Just How Cold Hardy is Lettuce Anyway?

Newly sprouted lettuce in my
garden on Nov 25th.  It
easily survived the freezing
temperatures last night.

Here in Southeastern North Carolina lettuce is a winter crop, or at least a cool-season crop avoiding the 5 months of the year (May - September) that can rightfully be called summer.  I first grew winter lettuce several years ago and treated it like it was a tender summer garden plant, dutifully covering it with glass frames and old bed sheets on sub-freezing nights.  The lettuce grew and prospered under that treatment, seemingly confirming my belief that I had to do all that work to get lettuce to grow well in the winter.

Then last winter I neglected to do anything to my lettuce on the coldest night of the year.  The temperature got down to 20 degrees Fahrenheit. (-6.7 degrees C)   What did I find the next morning?  Perfectly healthy lettuce plants that continued to grow and grow into the Spring.

I don't know if this is true with all varieties of lettuce but it seems that for the varieties I'm growing, their ability to handle cold winter temperatures is nearly as good as hardy brassica and related species like Mustard Greens, Collard Greens, and Kale.  I've also grown Spinach under the same cold-weather conditions with an equal degree of success.

Once the weather warms in earnest by late April, each lettuce plant grows a tall flower stalk that produces a copious number of seeds.  This flowering process is often called bolting.  Once ripe and dry, the seeds can easily be stored over the summer and replanted in the fall.  Many authorities suggest that lettuce leaves acquire a bitter taste when the plant is bolting.  I haven't observed this bitter taste develop myself, but when the plant is old and large enough to bolt the remaining leaves are usually so thick and tattered you wouldn't try to eat them in the first place.

Lettuce growing in my garden in mid-February, 2012

Mixed lettuce varieties, mid-February 2012.