You'll notice the second covering of plastic over a couple of the planters. This keeps the temperature in those planters up to 5 degrees warmer than the greenhouse temperature in the cold of the winter nights. Another technique I used to raise the temperature in the greenhouse and in the planters at night was water jugs and barrels. I have four 55 gallon water barrels in the greenhouse, the blue barrels in the pictures. I have heard that these larger containers do not work as well as smaller ones. The idea though is that the water heats up during the day and then slowly releases its heat during the night. The more water you can get in the greenhouse, the warmer you can keep it during the night.
The challenge is to find ways to keep your crops from freezing at night. Obviously there are artificial ways of heating greenhouses, but I wanted to avoid the cost of these and also the dependence upon them. With the water jugs and double plastic covering, I was able to keep the temperature next to the plants up to 14 degrees warmer that the outside temperature. I measured temperatures throughout the month of December. There was a streak of about 12 days where the low temperature in the night was in the teens outside and the greenhouse was always in the 20's. I did measure a few days as low as 21 degrees in the double-covered, water-jugged planter, but everything survived. Of course, during the day, the temperatures are significantly warmer, up to 40 degrees warmer. Even on cloudy/smoggy days, it was at least 25 degrees warmer.
The book I mentioned above The Winter Harvest had many great suggestions, many of which I have talked about already. The other thing I learned from this book was what types of vegetables I could grow in the winter time. They must be cold-hearty vegetables, since temperatures still got down to below freezing inside the greenhouse at night sometimes. The cold-hearty vegetables are the root crops (beets, turnips, carrots, onions, radishes, etc.) as well as the leaf vegetables (lettuce, spinach, celery). I also planted broccoli, kohlrabi, and celery. Everything came up, nothing died due to cold.
Things do grow much slower in the cold weather, but they do grow. Out of everything I planted, the radishes and turnips performed the best in that they grew the quickest. Here is a picture I took today:
Far left - onion (can't see them).
Near left - spinach, lettuces.
Near right - turnip (harvested all) lettuce, beets, carrots, broccoli (bolted to seed).
Far right - radishes, kohlrabi, carrots, celery.
One problem I hope to solve this year is to have onions ready when my other salsa vegetables are ready. Every year my onions are so far behind the rest of my vegetables that I can never use them for salsa making. I have plenty of onions already growing in the greenhouse so they should be ready with the other vegetables this year.
