This discussion of cold frames will focus on how to improve cold frame function, as well as vegetables that lend themselves to cold frames.
Why do I think it’s worth considering a cold frame in Virginia? I’ve read books written by people who live in Maine and Nova Scotia who use cold frames all winter long. If they can do it, why can’t we? At least it’s worth a try.
Cold frame function improvements can be explained using the analogy of layering clothes for the coldest days in the winter – each layer & the tiny airspace in between keeps you toasty warm. By using floating row covers inside your cold frame or sheets, blankets, or tarps over the cover, you may be able to improve cold frame heat retention. This will only be necessary for periods of extreme cold.
Researchers at Colorado State University (CSU) discovered the following:
- Floating row covers provide 2 – 4 degrees of frost protection and allow sunlight to penetrate, therefore can be left in place permanently.
- Clear plastic sheeting added 3 – 6 degrees of frost protection.
- When reflective space blankets, with the shiny side facing the ground (so that heat is reflected back), were placed over plastic sheeting on top of a cold frame, freezing was prevented even when outside temperatures dropped below zero the night following a sunny day. However, space blankets (or sheets or blankets) must be removed daily to allow sun to reheat the ground and air inside the cold frame.
Another idea that’s been tested is to string incandescent Christmas tree bulbs inside your frame and plug them in on those super-cold nights. CSU found that another 6 -18 degrees of protection was provided by a string of lights, and when a space blanket was combined with lights, 18 – 30 degrees of protection was added. If using lights in your cold frame, as a safety precaution be careful not to have hot incandescent bulbs touch plants, floating row covers, plastic, or space blankets. I made wire wickets to drape space blankets over.
During extreme cold, use your judgment to determine whether to uncover your cold frame during the day. The critical factors are air temperature and cloud cover because solar re-heating of soil must happen for the box to retain warmth at night. Making these decisions is more of an art than a science and can just be a matter of trial and error.
If you can find an inexpensive thermometer to hang inside your cold frame, you can check daytime temperatures yourself. For the curious, a remote wireless thermometer inside a cold frame will enable you to see exactly how cold the cold frame gets during extreme cold.
Another consideration is wind. We get many windy winter days where reported wind speeds can be 60 mph or greater. After investing the effort to build and use a cold frame, you probably don’t want it to blow to Richmond! Lashing the lid on the cold frame is wise.
Some of my plants are taller than my cold frame and their foliage touches the cover. Plant tissue may be damaged if in direct contact with the cover. In the past, I have cut off some foliage of carrots, so that it will not come in contact with the cover, leaving just enough to allow photosynthesis to continue (i.e. 5 – 6” tall). There was no apparent negative effect of doing this. In subsequent years, I trimmed back foliage from beets as well.
Finally, resist the temptation to water your cold frame. Condensation will build up daily and I personally have found it rarely necessary to water. At most, you can include your cold frame in once a monthly watering. Remember, we want the veggies to remain dormant; we are not trying to get them to actively grow because they are already mature.
What veggies work best in a cold frame? Forget about tomatoes, peppers, or other warm season vegetables. While a cold frame can enable seed starting earlier than normal in the spring by warming the soil, a cold frame will not support a hothouse garden during winter. Instead, a cold frame is a way to keep mature cool season plants in a dormant state until you are ready to harvest them. Good candidates are: lettuce, spinach, and other greens; beets; parsnips; kale; chard; carrots; radishes; turnips; cabbage; Brussels sprouts; and leeks. Most of these vegetables can survive if temperatures are kept in the mid- to high-20s; some can withstand temperatures in the teens. Some, particularly root crops, often become sweeter as a result of cold temperatures and can actually withstand a freeze and remain edible as long as they are not subjected to freeze-thaw cycles.