Fennel, with its feathery fronds, anise-flavored bulb, and aromatic seeds, is a versatile plant cultivated for distinct culinary purposes. Successful growth of the bulb variety, Florence fennel, relies on precise timing to manage its sensitivity to environmental stress. The goal is to ensure the plant matures during moderate temperatures and consistent moisture, achieved by carefully selecting the planting date to avoid intense summer heat.
Primary Planting Window Spring and Early Summer
The first opportunity to establish fennel is in the spring, timed for an early summer harvest. Seeds can be started indoors four to six weeks before the last expected frost date. This indoor head-start allows the plant to gain strength, but requires careful handling due to the plant’s sensitivity to root disturbance.
Direct sowing is recommended only after all danger of frost has passed. Seeds require a minimum soil temperature of 50°F to 60°F for reliable germination. Transplanting is tricky because the plant forms a deep taproot; injury can shock the seedling and trigger premature flowering. For this reason, many growers prefer direct sowing or using biodegradable pots to minimize transplant shock.
Succession Planting and Fall Timing
Fennel is a cool-season crop, and its biggest challenge is bolting, the premature growth of a flower stalk. Bolting is typically triggered by stress, most often heat or inconsistent moisture, which causes the plant to bypass bulb formation. Once the plant bolts, the bulb becomes woody, tough, and inedible, making timing the primary defense against crop failure.
To mitigate the risk of bolting, succession planting involves sowing small batches of seeds every two to three weeks in early spring. This method ensures that if one batch is lost to an unexpected heat wave, another is following, extending the harvest window.
The most reliable planting time for Florence fennel is mid-to-late summer for a fall or early winter harvest. By planting in mid-summer, the plant’s vegetative growth occurs during the heat, but the crucial bulb development happens as days shorten and temperatures cool. For a fall crop, seeds should be direct-sown 80 to 90 days before the first expected frost date. This late-season timing avoids the long, hot days that induce bolting, allowing the plant to concentrate its energy on forming a crisp, sweet bulb.
Knowing When to Harvest Each Plant Part
The timing of harvest depends on which part of the fennel plant you intend to use. The bulb, the swollen base of the leaf stalks, is ready for harvest approximately 12 to 14 weeks after seeding. The optimal time to cut the bulb is when it reaches two to four inches in diameter, feels firm, and is sized between a tennis ball and a softball.
Harvesting the bulb must occur before the plant begins to bolt, as flowering causes the bulb to become stringy and fibrous. The bulb is cut at the base of the plant just above the taproot, which removes the entire plant.
The feathery fronds can be trimmed lightly and continuously once the plant is well-established, usually after about 12 weeks of growth. Avoid removing more than one-third of the foliage at any one time to prevent stunting the plant. If growing fennel for seeds, they are harvested late in the season, typically late summer or early fall. The flower heads should be allowed to completely dry and turn brown before being cut, indicating the seeds have fully matured.