Cheating Winter: The Little-Known Truth about Frost and Freeze Tables
My summer garden is beginning to show signs of fading. My poor tomatoes have surrendered to early blight, and my squash are just plumb tuckered-out from cucumber beetles and powdery mildew. But that’s okay! I’ve canned quarts upon quarts of tomato sauce and salsa verde, I’ve run out of room in the chest freezer, and the pantry is full.
Besides, I’m not saying goodbye to my garden anytime soon (though I will be putting some plants out of their misery!). In fact, I’m working as fast as I can to get new seeds and transplants in the ground. I plan on growing vegetables continuously through fall and winter, not in an expensive greenhouse or high tunnel, but right in my backyard garden.
If you’ve had a look at the tables floating around the web that show the minimum temperature limits for garden vegetables, you might quickly conclude that winter gardening is impossible for you in your zone. Here’s one of these tables that I’ve shared before, which I based on published literature and cooperative extension publications.
Based on this table, you might think that gardening ends when winter temperatures plummet to 20 degrees Fahrenheit or below, but I’m here to tell you that there’s no need to throw in the trowel. I’ve grown most of these crops on the right side of this table through our zone 6b winters, with repeated dips below 0 degrees F. These cold-tolerance lists are just guidelines — you can cheat winter!
It’s More Than About Temperatures
Cold-tolerant plants have unique adaptations that help their tissues better respond to freezing conditions. They compartmentalize freezing, by moving most of the water outside of the plant cell to freeze (which is why frozen plants appear wilted but then quickly spring back to life when temperatures thaw). They also accumulate cold-tolerance proteins and sugars (their own sweet antifreeze), which act to lower the freezing point of water inside the plant cell and prevent the initiation of ice crystals. They’ll even change the composition of the fats in their cell membranes, giving them a little more flexibility should any ice crystals form. Warm-season crops like tomatoes and corn don’t have these adaptations, which is why they turn to mush after a freeze, a result of their cells bursting as water expands (akin to a bottle of water exploding in your freezer).
So if cold-tolerant plants can handle some freezing, what is it that really damages them? In most parts of the United States, it’s simply desiccation — plants have already moved water out of cells, and the low humidity and harsh winds of winter dries out their tissues even further. Early freeze damage frequently shows up first at leaf margins, producing tip burn, or dried-up edges. And cold-tolerant plants don’t turn to mush when pushed beyond their limits — they simply look dehydrated and brown. So it’s desiccation, not cold temperature per se, that damages plants.
Freeze-Wrap Your Garden
With that little bit of understanding of how winter actually injures plants, it’s not hard to see how to cheat the system. You can push the minimum limits of those cold-tolerance tables by simply giving your plants some protection, either in a cold frame or in low tunnels (my personal favorite). It’s easy to think that these protective structures work simply by trapping some heat from sunlight during the day and releasing it at night, but there is much more to them than that. Think of it as freeze-wrapping your garden. In your chest freezer, freezer-burn occurs due to the sublimation of frozen water from the tissues of meats and veggies — you can prevent it by increasing the humidity in your freezer (hard to do), or more commonly by just making sure you have an air-tight seal in the packaging around your foods, or encasing foods in frozen water. You can do the same by “packaging” your garden beds into low tunnels or cold frames, which increases the humidity inside the structure (reducing sublimation of water from plant tissues), and protects the plants from drying winds — very much the way a good snowfall can is known to “insulate” plants.
How big of a cheater am I? Last winter, I had multiple test plots of lettuce in garden beds — some grown in the open, and others in low tunnels under various thicknesses of Agribon fabric row cover or clear plastic sheeting. Lettuce should die at temperatures below 28 degrees Fahrenheit according to the table, and some varieties did in the unprotected beds, but lettuce in all of the low tunnels easily survived temperatures into the upper teens. The real test occurred during the “Polar Vortex” that hit in January 2014, plummeting temperatures in my garden to -8 degrees F. Prior to the Vortex’s arrival, I covered some of the fabric-covered beds with either one or two additional layers (Agribon-30) as extra insurance. With the exception of the low tunnels covered with only a single layer of light-weight fabric cover (AG-19) or medium-weight cover (AG-30), lettuce in the tunnels survived (though a few varieties showed tip burn). How cold did it get in the tunnels? I happened to have a temperature sensor in a tunnel covered with fabric AND clear plastic row cover, and in this most-insulated tunnel, it got down to 10 degrees F (the fabric-covered tunnels may have dipped much colder). Temperature clearly isn’t the only factor that determines winter vegetable survival.
Don’t Fight the Cold
If you choose to protect your plants with low tunnels or cold frames in winter, don’t baby them. Just as the structural changes and sugars they accumulate are induced by cool weather, they quickly lose them when they are exposed to heat. Fabric row covers are a great option if you can’t be at home to vent tunnels covered with clear plastic, which you’ll need to do when temperatures are in the 40s or above. Cold frames or plastic-covered tunnels can easily reach the 80s or 90s F on a sunny 40 F winter day, which causes plants to lose those sugars and antifreeze proteins they’ll need to survive when temperatures take a plunge.
In my tunnels covered with plastic, I monitor them closely and vent them when it approaches 60 degrees F inside. I’m convinced that keeping plants cool is a key ingredient to successful winter gardening. Many of my farming friends are surprised at how well my low tunnels survived the Polar Vortex this past winter, when many farmers lost everything in their expensive and sophisticated high tunnels. Although some farm losses were due to collapses or tears in high tunnel covers, or from not having an extra layer of row cover immediately over plants inside the high tunnel, some losses may also been due to simply keeping tunnels too warm in the weeks preceding the Vortex (one remarked about how nice it had been to spend the New Year in their high tunnel wearing shorts!). Your plants will be hardier, and will also taste sweeter, if you keep them cool.
Additional Winter Gardening Resources:
These books are in my personal library, and I highly recommend them (these affiliate links cost nothing extra to you, but generate advertising fees that help me support this blog):
What vegetables could you be growing this winter? Check out this extensive list: my Winter Vegetable Planting Guide. And for more winter gardening resources from me, including how to build an inexpensive low tunnel, click the winter gardening tab above.
Want to learn more? Check out the following posts about winter growing, or subscribe to the blog — I’ll soon be posting reviews of tunnel row covers, a list of the most cold-hardy veggies for your garden based on my experience, recommended winter cultivars, and more!
Other Posts about Winter Growing from Mother of a Hubbard