Growing Bulb Fennel in the Pacific Northwest

Bulb fennel can be directly seeded in the field or transplanted as seedlings started in a greenhouse (Miles et al. 2019). There are a few considerations when determining which method to use.

Direct seeding can provide a shorter seed-to-harvest time which can be beneficial for bulb tenderness especially if planting later in the season. Transplanting removes many variables that can negatively impact seed germination and early establishment, such as weather fluctuations, pests, weeds, etc. However, this method is more labor and resource intensive and requires suitable greenhouse space. For both planting methods, seeding date can impact the quality of fennel bulbs, with earlier-seeded plants producing more tender bulbs than plants seeded later. 

Plant bulb fennel in a double row on a raised bed that is 2–3ft wide. For weed control, cover beds with black plastic mulch. Space the rows 12 inches apart on the bed, and space the plants 8 inches apart within the row.

Apply 100 lb N/acre to the soil before bed shaping. Apply supplemental fertilizer (e.g. 2.1 lb N/acre) via the drip irrigation system three times throughout the fennel growing season to improve plant productivity and bulb size. Ensure the supplemental fertilizer product is labeled for fertigation before applying.

See Table 1 for varieties that have been tested in Mount Vernon, WA. Bulb fennel thrives in the long, warm days of early summer in western Washington. The decrease in daylength following the summer solstice (late June) signals the fennel plants to form its bulb. Earlier seeding leads to larger and more tender bulbs. Varieties that bolt (form flower stalks) in the long daylength of summer in this region should not be planted. Note that drought stress can cause bulb fennel to bolt, so ensure plants are irrigated each week.

Use drip irrigation to grow bulb fennel as it conserves water and keeps the canopy dry, which helps reduce disease incidence. Install drip tape with 8-inch emitter spacing (e.g., T-Tape model 508-08-340; Rivulis Irrigation, San Diego, CA) between the two rows. If using plastic mulch, place the drip tape on the center of the beds under the mulch as the beds are formed. After seeding or transplanting, irrigate three times per week for approximately 75 min each irrigation, and reduce application with precipitation. After 2-3 weeks or whenever plants are established, reduce irrigation to once per week.

Weed management can be challenging early in the season as bulb fennel is difficult to weed by hand due to its small, slow-growing seedlings, similar to carrots. Plastic mulch laid over the bed helps minimize weeds, and hand-weed planting holes as needed.