Plants need fresh air and good air circulation. This is true even for winter garden crops grown under cover. No matter what form of protection you use for your plants, you’ll still need to insure they get enough ventilation.
Fortunately your plants can survive short periods of being “closed up” when weather makes it impossible to do otherwise.
When Lettuce Needs Protection in the Winter Garden
When temperatures are forecast for 29ºF or below I cover my fall/winter lettuce beds.
Although lettuce can survive 28ºF , I’ve found the forecast is never exact.
Temperatures can easily slide to 27º when 28º or 29º was forecast. Below 28º without protection and you’ll most likely lose your lettuce.
3 of my 4 winter beds covered. I use concrete reinforcing wire and bend it to make the frame that holds the plastic.  It makes a low tunnel that withstands wind a lot better than the higher tunnels I use to use. And they don’t cave in from heavy snow either. They’re light weight and easy for me to place in and/or remove from the garden.
(My) First Step to Get Air Circulating After a Cold Spell
One year was unusually cold with lots of ice and snow. Winter garden crops remained covered and “closed” for more than two weeks. At the first opportunity I broke up the ice around the ends of the beds and opened each end of my low tunnels to let fresh air circulate.
If night time temperatures were expected to fall below 30º or so, I closed the ends again.
Your first step after a cold spell in your winter garden will depend on what type of system you have.
I open both ends of the tunnels if temperatures are ok in the day. When temperatures below 30º are forecast I can easily close the ends. When the forecast is for several or more days and nights above 30ºF, I just roll the plastic off to one side to expose the beds.
Lettuce inside the tunnel. This gives you a better look at the concrete reinforcing wire I use.
Reason Plants Need Air
They need air to synthesize their food from carbon dioxide (from the air) and water (from the soil).
Reasons for Good Air Circulation
Prevents heat from building up under plastic coverings.  This can happen even on cold days when the sun is bright.
Allows evaporation of excess moisture.
Keeps plants strong and rigid.
These things help prevent plant diseases and various pests that would otherwise thrive.
Uncover  the Winter Garden When Weather Permits
A bright sunny day with light winds and temperatures of 39º might feel too cold to you, but your plants will love it. It’ll help keep them strong and healthy.
Rain – Another  Benefit of Uncovering Your Beds
Plants need the rain in winter as well as summer.
If for some reason your soil becomes dry and temperatures drop below freezing, it can kill your plants even if they’re under protection. To withstand the cold, winter garden crops need sufficient moisture.  (Not soggy soil, but not totally dry either.)
As I mentioned in the post on Watering Guidelines “Don’t go by the top of the soil being dry. Push your finger into the soil.  If it’s moist 1 1/2 to 2 inches down — don’t water.”
In case you’ve not experienced this:
With temperatures at 32º, freezing rain, and their leaves encased with ice, most lettuces remain unscathed when temperatures rise again. (I’ve had this happen many times.)
Common sense will prevail here.  If you have any doubt about what the temperatures will be, cover the beds again.  Better safe than sorry.
If you have a system that’s easy to handle like mine, it’ll be easy to pull the plastic over the beds when unfavorable conditions are forecast.
Final Thoughts
Nature’s perfect covering for cold hardy plants is snow. It protects and still allows for needed air.
So if you get an unexpected snow that covers your lettuce (and temperatures are between 30º and 32º or warmer), it should be just fine. You can cover the beds again and not bother about the snow on the lettuce.
Man-made protection for plants in a winter garden is not as accommodating as nature’s, but with a little thought and effort we can make it work.
Lettuce and parsley inside the tunnel at the open end.
Lettuce before I had to cover it. You can better see the concrete reinforcing wire here.
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