years. Unfortunately, every year is different so what worked one year won't always work the next. I've tried black plastic, but it didn't work well for me. It seemed to shade the snow I was trying to melt. Clear plastic worked better, but there is only so much that can be done while the sun is still low on the horizon. I'm just above 60 degrees north latitude and it seems like it isn't until early April that melting really starts to happen. Even then, the ground is too wet to work and too cold for germination. I still have about 3 or 4 feet of snow, so it may be May before I get to see my beds.
In years when I can get a snowblower into the garden, I like to remove as much of the snow as I can without tearing up my slightly raised beds. They're frozen fairly solid, so no worries about compaction. I then sprinkle wood ashes on the beds to darken the remaining snow. This melts the snow quickly, assuming you don't get a late spring snow that covers the ashes. Once the bed is exposed, the dark, moist soil is still too cold and wet for germination, but its weeks ahead of the non-treated spots. A low tunnel over the beds has helped me in the past.
It's not my favorite thing to do, but I also grow potatoes in plastic growbags placed on a large gravel area the previous owners created. I get good yields and the soil warms up quicker than does the ground. Plus, I don't have to invert my soil layers hilling or digging up the spuds. Crop selection is critical for cold-climate gardening. Long day onions, hard neck garlic, brassica crops, greens, potatoes, some kitchen herbs, peas, and the most amazing carrots and beets I've ever had make up the majority of my outdoor crops.
I have a small greenhouse with in-grpunds beds. I put cold hardy greens in there a couple weeks ago. I know it's cheating, but I also have a heater in there to keep it above freezing at night. I'm tired of grocery store greens and the overnight warmth breaks the momentum of the cold. I can generally get the greenhouse 20-30 degrees Fahrenheit warmer than outside temps during the day. I can't recommend enough a high tunnel or greenhouse. They not only help extend the season, they add growing degree days to those heat-loving crops like summer squash and tomatoes.
I start most of my crops inside well before the snow melts. I put LED growlights under the kitchen cabinets and have taken over my wife's kitchen. Once the seeds germinate, the seedlings goes out to the greenhouse to grow their sun leaves. Some crops just take too long to not start early. Things like leeks, tomatoes, and hard squashes have a hard time maturing if I don't start them early and/or we have a cool wet summer. I plant a lot knowing some crops won't perform.
Worst case scenario, have some sheets or row cover cloth on stand-by if you get bedeviled by a June or August frost. They suck, but picking frost tolerant crops helps that be less of a concern.
Good luck,
Ben