The question of whether kale is a perennial vegetable is common, stemming from its ability to survive multiple seasons in many gardens. Common varieties of kale, such as curly and lacinato, are botanically classified as biennials. A biennial plant completes its entire life cycle over two growing seasons, focusing on vegetative growth in the first year and then flowering, setting seed, and dying in the second. Many gardeners cultivate kale as an annual, harvesting the leaves and removing the plant before it enters its second-year reproductive phase. This confusion arises because kale’s cold tolerance allows it to behave much like a perennial in specific climates.
Understanding Plant Life Cycles: Annual, Biennial, and Perennial
Plant life cycles are defined by the time it takes for a species to grow from seed, produce new seeds, and finally die. An annual plant executes this entire process within a single growing season, often completing its cycle in just a few months before dying with the first frost. Common garden vegetables like corn and many types of lettuce fall into this category.
Biennials require two full seasons to complete their reproductive cycle. The first year is dedicated to vegetative growth, producing leaves, stems, and a strong root system. The plant then enters a dormancy period, often triggered by winter cold, which is necessary to induce flowering the following spring.
Perennial plants, by contrast, live for more than two years, and many complete a reproductive cycle annually, returning year after year from the same root structure. While most kales are biennials, specific, less common varieties like Tree Kale or Daubenton’s kale are true perennials, surviving for many years and often grown from cuttings rather than seed.
Why Kale Appears to Be a Perennial
The biennial nature of kale is governed by vernalization, a period of sustained cold required to trigger flowering. For most varieties of Brassica oleracea, this involves exposure to temperatures below 50°F (10°C) for eight to ten weeks. Once the plant is large enough, this cold exposure signals that it is time to shift energy from leaf production to reproduction when warmer weather returns.
Kale’s cold hardiness is the main reason it is mistaken for a perennial. It can tolerate temperatures as low as 5°F (-15°C) and often continues to grow and produce leaves through the winter in milder regions. In USDA Hardiness Zones 7 to 10, winter temperatures are often cold enough to induce vernalization but not severe enough to kill the plant.
When the kale plant survives the winter and the weather warms up in its second year, it rapidly sends up a central flower stalk, a process known as bolting. The plant dedicates its energy to producing flowers and seeds, and the leaves become tougher and more bitter. This bolting marks the end of the biennial’s life cycle, confirming it only lasts for two seasons, unlike a true perennial.
Practical Tips for Multi-Season Production
Gardeners can maximize the longevity of their kale plants by utilizing specific harvesting methods. The “cut-and-come-again” technique involves picking the older, outer leaves while leaving the central growing point, or terminal bud, intact. This encourages the plant to continually produce new leaves from the center, prolonging the vegetative phase and ensuring a steady harvest.
Avoid removing more than one-third of the leaves at any single harvest to prevent stressing the plant. Regularly harvesting the older foliage redirects the plant’s energy toward new growth rather than maintaining aging leaves. This practice, combined with consistent fertilization, helps sustain the kale for a longer period.
To successfully overwinter kale in colder climates, such as USDA Zones 4 to 6, growers often use protective measures. Strategies include applying a thick layer of organic mulch around the base or covering the plants with a row cover or cold frame. These methods insulate the roots and foliage, allowing the plant to survive the deep freeze and resume production in the spring until it eventually bolts.