Washingtonia filifera, also known as desert fan palm, California fan palm or California palm, is 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.
This is the only palm native to the Western United States and the country's largest native palm. 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).
Washingtonia filifera grows to 18 metres (59 ft) in height (occasionally to 25 metres (82 ft)) in ideal conditions. The fronds are up to 3.5–4 metres (11–13 ft) long, made up of a petiole up to 2 metres (6.6 ft) long, bearing a fan of leaflets 1.5–2 metres (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 there is any red color present on petioles or trunk it is not a pure filifera but a fila-busta hybrid. Washingtonia filifera can live from 80 to 250 years or more.
California 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 as well as 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, it seems that this species is resistant to the red palm weevil through a mechanism based on antibiosis. Currently the California fan palm is experiencing a population and range expansion, perhaps due to global warming.
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.
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 as well as other Southern Paiutes have written memories of using this palm's seed, fruit or leaves for various purposes including starvation food