Kale, a member of the Brassica oleracea species, is a popular garden green known for its high nutritional value and remarkable resilience. Gardeners often wonder how much cold this leafy vegetable can handle, especially as the first frost approaches. Kale’s reputation as a champion of the cold is well-earned, as it possesses inherent mechanisms that allow it to survive temperatures that would destroy many other common garden vegetables.
Kale’s Tolerance to Freezing Temperatures
Kale is highly frost-tolerant and belongs to the family of hardy cool-season crops, meaning it thrives when temperatures drop. A mature, established kale plant can typically withstand temperatures down to approximately 10°F (-12°C) without suffering significant damage. Some of the most cold-tolerant varieties, such as ‘Winterbor’ or ‘Redbor,’ can survive brief dips closer to 0°F (-18°C), provided they have been gradually exposed to cold. The ability of kale to endure freezing temperatures is significantly increased by a process called cold acclimation, or “hardening off.” This involves the plant being slowly exposed to lower temperatures, which triggers its internal defense mechanisms. Younger seedlings are more susceptible to damage and may be harmed by temperatures below 25°F (-4°C). For best survival, kale should be planted in late summer or early fall, allowing it to mature and acclimate before the coldest weather arrives.
How Cold Weather Changes Kale’s Chemistry
The plant’s resilience in freezing conditions is rooted in a biological process known as cryoprotection. When the ambient temperature begins to fall, the kale plant responds by modifying the composition of its cellular fluids. This adaptation involves the conversion of complex starches stored in the plant into simpler, soluble sugars like glucose and fructose. This sudden increase in sugar concentration acts as a natural antifreeze, lowering the freezing point of the water inside the plant cells. By preventing the formation of sharp, damaging ice crystals within the cell walls, the plant protects its internal structures from rupture and desiccation. This shift in chemistry is the science behind kale’s ability to survive a hard freeze.
Maximizing Flavor After the First Frost
The same conversion of starches into sugars that provides cryoprotection also has a delicious culinary consequence. The resulting higher concentration of soluble sugars imparts a noticeably sweeter flavor to the leaves. This process also tends to reduce the levels of certain compounds, such as glucosinolates, which are responsible for the vegetable’s slightly bitter taste. Therefore, the optimal time to harvest kale for the best flavor is after the first substantial frost or a series of cold nights. Waiting until a light to medium frost has occurred ensures the leaves have undergone this sweetening conversion, resulting in a milder, more palatable green. Harvesting can continue throughout the winter, as the cold temperatures maintain the enhanced flavor profile.
Winter Protection for Extended Harvesting
While kale is a tough plant, its natural tolerance limit can be exceeded during extreme cold snaps. To push the harvest window well into deep winter, gardeners can employ various physical protection methods. A simple and effective technique is covering the plants with a row cover, which is a lightweight fabric draped over hoops to create a low tunnel. This structure traps heat radiating from the soil and can provide several degrees of frost protection. For more severe weather, a cold frame or a high tunnel offers a more robust shield against sub-zero temperatures and wind. Applying a thick layer of organic mulch, such as straw or shredded leaves, around the base of the plants also helps to insulate the roots and stabilize the soil temperature. These layers of protection mitigate the effects of extreme cold, allowing the kale to remain harvestable.