Sweeter After a Frost
How Winter Veggies Conquer the Cold
STORY AND PHOTOS BY BILL THORNESS
In my winter garden, I often hope for a frost. Just a light radiation frost — the kind that twinkles off the tips of the plants.
Not advection frost, the deep-freeze killer.
A light frost is literally a sugar coating for my winter vegetables, as it triggers a process inside the plant that makes the produce sweeter. The process begins in the fall, when the plants respond to shorter day lengths and nights that get progressively cooler. Those two effects must happen together to signal the plant to protect itself by “hardening.” (Not all plants have this ability; hence we don’t try to overwinter basil.) Also known as cold acclimation, hardening works on many levels, from cellular to physiological to biochemical.
An easy example is the thickness of the cell wall. Most fruits and vegetables have large, thin-walled cells with a high moisture content.
When we preserve our produce by freezing it, the process often ruptures those cell walls. When the frozen produce is thawed, it will be limp or mushy.
But during hardening, the plant slowly creates thicker cell walls, which are smaller and contain less moisture. There is water outside the cells (intercellular) that can freeze, but the cells themselves are better protected and so can continue to do what plant cells do: perform photosynthesis and respiration.
That’s right: Even during cold winter, the plants are still working. Of course, I see this as my Brussels sprouts, kale leaves, cabbage heads, and broccoli florets develop. It has always astounded me to see them growing in our short, cold, rainy winter days. How can they keep at it when the rest of the garden is dormant?
Turns out, the key lies in that sweetness that we crave from our winter vegetables. You can thank sugar.
Sugar is produced, along with oxygen, during photosynthesis. The plant takes in carbon dioxide and water and uses light to convert it to starch, which is how it stores energy. Enzymes convert the starch to sugar. So our vegetables have varying levels of sugar, mostly sucrose. In winter-grown root vegetables like turnips, parsnips, and carrots, the sugar-to-starch ratio is greater than in ones grown in the higher temperatures of summer.
Those boosted sugar levels are there to protect the plant. In cold-tolerant plants, the metabolism of sugar produces energy, which is given off when the external conditions are cooler. This radiational effect helps the plant “stay warm.”
But that’s not all. As sugars accumulate, they can decrease the amount of ice formed in the cells and thus reduce dehydration of the plant. Sugars can also protect the cell membrane from freezing. Amazing, yes?
Standing in my winter garden, I can see the varying effects of cold temperatures and frost on the edibles. The strap-like leaves of Lacinato kale will shrink and curl during an extreme cold spell. Broccoli and Brussels sprouts leaves will droop. But those reactions are defensive, and as the weather warms a bit, the plants perk right up. Sugar protected them, and their metabolic changes for winter acclimation have given them strong defenses.
A frost-tolerant plant can reduce or increase its respiration rate based on the surrounding conditions. A reduced respiration also reduces photosynthesis, causing the plant to grow more slowly during that period. But in the fall, when plants are in the process of hardening, a higher rate of photosynthesis than respiration triggers the production of higher levels of sugars, at the expense of vegetative growth. So even if the winter garden slows down, processes are still occurring that allow the plants to not just stay alive, but create energy in the form of tasty, nutritious sugar.
Other unseen processes are also at work to make a winter vegetable plant hardy.
The root system is developing at a higher rate than in summer plants. The root-to-shoot ratio will change, because a larger root system allows the plant to better take in nutrients when the colder winter soil is less microbially active.
The plant is also protecting itself against the wind. Winter winds can have a desiccating effect, drawing moisture from the plant and causing it to wither. One of my most delectable winter greens, tatsoi, grows in a low rosette form, and its layers of spoon-like leaves, sitting just a couple of inches above the ground, are below the layer of high wind speed.
Sometimes, though, the winter vegetables need help. Lettuce, chard, and bok choy have less frost-hardiness and so must be covered or contained in a cloche or cold frame to survive. Even some frost-tolerant plants, like beets, will benefit from a loose mulch-like straw around the crown of their roots so that wind won’t dry out the soil or pounding rains compact it. If you’re in a high-wind area, erecting a permeable windbreak — what the Brits call a “hurdle” — can give exposed plants a boost.
And in very cold conditions, like a week or so of persistent cold temperatures that drop below freezing, expect the beet tops to wilt beyond recovery. You can slow or prevent this by covering them with floating row cover or a cloche when a cold snap is predicted. Even if the leaves turn into a slimy mess, you may see new leaves emerge after a few weeks, or you may just have to be content with eating the sweet roots that are still being stored in nature’s refrigerator, your winter garden soil.
I’m often out in my garden on frosty days, enjoying the crispness of the air and the glittering grayness of the hoary ground. I’ll take care not to peek at the covered plants during the coldest spells so I don’t reduce the protective effect. Snow, especially, is a great insulator and should be left in place until it naturally melts.
On those days, I am mostly an observer of nature’s incredible adaptation. I get a sense of the sweetness in those winter vegetables, even if not on the taste buds. It is there, developing in the chill air.
Bill Thorness is the author of Cool Season Gardener and Edible Heirlooms. He is a Master Gardener in King County and lives in Seattle, where he spends winter days reading gardening books. Two 2015 releases that were helpful in creating this article are How Plants Work by Linda Chalker-Scott and The Gardener’s Guide to Weather & Climate by Michael Allaby, both in Timber Press’s Science for Gardeners series.