As of February 22, here are our winter gardening findings so far. The tunnel did indeed do better at sloughing off snow, but it added a lot of air mass for the water to warm and I think it was quite a bit colder than the make shift cold frame was. This along with the colder temps in December and January resulted in the cessation of growth in all plants. However, all plants survived the coldest months of the winter: the end of December and the beginning of January.
As expected, the Miner's lettuce was the most cold tolerant. It lost no leaves and indeed showed no evidence of damage! It was also very delicious and is still making great salads! I anticipate if I planted enough of this we could have fresh garden salads all winter. The chard lost it's biggest, outer leaves when the temperatures reached about -17. The inner leaves however, are doing quite well and I expect them to grow and provide an early season harvest as soon as things warm up in the next couple months. The beets were a touch more cold hardy than the chard. They didn't loose their large, outer leaves, but they have stopped growing. I assume they will be ready for a spring harvest as well.
I planted some early, frost hardy peas in mid-January as the soil was still workable and warm in all but one small corner of the cloche. The soil seems to be staying warm, but the air gets very cold when a cold front comes through. We will see how they do.
I have also figured out what happened to my spinach. The slugs!!!! They eat it as soon as it sprouts! I've set out traps, but they don't seem to be enough. Though I catch quite a few, one slug can eat an entire spinach seedling in no time. I may make more traps, or start the spinach inside, though the seeds say "not recommended to start indoors. It's my hope that starting them indoors will give them a jump on the slugs so they will survive a slug attack, rather than being eaten in a single night.
After this winter's experiments, here are the changes I am planning on for next year:
1. Re-orient garden beds so the long side faces south. In addition I plan on putting the winter garden in the southern most beds. Their current location is shaded by the house during a good part of the day. I didn't anticipate this because they are not shaded much in the summer, but the angle of the sun changed in the winter.
2. Create new insulated cold frames (perhaps wood lined with foam so we don't have to use the bulky straw, and so that the top "glass" can get a better seal than with straw.) Insulate the north east and west sides. The south side will be glass, plexi or polycarbonate.
3. Modify the shape of the cold frame so the "glass" is at an angle to slough off snow, but avoid the hoop or a tall cold frame so there is not too much air space for the thermal mass to heat.
4. Maintain a ratio of water (thermal mass) to glass of 1.7 gallons per 1 sq ft glass. (or something like this, Anthony knows the ratio better than I do.)
5. Reduce the surface area of the "glass" or plastic so all glass is facing south and the rest of the coldframe is insulated and lined with thermal mass.
6. Plant our winter garden sooner. The books I read said to plant the winter veggies in "late summer". To me, that means late August. That didn't work so well. The garden center here said that "late summer" for Colorado is mid-July. So I will plant sooner next year. The veggies keep well over winter but as Elliot Coleman explained so beautifully in the "Four Season Harvest", we are trying to extend the harvest time, not the growing season. When it gets cold outside, your plants need to be of an edible size because they will not continue to grow much over the winter.
We had no trouble with over-heating, even on very sunny days here in Colorado. The water did an excellent job of moderating temperature. This was one of our greatest concerns as people constantly told us that a winter garden was impossible here due to the temperature swings. However, the thermal mass was sufficient to counter act this problem. I am glad we were not deterred by those opinions, but rather took them into account and worked them into our plan.
Over-all I am very pleased with our experiment. I think we will be much better equipped next winter. :)
Thanks for reading, God be with you on your journey!