Common Name: Sweet fennel
Scientific Name: Foeniculum vulgare
Code: FOVU
Group: Dicot
Family: Apiaceae
Growth Habit: Forb/herb
Duration: Biennial/ Perennial

What does it look like?

Fennel is a licorice-scented plant that sends up tall, 4-10 foot stalks. The leaves have a feathery or wispy look (BLM 2003). Stems have distinct joints where branches grow and the leaves extend from either these stems or the root crown. The small yellow flowers are clustered into large, rounded, umbrella-like groups (about 4 inches across). Fennel blooms from April to July. Flower production generally occurs when plants are eighteen to twenty-four months old, and seed production peaks in August and September. The seeds of wild fennel are oblong, dorsally compressed, and ribbed, similar to the fennel seed that is used as a common spice (Bossard et al. 2000).

Reproduction occurs from both root crown and seeds, via water, vehicles, clothing, and birds and rodents who eat the seeds. Seeds can persist in soil for several years before germinating at almost any time of the year. Vegetative growth begins in mid-winter and peaks in July to August. Initial growth is slow and then becomes rapid in early summer (Bossard et al. 2000).

Where does it grow?

Fennel is native to southern Europe and the Mediterranean region but has been documented in California for at least 120 years and is notorious for repeatedly escaping cultivation (Bossard et al. 2000).

Fennel thrives in a Mediterranean climate beginning at sea level to 2,000 feet in well-drained, sandy soil but can also flourish in sites with high clay content. Areas of infestation include disturbed, weedy sites, usually adjacent to water sources, pastures, abandoned lots, or roadsides. Fennel grows particularly thick in sites that have been grazed, plowed, or recently abandoned (Bossard et al. 2000).

Is it in our watershed?

Widely distributed in the Mattole watershed, it tends to grow thick along roadsides, in open fields and other disturbed areas.

Why is it a problem?

Fennel tends to invade disturbed areas and prevent reestablishment of native species, by outcompeting native species for light, nutrients and water. Fennel also develops dense uniform stands, sometimes reaching fifty to ninety percent cover. Its presence drastically changes plant communities in various ecosystems, including grasslands, coastal scrub, riparian, and wetland areas. Once established, fennel is extremely difficult to control, especially because of its long-lived seedbank that builds rapidly.

How do you get rid of it?

Removal and management of large fennel stands requires a long-term commitment of time and resources. During the removal process, it is critical to minimize disturbances that lead to further spread of fennel. Research shows that a variety of non-native species quickly invade fennel removal sites, regardless of control technique. Removal should be the first step in a long-term restoration process. Further actions are necessary to promote recolonization of native plants. Due to the tenacious nature of fennel, it is unlikely we can remove it completely from California’s wildlands, but it can be managed by thinning stands and reducing disturbances (Bossard et al. 2000).

Manual Control: Digging out individual plants is a labor-intensive but effective removal technique when infestations are small. This is preferable to plowing or bulldozing because it minimizes soil disturbance.

Mechanical Control: Cutting, mowing, and chopping only temporarily reduces the height of fennel plants but is not effective for removal. This is obvious along county roads where sides have been mowed and fennel quickly resprouts from roots. This technique leaves roots intact, alive, and able to regrow. It is possible to exhaust the resources of the taproot through repeated mowing over time, but these cuts must occur in short intervals to prevent replenished energy supplies (Bossard, 2000). Cutting plants in seed will promote dispersal.

NOTE: Grazing and prescribed burns have not been successful for removal in the past. Grazing often leads to denser, thicker infestations (Bossard, 2000).

Author:
Unity Peterson, Invasive Plant Program Coordinator, Mattole Restoration Council, Petrolia, CA.

Photographs:
Unity Peterson

References:
BLM 2000. Noxious Weeds of the Arcata Field Office. Bureau of Land Management. http://www.ca.blm.gov//arcata/arcweeds.html

Bossard, CC., J.M. Randall and M. Hoshovsky. (eds.) 2000. Invasive Plants of California’s Wildlands. University of California Press, Berkeley, CA. Online version: http://groups.ucanr.org/ceppc/Invasive_Plants_of_California's_Wildlands/