If you live in one of the more northern climates, you might want to get a jump on the growing season. I live in Michigan. I try to plant as soon as possible. This year, since the winter was mild, the ground never really froze. We had an early start to spring like temperatures. The next 10 days it's not even predicted to fall below freezing at night. So I am ready to garden. But still wanted to give my seeds the best chance. To do this I decided to help warm up the soil a bit. The easy way to do it - black garbage bags. By laying down black bags on top of my soil, I am drawing in heat, and this will help give the soil temperature an additional boost. It is also good for keeping weeds at bay before you are ready to plant.
The day I put them down it was really windy, which was good because I got to test what would keep the bags from flying away. I am lucky that I have a lot of bricks and stones on my yard left behind by a former tenant. I just put a stone at each corner.
Why Garbage Bags?
They are cheap. It's a lot quicker than having a roll of black plastic, which you are going to have to cut. The beauty of the garbage bag is you can remove them one at a time. Say you only want to plant in one spot, you can just remove the bag(s) from that spot.
How Long Do I Need to Have the Bags Down For?
I would say as long as you can. As long as you don't think it's really going to snow a lot again, I would put them down ASAP. Then when you are ready you can remove them.
What About Drainage?
You should poke some holes into your bags, so that water can drain down into your soil.
When Will It Not Be Necessary?
Once you pass the frost free date in your area, then I wouldn't bother putting them down anymore, your soil should be ready. I am most using them for early crops, like peas, broccoli, lettuce, and cabbage.