Are Brussels Sprouts Perennial? A Biennial Explained

Many gardeners treat Brussels sprouts as annuals, planting them for a single season’s harvest. However, Brussels sprouts are biennials, meaning their natural life cycle spans two years. In the first year, the plant focuses on vegetative growth, producing a tall, leafy stalk and the edible sprouts that form in the leaf axils. If left in the garden, the plant undergoes a second stage to complete its cycle.

The Brussels Sprout Life Cycle

As members of the Brassica oleracea species, Brussels sprouts require a long, cool growing season to mature properly. During the first year, the plant establishes a strong root system and a tall, sturdy stalk, with sprouts developing progressively from the bottom upwards. For the best flavor, many gardeners wait to harvest until after the plants have experienced a light frost, which makes the sprouts taste sweeter.

After surviving the winter, the plant’s biological imperative shifts for its second year. Triggered by the cold period, a process known as vernalization, and the return of warmer weather, the plant enters its reproductive phase. Its energy is redirected toward creating flowers and seeds. A tall flower stalk emerges from the top of the plant, a process called bolting, which yields flowers and, after pollination, mature seeds.

Overwintering for a Second Harvest

While the primary harvest is in the first year, it is possible to overwinter Brussels sprout plants for a smaller, secondary crop. Gardeners in milder climates, like USDA Zone 7 and higher, can leave plants in the ground with some protection. Applying a thick layer of mulch, such as straw or hay, around the base of the plants helps insulate the soil and protect the roots from harsh freeze-thaw cycles that can cause rot.

The harvest from a second-year plant differs significantly from the first. Instead of large, tight sprouts along the main stalk, you will get a crop of smaller side sprouts or tender, edible leaves that emerge before the plant bolts. Some plants may also produce a second, smaller set of sprouts at the base of the stem. These secondary growths are not as firm as the initial crop but are perfectly edible.

Managing Second-Year Growth and Bolting

As temperatures rise in the spring of the second year, the overwintered plant will inevitably begin to bolt. Bolting is a response to the lengthening days and warmer weather following the cold of winter. Once this process begins, the plant directs its resources away from sprout growth and toward reproduction, which can cause any remaining sprouts to develop a bitter taste.

When a plant starts to bolt, the gardener has a couple of options. The young, flowering shoots and buds that appear at the top of the plant are edible and can be harvested and cooked much like broccoli rabe, providing a final, unique harvest. Alternatively, if you are interested in saving seeds, you can allow the plant to flower fully. The flowers give way to seed pods that, once mature and dry, can be collected for future planting.