Kale is a highly nutritious, leafy green vegetable that thrives in cooler temperatures. Proper timing is a significant factor in successfully growing this Brassica oleracea crop. Planting at the right moment maximizes the harvest and ensures the leaves develop their best flavor.
Understanding Kale’s Temperature Tolerance
Kale is recognized for its cold tolerance, making it a cool-season crop. The plants are robust and can generally survive temperatures as low as 10°F (-12°C), with some varieties withstanding 0°F (-18°C) for short periods. This hardiness is due to cold acclimation, where the plant accumulates sugars in its cells, acting as a natural antifreeze.
This accumulation of sugars is why the flavor of kale improves and becomes sweeter after exposure to a light frost. Conversely, kale struggles in prolonged high temperatures, typically above 80°F, which causes the leaves to become tough and bitter. When exposed to summer heat, the biennial plant may prematurely “bolt,” or send up a flower stalk, attempting to complete its lifecycle.
Timing for Spring Planting
The spring planting window is designed to allow the kale to mature and be harvested before the summer heat arrives. For those who want an early start, seeds should be sown indoors approximately four to six weeks before the average last expected spring frost date in your area. Starting seeds inside provides a jump-start, ensuring the young plants have a strong root system before moving outdoors.
Seedlings should be hardened off and transplanted into the garden relatively early, about one to five weeks before the last expected frost. Kale can tolerate the remaining light frosts of early spring, which helps them establish. Direct sowing seeds outside is also possible once the soil temperature reaches at least 45°F, though this results in a later harvest.
The main challenge with spring planting is the limited harvest window, which is cut short once daily temperatures consistently climb above 75°F. As temperatures rise, the leaves can become bitter, and the risk of bolting increases. Gardeners should aim to harvest the majority of their spring crop before the peak summer heat to maintain the best quality and flavor.
Timing for Fall and Winter Harvests
The fall planting schedule is often preferred by gardeners because the cooling temperatures enhance the kale’s flavor, providing a sweeter, more tender leaf. To calculate the correct planting date, one must work backward from the average first expected fall frost date in their region. The general rule is to plant seeds or transplants six to eight weeks before this date to ensure the plants reach maturity before the deep cold sets in.
To ensure a continuous supply of fresh leaves, succession planting is recommended throughout the summer, starting in mid-summer and continuing until approximately two months before the first frost. This strategy involves sowing small batches of seeds every two to three weeks, which staggers the harvest and maximizes the yield. Fall planting also benefits from naturally decreasing pest pressure as the weather turns colder, resulting in less damage from common insects like cabbage worms.
The plants will continue to grow until temperatures drop to about 20°F. In mild climates, the harvest can often extend throughout the entire winter, especially with the use of row covers or cold frames.