What Causes Kale to Bolt?
When you see your kale plant flowering, it’s essential to remember kale is a biennial. Like many two-season plants, kale spends its first season growing vegetatively. Once the kale plant is exposed to wintry weather, it’s programmed to flower. Most biennials do this in their second year. So what causes kale to bolt in its first year? The simple answer is human eagerness. As a cool-season crop, we are eager to get this biennial into the ground as early in the spring as we can. We may even start our kale plants indoors to get a jump on the growing season. But cool weather can trick kale into thinking winter has come and gone. This process of initiating kale plant flowering is called vernalization. And unfortunately, there are no hard and fast rules when it comes to avoiding vernalization of kale plants. For many biennials, vernalization requires exposure to temperatures below 50 degrees F. (10 C.) for 8 to 10 weeks. The effect of low temperature is additive, so a few cold snaps with warm weather in between can be sufficient to cause bolting kale plants to develop later in the season. Variation in vernalization requirements can also vary depending upon the age, species and variety of the plant.
How to Stop Kale Bolting
Understanding what causes kale to bolt is one thing, preventing bolting kale plants is another. Here are few tips to try in order to avoid kale plant flowering in the first growing season:
Direct seed – Young kale plants are immune to the effects of vernalization, so directly-sowing kale seeds into the garden 6 to 8 weeks before the last frost date can prevent bolting kale plants.
Delay transplanting seedlings – Once kale seedlings have approximately 8 leaves, they become receptive to the effects of vernalization. If you start your kale seedlings in the house or buy your plants, hold off transplanting kale into the garden until the last frost date has passed.
Warm the garden soil – Use black plastic or row covers to raise soil temperature and keep those eight-leaved kale seedlings warm. With this clever hack, gardeners can take advantage of early planting, but still prevent kale plant flowering in the first year.
Choose bolt-resistant varieties – Select kale varieties like Red Ursa, Premier (aka early hanover) or Vates. These varieties have shown resistance to summer bolting.
