|Washingtonia filifera in native grove near Twentynine Palms, California|
Washingtonia filifera, also known as desert fan palm,California fan palm or California palm, is a flowering plant in the palm family (Arecaceae) native to the far southwestern United States and Baja California. Growing to 15-20 m (49-66 ft) tall by 3-6 m (10-20 ft) broad, it is an evergreen monocot with a tree-like growth habit. It has a sturdy, columnar trunk and waxy, fan-shaped (palmate) leaves.
Other common names include California fan palm and petticoat palm. The specific epithet filifera means "thread-bearing".
Primary populations are found in desert riparian habitats at spring-fed and stream-fed oases in the Colorado Desert and at a few scattered locations in the Mojave Desert. It is also found near watercourses in the Sonoran Desert along the Gila River in Yuma, along the Hassayampa River and near New River in Maricopa County, and in portions of Pima County, Pinal County, Mohave County (along the Colorado River), and several other isolated locations in Clark County, Nevada. It is a naturalized species in the warm springs near Death Valley and in the extreme northwest of Sonora (Mexico). It is also reportedly naturalized in the Southeast, Florida, Hawaii, the U.S. Virgin Islands, and Australia (New South Wales).
W. filifera grows to 18 m (59 ft) in height (occasionally to 25 m (82 ft)) in ideal conditions. The California fan palm is also known as the desert fan palm, American cotton palm, and Arizona fan palm.
The fronds are up to 4 m (13 ft) long, made up of a petiole up to 2 m (6.6 ft) long, bearing a fan of leaflets 1.5-2.0 m (4.9-6.6 ft) long. They have long, thread-like, white fibers, and the petioles are pure green with yellow edges and filifera-filaments, between the segments. The trunk is gray and tan, and the leaves are gray green. When the fronds die, they remain attached and drop down to cloak the trunk in a wide skirt. The shelter that the skirt creates provides a microhabitat for many small birds and invertebrates. If any red color is present on petioles or trunk, it is not a pure W. filifera, but a W. fila-busta hybrid.
W. filifera typically lives from 80 to 250 years or more.
Desert fan palms provide habitat for the giant palm-boring beetle, western yellow bat, hooded oriole, and many other bird species. Hooded orioles rely on the trees for food and places to build nests. Numerous insect species visit the hanging inflorescences that appear in late spring.
The palm boring beetle Dinapate wrightii (Bostrichidae) can chew through the trunks of this and other palms. Eventually, a continued infestation of beetles can kill various genera and species of palms. The recent discovery of the red palm weevil (Rhynchophorus ferrugineus) in southern California may pose a threat to many palms, with coastal garden W. filifera specimens already a known host. However, this species deems to be resistant to the red palm weevil through a mechanism based on antibiosis.
Historically, natural oases are mainly restricted to areas downstream from the source of hot springs, though water is not always visible at the surface.
Grazing animals can kill young plants through trampling, or by eating the terminus at the apical meristem, the growing portion of the plant. This may have kept palms restricted to a lesser range than indicated by the availability of water.
Today's oasis environment may have been protected from colder climatic changes over the course of its evolution. Thus, this palm is restricted by both water and climate to widely separated relict groves. The trees in these groves show little if any genetic differentiation, (through electrophoretic examination), suggesting that the genus is genetically very stable.
Joshua Tree National Park in the Mojave Desert preserves and protects healthy riparian palm habitat examples in the Little San Bernardino Mountains, and westward where water rises through the San Andreas Fault on the east valley side. In the central Coachella Valley, the Indio Hills Palms State Reserve and nearby Coachella Valley Preserve, other large oases are protected and accessible. The Santa Rosa and San Jacinto Mountains National Monument, and Anza-Borrego Desert State Park both have large and diverse W. filifera canyon oasis habitats.
The fruit of the fan palm was eaten raw, cooked, or ground into flour for cakes. The Cahuilla and related tribes used the leaves to make sandals, thatch roofs, and baskets. The stems were used to make cooking utensils. The Moapa band of Paiutes and other Southern Paiutes have written memories of using this palm's seed, fruit, or leaves for various purposes including starvation food.
W. filifera is widely cultivated as an ornamental tree. It is one of the hardiest Coryphoidiae palms, rated as hardy to USDA hardiness zone 8. It can survive brief temperatures of -10 °C (14 °F) with minor damage, and established plants have survived, with severe leaf damage, brief periods as low as -12 °C (10 °F). The plants grow best in Mediterranean climates, but can be found in humid subtropical climates such as eastern Australia and the southeastern USA. It has gained the Royal Horticultural Society's Award of Garden Merit.
Mark Fleming, curator of botany at the Arizona-Sonora Desert Museum of Tucson, says Washingtonia filifera, or the California fan palm, is the state's only naturally occurring variety and that they are found in pockets around southern California, Northern Mexico, and one or two pockets in Arizona.