Species Description and Current Range
Fennel, an anise-scented herb, grows grayish-green, feathery stalks, and tiny yellow flowers in umbrella-shaped
clusters. Plants establish long taproots, and can attain heights of 1.5-2 meters after two growing seasons. Invasive
Fennel is common on roadsides, construction sites, and other disturbed areas in WA and OR, mainland CA and on
the CA Channel Islands. Fennel has invaded much of the coastal US and its territories, including the East Coast,
West Coast, HI, PR, and the VI.
Impacts to Communities and Native Species
Fennel thrives in disturbed areas, but generally does not invade established native plant communities. Still, its roles
in the decline of the CA island fox, and in the changing life habitat of the anise swallowtail butterfly warn of its
potential to wreak ecological havoc.
Ranchers brought Fennel and livestock to the CA Channel Islands in the 1850s. Domestic Pigs became feral and
decimated native plant communities. Fennel took over the disturbed land, and is currently thought to cover some 5%
of Santa Cruz Island. Fennel stands provided ideal habitat for pigs, and pig populations grew rapidly. Pigs
contributed further to Fennel’s spread by transporting seeds on their coats and in their feces. Meanwhile, use of
DDT as an agricultural pesticide in the 1940s-1960s drastically reduced the Islands’ Bald Eagle populations. With
native eagles largely gone, mainland Golden Eagles colonized the Islands sometime in the 1990s. Bald Eagles had
preyed largely on fish, and had posed no significant threat to Island Foxes. Today, Golden Eagles prey on piglets in
the spring. However, as piglets grow larger and more difficult to catch, the eagles switch to Island Foxes. About the
size of small house cats, Island Foxes are so unaccustomed to predation that researchers can perform examinations
without sedating them, or even using protective gloves. The foxes also hunt during daylight, when they are
especially visible to Golden Eagles. This lack of fear has been disastrous. Since 1995, when Golden Eagles arrived
on San Miguel Island, fox populations have declined from 2000 to 70 individuals. Today, the Island Fox is a
candidate for endangered species status. The Nature Conservancy, National Park Service and other organizations are
working to control or remove Fennel and Pig populations, and to return Bald Eagles to Santa Cruz Island.
Other observations from CA suggest that successful removal of Fennel would not be without complications. The
larvae of the Black and Yellow Anise Swallowtail Butterfly (Papillo zelicaon), a species common throughout the
Pacific Coast, historically fed on native members of the Apiaceae, which are relatives of Fennel. Habitat destruction
has sharply reduced populations of these native plants, and swallowtail larvae now depend heavily on invasive
Fennel as a nesting site and food source. Some naturalists fear that removing Fennel without restoring native plants
would have negative consequences for the butterfly.
Control Methods and Management
1. Do not use Fennel in your garden. Use native plants or non-invasive non-native species in your home garden.
2. Mow or cut existing Fennel plants before seeds brown and mature in early fall. Preempting seed production
prevents spread by sexual reproduction.
3. Remove the entire taproot when digging out the plants, since they can regenerate from root fragments.
4. Controlled burning can be used in large stands to remove dead vegetation and render regenerating Fennel
vulnerable to pesticides. However, fire should be used with caution, since wildfire may be a significant risk in
vegetated areas.
Life History and Species Overview
Fennel is a member of the family Apiaceae, which includes Carrot, Celeriac, Parsnip, Celery, Parsley, Dill, Caraway
and Anise. Though Fennel prefers well-drained, limestone-rich soils, it is robust in pH levels ranging from acid to
basic, temperatures from 4-27 °C, and precipitation from 0.3-2.6 m annually. Under conditions too dry for
germination, seeds remain dormant in soil for up to seven years. Fennel requires two growing seasons to reach full
sexual maturity; younger plants may spread asexually when flood waters pick up taproot fragments and deposit them
elsewhere. Fennel grows most quickly during summer, but can produce leaves year-round if the winter is mild. In
the PNW, Fennel typically blooms in July and August and sets its ridged, oblong seeds in the early fall.
History of Invasiveness
Fennel is native to Mediterranean Europe. The wild Fennel common in the PNW generally is a bit less sweet and has
a smaller bulb that the cultivated variety available at groceries and farmers markets. Fennel has been used for
centuries: its leaves as a garnish, its celery-like stalks for cooking, its seeds for their medicinal properties, and the
entire plant as a perennial ornamental. Various cultures have credited Fennel with promoting lactation, suppressing
appetite, healing snake bites, curing colic, repelling flies and ticks, improving eyesight, relieving nausea, and
silencing stomach growling during long church services. Roman gladiators were awarded a Fennel garland when
they triumphed in a bout; sweet Fennel seeds are often offered as a breath-freshener in Indian cuisines. Fennel was
separately introduced to the East and West Coasts of North America. During the 1600s, Puritan colonists in VA
began to cultivate Fennel for use as an appetite suppressant and digestive aid. Spanish priests planted it over two
centuries ago in missionary gardens in present-day CA, and used its feathery fronds to perfume mission buildings.
Fennel escaped Puritan and missionary gardens, and began to proliferate in the wild.