The leafy green kale, a cultivar of Brassica oleracea, is one of the most resilient vegetables a gardener can grow. This classic cool-season crop thrives when many other plants struggle. Its ability to withstand cold temperatures allows for harvests well into the winter months. Understanding the plant’s temperature preferences is necessary for maximizing both yield and flavor.
Ideal Growing Conditions
Kale achieves its most vigorous growth and best flavor profile when air temperatures are consistently cool. The optimal range for peak productivity is generally between 60°F and 70°F, though it tolerates a wider range from 55°F to 75°F. Growth slows significantly once temperatures move outside this preferred bracket, whether too cold or too warm. To encourage strong root development, the soil temperature for planting should ideally be around 60°F to 65°F. Planting in the spring or late summer allows the kale to mature during the naturally cooling conditions of fall, when its flavor is often at its best.
Surviving Freezing Temperatures
Kale’s exceptional cold tolerance allows it to remain a garden staple long after the first frost. Most mature plants can survive temperatures as low as 10°F without severe damage, and some varieties endure brief dips toward 0°F. Young seedlings are more vulnerable but generally withstand temperatures around 25°F.
This hardiness is due to cold acclimation, where the plant prepares for freezing conditions by accumulating sugars within its cells. These sugars act like an internal antifreeze, lowering the cell sap’s freezing point. This sugar concentration increase is why a mild frost improves the flavor of the leaves, making them sweeter.
Gradually exposing plants to cold, known as “hardening off,” prepares them for deep freezes. A sudden, sharp temperature drop without this preconditioning can still damage the plant. While light frost improves flavor, prolonged hard freezes cause water in the tissues to freeze, leading to cell rupture and leaf collapse.
Managing High Heat
While kale excels in the cold, it struggles significantly with sustained high temperatures, which is why it is not a summer crop in many regions. Heat stress begins when air temperatures remain consistently above 80°F or 85°F. High heat causes the leaves to become tough, fibrous, and noticeably bitter, reducing their quality. The most significant negative effect is the triggering of premature bolting, where the plant rapidly sends up a tall central flower stalk. Once the plant diverts its energy into seed production, the leaves become highly bitter and the harvest season ends.
Protecting Kale from Temperature Stress
Gardeners can take proactive measures to extend the kale harvest through both temperature extremes. To protect plants during severe winter cold, apply a thick layer of organic mulch, such as straw or shredded leaves, to insulate the roots and stabilize soil temperature. Simple row covers or cloches can also provide a sufficient boost in temperature to keep the plant safely above the freezing point.
In hot weather, mitigating heat stress involves strategic placement and shading. Planting kale in a location that receives morning sun but is shaded during the harsh afternoon hours prevents leaves from scorching. Using a shade cloth during the hottest summer months diffuses intense sunlight and helps keep the air and leaf temperature lower. Consistent and deep watering in the early morning also helps cool the roots and soil, preventing drought stress that accelerates bitterness and bolting.