How Hardy Is Lettuce?

Lettuce is generally regarded as being a hardy annual vegetable. But how hardy is it really? This year I have been experimenting to try and find the answer to that question. And a colder than usual November and December has been helping me in my research.

My amateur experiment involves growing several different lettuce varieties in my homemade cold frames, along with some other hardy greens like spinach, arugula, komatsuna, tatsoi and mizuna. The cold frames are 4 foot square, and about 8″ tall in front tapering to 12″ tall in the back. The top is a hinged wooden frame covered in spunbonded polyester row cover fabric (like Reemay or Agribon).

Why not cover the tops with plastic? That’s a good question, and one my wife asked recently. Covering the tops with plastic turns the cold frame into a mini greenhouse that can quickly overheat on a sunny day, even in winter. Since I’m not always at home to open them, if I went that route I would need to put an automatic opener on the cold frames.

For this experiment, the polyester cover is ideal. It provides only a few degrees of frost protection, but its main purpose is to keep the wind and snow out. The greens can handle the cold better than the wind or moisture. My goal this year was to find out just how much cold they can take.

The beds under the cold frames were planted in mid October. Six weeks later, in late November, the cold frames were full of greens big enough to begin harvesting. In the bed in the photo below (taken November 30th) from left to right we have lettuces Winter Density, Oak Leaf, Sea of Red, Radichetta, and Ruby. To the far right we have the Spinach Spargo.

Through the month of November we had 10 nights of temperatures below freezing, the lowest temperature being 23F. That’s 10 cycles of being frozen at night, then gradually thawing the next day. It didn’t seem to bother the plants any. On December 1st we started harvesting some of them. The lettuces were still not quite full sized, but there was plenty to eat, and the leaves were tender and mild. It’s hard to believe they have spent that much of their lives frozen!

It’s still a matter of scientific debate regarding the exact mechanisms plants use to survive freezing temperatures. It’s thought that lettuce and other hardy plants have the ability to move water out of their cells into the intercellular spaces so that the cell walls don’t rupture when they freeze. Of course different species vary greatly in their adaptability to cold.

The secret to harvesting lettuce in winter is to not harvest while it is frozen. Instead, wait until temperatures get above freezing and the plants can naturally defrost themselves. If you harvest while it is frozen you will wind up with limp and slimy lettuce.

The other cold frame in my experiment was planted with (from left to right) arugula, komatsuna, Yukina savoy (tatsoi), Spotted Trout lettuce, and Gigante Inverno spinach. The photo below was also taken on November 30th.

So far we have harvested all the tatsoi and some of the arugula and Spotted Trout lettuce. All were in great shape and seemingly unaffected by the cold. The spinach is not quite big enough to harvest, so it will likely have to wait until late winter or early spring. The komatsuna is reportedly very hardy, so we left it growing longer.

In early December the weather turned even colder. So far it has been below freezing every night, with the lowest temperature being 14F. For three days last week the high temps did not even get above freezing. When it finally did get above freezing, I harvested some more lettuce. This lettuce is just as good as that harvested before the weather turned colder. The lettuce is not growing much in the short days of winter, but it is surviving the cold. In the bowl below we have Winter Density, Oak Leaf, Ruby and Spotted Trout.

I have now begun replanting some of the areas where plants have been harvested. I have been planting with seedlings of mizuna and tatsoi, as well as Rouge d’Hiver lettuce. Given their late planting, it will be interesting to see if any of these plants survive the winter. The photo below was taken this morning, while temps were around 20F. The plants in there were quite frozen.

So were the plants in the other cold frame. It will be interesting to see how the komatsuna plants taste when we finally harvest them.

After the last two photos were taken, it started snowing. Tonight and later this week the low temperatures are supposed to be below 10F. Now the plants and the cold frames will really get tested!

I will report later in the month on how the lettuce and other plants are doing. In the meantime, we’ll enjoy salads from the lettuces we’ve already harvested. Good night, little lettuce plants, and sleep tight under those cold frames!