Turnips are a resilient, cool-season crop belonging to the Brassica family, cultivated for both their leafy greens and fleshy taproots. Unlike tender summer vegetables, turnips possess a natural hardiness that allows them to thrive in the crisp air of autumn and early winter. Understanding the turnip’s physiological response to cold is the key to successfully extending the growing season far beyond the first autumnal frost.
Defining the Turnip’s Cold Tolerance
Turnips are classified as a frost-hardy vegetable, possessing a significant tolerance for temperatures below the freezing point of water. Turnips can often survive brief dips down to about 20°F (-6°C) without suffering catastrophic damage. The primary danger comes not from a light frost (28°F to 32°F), but from a hard freeze, which is a sustained period where temperatures fall below 25°F (-4°C).
The different parts of the plant exhibit varying degrees of vulnerability to the cold. The leafy greens are the most susceptible to frost damage and may wilt or rot if exposed to repeated hard freezes. Conversely, the root is insulated by the surrounding soil, allowing it to remain viable even when the foliage is damaged, provided the ground does not freeze solid. Turnips can continue growing, albeit very slowly, until temperatures drop to approximately 15°F (-9°C).
The Flavor Transformation: How Cold Improves Taste
Exposure to cold temperatures triggers a biochemical change within the turnip root that improves its flavor profile. This survival mechanism, known as cold acclimation or hardening, is essential for the plant to withstand freezing conditions. The plant’s cells respond to the stress of the cold by converting complex starches stored in the root into simple sugars.
This increase in sugar acts as a cryoprotectant, serving as “nature’s antifreeze” by lowering the freezing point of the water inside the cells. This prevents the cell water from turning into damaging ice crystals, protecting the cellular structure from rupturing. This elevated sugar content results in a root that tastes noticeably sweeter and less bitter or pungent than a turnip harvested before the first frost.
Extending the Harvest Window with Protective Measures
Gardeners can intervene to maximize the turnip’s natural cold tolerance and extend the harvesting season well into winter. The most effective strategy for protecting the root is applying a thick, insulating layer of deep mulch over the planting area. Materials like straw, shredded leaves, or hay should be piled several inches deep to prevent the soil from undergoing a deep, solid freeze.
This layer of mulch acts like a blanket, trapping the residual heat of the earth and protecting the roots from the most damaging extreme cold. Protecting the greens, if desired for harvest, requires a different approach, such as using floating row covers or a cold frame. These covers provide an air buffer and shelter the foliage from wind and direct contact with frost, preventing the leaves from succumbing to rot.
Lightly watering the turnip bed a day or two before a predicted hard freeze can also assist in root protection. Moist soil retains heat much better than dry soil, which helps to insulate the ground and keep the root zone slightly warmer through the cold night. By employing these techniques, turnips can often be left in the ground and harvested as needed until the soil becomes too solidly frozen to dig.