The key to cold weather crops is to not let the roots freeze at the point where the leaves form. The plastic cups I used at night I also kept on during the days to create a heated micro climate. I took them off every third day to let the moisture and humidity out. At night I used a plastic painter's drop cloth to cover everything. I put the drop cloth on around 5 pm. Some day temperatures just broke 40, I kept the drop cloth on there 1 or 2 times for a full day. This created some heat for the bed. It did this on full sun days.
This method saves a few degrees of heat. That is all you need.
I only had to protect one main bed. The plastic sheet is a painter's drop cloth from Home Depot. It was under $5. It is translucent. It only allows some light to pass through it. If it was totally clear plastic, you would run the risk of over heating the plants and killing them.
I just covered the whole bed with the sheet and used the tomato cages to weight it down.
I still have three days until the temperature gets to the upper 30's at night.
Thanks! I live at 2,000 feet near Oakhurst, CA in the Sierra foothills -- light November frosts. I built frost protection for an 8'x8' plot with turnips and spinach by planting four 12" oak branches (1" diameter) at the four corners and a taller 18" oak branch with a curved top in the middle, running string around the four corner posts and then running it diagonally across the middle oak branch. I then placed a 0.7 mil clear plastic painter's drop cloth I got at Dollar Tree (one dollar) and attached the plastic to the outside string with ordinary vinyl-covered paperclips. Metal binder clips work too. The excess plastic hangs down, making walls for the enclosure, but any breeze will lift them so it doesn't get too hot during the day. We seldom have frosts with wind, so at night the sides hang down. It has taken up to 20 mile an hour gusts, with the wind going under and over the plastic. The paperclips are extremely versatile and easy to attach. You can sort of triple-wrap the plastic around the string before installing the paperclips. I noticed a cloud of small flies and a jack bee trapped at the corner closest to the sun and I made a dime-sized hole which they all found easily. I've also used plastic to wrap the top of a 2.5 x 8 French intensive bed that I wrapped under and over with 1/2" chicken wire. It has gaps close to the ground to let out hot air and creates a nice warm environment, gopher-free. Milton
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