Last fall I built a small "hoop house" with PVC pipe and plastic sheets from the hardware store. A hoop house is a mini greenhouse, and they are easy and cheap to build. If you Google "hoop house PVC" you will find many plans for every possible size and garden situation.
The PVC pipe is white plastic pipe that comes in 10-foot lengths, and I used the 1/2 inch diameter pipe. The plastic sheets are the kind that people use as dropcloths when they paint, the thickest ones I could find. The sheets aren't clear, they're translucent white, and at first I was worried that they wouldn't let in enough light, but they did just fine.
Last September, inspired by the book "Four Season Harvest" by Eliot Coleman, I planted kale, parsley, carrots, onions, arugula, and some unusual greens like mache and miner's lettuce. When it got cold I put the plastic sheet on. I had fresh greens from about October onward, and was picking salads until the beginning of January.
In January it started to snow, and snow, and snow. We had the snowiest, coldest, iciest winter I've ever seen on Long Island. The hoophouse got covered with snow and ice and partially collapsed. I figured everything in it was probably dead.
A few days ago, after all the snow finally melted, I got around to opening it and this is what I found:
I just stood there in awe: it was like a little piece of permanent spring. The plants were totally untouched by the winter, even though they had just gone through six weeks of ice, snow, and below-zero temperatures in an unheated, snowed-under, unwatered plastic cave.
This is a view of half of the hoop house: kale, arugula, green onions, and mache. The other side was filled with parsley and carrots. I had never had homegrown carrots before and they were amazing, with a tender crunchiness and delicate flavor--they almost tasted perfumed, like Earl Grey tea. The baby kale was so tender and fresh, I made soup with it right away!
Next fall, I'm making an even bigger hoop house, one big enough to walk into (and since my yard is so small, it will take up half my front yard). Imagine in the winter, being able to walk into that sweet, fresh place, filled with the smell of fresh greens. And a big plus of winter gardening: no weeds, and no bugs!
