Fennel (Foeniculum vulgare) is a fragrant plant appreciated worldwide for its aniselike flavor, used both as an herb and a vegetable. Whether it returns every year is a common source of confusion. The answer depends on the specific variety grown and the local climate. Understanding the difference between the two main types of fennel is the first step in predicting its behavior.
The Dual Identity of Fennel
The term “fennel” refers to two distinct forms that behave differently in the garden. Herb Fennel, often called common or wild fennel, is primarily grown for its delicate fronds and aromatic seeds. This variety (Foeniculum vulgare) is a short-lived perennial that thrives in moderate climates.
Florence Fennel (Foeniculum vulgare var. azoricum), also known as finocchio, is grown for its swollen, bulb-like base. This bulb is formed by the overlapping stem bases and is used as a vegetable. Although technically a biennial, Florence Fennel is typically treated as an annual plant by gardeners.
Perennial Nature and Self-Seeding
Herb fennel, the non-bulbing variety, is the true perennial form. Its deep, woody taproot allows it to survive mild winters, generally in USDA Hardiness Zones 6 through 10. In these areas, the foliage dies back in autumn and reliably regrows from the root crown each spring.
Even in colder regions where the root might not survive, herb fennel frequently appears to return due to its aggressive self-seeding habit. The plant produces a large number of seeds in umbrella-shaped clusters, called umbels, which readily fall and germinate the following season. This prolific dispersal can lead to fennel naturalizing, making it seem like the original plant is continually coming back.
Cultivating Florence Fennel
Florence Fennel is the exception to the perennial rule because of its intended harvest. Although botanically a biennial, gardeners treat it as a single-season crop. Harvesting the bulb requires cutting the plant at its base, which removes the swollen stem base and prevents the plant from returning the following year.
The bulb variety is highly sensitive to environmental stress, which can trigger bolting. Bolting is the premature formation of a flower stalk, causing the bulb to become woody, tough, and inedible. Inconsistent watering, high heat, or sudden temperature drops can cause this shift. Consistent moisture and planting during cooler periods are important for a successful harvest. The plant must be harvested when the bulb reaches the size of a tennis ball and before it begins to flower to ensure a tender, crisp bulb.
Controlling Growth and Spread
The self-seeding nature of herb fennel requires proactive management to prevent it from dominating the garden. The most effective method for containment is deadheading, which involves removing the flower heads before they set and drop seeds. Remove the seeds once the yellow flowers turn brown, as they are loosely held and can easily spread.
Gardeners should be careful when choosing a planting location, as fennel is known to cross-pollinate with other members of the carrot family, such as dill or cilantro. This cross-pollination can result in undesirable flavors in the harvested seeds of both plants. In colder areas, applying a thick layer of mulch around the base in late autumn can help insulate the taproot from deep freezing temperatures.