|Washingtonia filifera in native grove near Twentynine Palms, California.|
Washingtonia filifera (filifera – Latin "thread-bearing"), with the common names, Desert Fan Palm, California-palm, Fanpalm, Petticoat-palm, Cotton palm, Arizona Fan Palm and California Fan Palm. It is a palm native to southwestern North America between an elevation range of 100–1,200 metres (330–3,900 ft), at seeps, desert bajadas, and springs where underground water is continuously available.
Washingtonia filifera is the only palm native to the Western United States and 'palm iconic' California (although most palms known to Los Angeles and San Diego metro areas are W. robusta – and are easily confused by the layman with W. filifera)  It is the largest native palm in the contiguous United States. The primary populations are found in desert riparian habitats at spring-fed oases in the Colorado Desert (Low Desert) and throughout a major portion of the Mojave Desert; with all populations extant in isolated disjunct populations throughout the stated regions. It has been introduced to populated portions 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, Mojave County (along the Colorado River) and several other isolated locations in Clark County, Nevada. It is also known in northern Baja California of Mexico. It is an introduced, naturalized species in the warm springs near Death Valley and in the extreme northwest of Sonora (Mexico), and also in Florida and the U.S. Virgin Islands. As an ornamental tree it is cultivated in suitable temperate climates worldwide.
The Washingtonia filifera palm grows to 18 metres (59 ft) in height (occasionally to 25 metres (82 ft)) in ideal moisture and microclimate 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, filifera-filaments, between the segments. 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.
Washingtonia filifera can live from 80 to 250 years or more.
The genus name honors George Washington, the first President of the United States. The plant is popularly honored by its common name and habitat used in naming communities and landforms, such as Palm Springs, California which is home to large populations.
Fan palms provide a habitat for Desert Bighorn Sheep and the California endemic Peninsular Bighorn Sheep, Hooded Oriole, Gambel's Quail, Coyotes, and a rare bat species (Lasiurus xanthinus) that is especially fond of W. filifera groves. Hooded Orioles rely on the trees for food and places to build nests. Both Hooded Orioles and coyotes play an integral part in seed distribution.
Joshua Tree National Park (in the Mojave Desert) preserves and protects healthy riparian palm habitat examples in the Little San Bernardino Mountains section, and westward where water surfaces up 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. In the Santa Rosa Mountains on the west side of the valley, at 'Palm Canyon' in the Santa Rosa and San Jacinto Mountains National Monument, and in Anza-Borrego Desert State Park, both parks have large and diverse Washingtonia filifera canyon oases habitats.
The palm boring beetle Dinapate wrightii (Bostrichidae) can chew through the trunks of the trees. 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 Washingtonia filifera trees already a known host. However, it seems that this species is resistant to the red palm weevil through a mechanism based on antibiosis.
Today due to urbanization and ground water depletion, palm oases are retracting and disappearing. Increased agriculture irrigation needs of well water has lowered aquifers which decreases or stops water availability at seeps and springs in palm oases. This creates a threat to the only native palm of the western US, and the organisms which rely on the riparian palm oases habitat to survive.
Fossils of this palm are known to exist as far north as Colorado, Wyoming and Oregon. The palm apparently reached its current form by at least 50 – 70 million years BP making it one of the oldest known palm identifiable to taxon through morphological characteristics of the leaves, petioles and other fossilized parts.
Natural oases environments are mainly restricted historically to the area surrounding warm or hot springs, near the source, or shortly downstream from the source although water is not always visible as surface water in various loci such as KOFA AZ.
Grazing animals including deer and cattle and in more ancient times, Giant Sloths and other extinct herbivores, can kill young plants through trampling, or by eating the terminus at the apical meristem, which is the growing portion of the plant. This may have kept these palms restricted to a lesser range than would have been expected if one simply considers the availability of water sources.
Typically, the oasis environment found today is one which 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.
Native Americans 
The fruit of the fan palm was used by Native Americans. It was eaten raw, cooked, or ground into flour for cakes. The Cahuilla and related tribes used the leaves to make sandals, thatch roofs, and for making baskets. The fan palm was a valuable resource and the stems were used to make utensils for cooking. The Moapa band of Paiutes as well as other Southern Paiutes have written stated historical memories of grandparents using this palm's seed, fruit or leaves for various things including as a starvation food. The Southern Paiutes are related linguistically and by ancient trade routes to the Cahuilla.
Washingtonia filifera is widely cultivated as an ornamental tree. It is one of the hardiest of Coryphoidiae palms, is tolerant of considerable frost, and is rated as hardy to USDA hardiness zone 8. It will survive temperatures of −10 °C (14 °F) with minor damage, and established plants have survived, with severe damage to the foliage, brief periods of temperatures as low as −12 °C (10 °F). It is a favorite of cold-hardy palm enthusiasts. The plants grow best in warm temperate climates with dry summers and wetter winters. Specimens outside of Mediterranean climates do not grow as large, rarely exceeding 15 metres (49 ft). It is often seen in California with the closely related species Mexican Fan Palm (Washingtonia robusta) – a less hardy palm needing slightly milder winters, that may be visibly damaged at −7 °C (19 °F) and is more amenable to humidity making it more favored along the Gulf Coast. 
- Flora of North America @ efloras.org: Washingtonia filifera
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- C. Michael Hogan. 2009. California Fan Palm: Washingtonia filifera; "The only palm native to the contiguous United States west of the Balcones Fault Zone in Texas (except for Sabal minor);" GlobalTwitcher.com, ed. Nicklas Stromberg
- Clover, E. U., 1937. Vegetational survey of the lower Rio Grande valley, Texas; "Isolated stands of Sabal minor in the Texas Hill Country.;" Madrono 4:41–72.
- Little. Atlas of United States Trees, Volume 3, Minor Western Hardwoods, Little, Elbert L, 1976, US Government Printing Office. Library of Congress No. 79-653298. Map 201, Washingtonia filifera.
- USDA – W. filifera
- Wendland. Herman. Ueber Brahea oder Pritchardia filifera Hort. Botanische Zeitung (Berlin) 37(5): 68. 1879.("als Erinnerung an den grossen Amerikaner")
- Gudde, Erwin Gustav; Bright, William (1998). California Place Names: The Origin and Etymology of Current Geographical Names (4th ed.). Berkeley, CA: University of California Press. p. 277. ISBN 0-520-24217-3. LCCN 97043168. "Washington ... found 29 'cabbage trees' [at Twentynine Palms ... ] the common name for the Washington palm before German botanist Wendland named it in 1879."
-  Orange County Register, "Destructive exotic beetle found in Laguna Beach."
-  Center for Invasive Species Research, "Red Palm Weevil."
-  Journal of Applied Entomology, "Are the palms Washingtonia filifera and Chamaerops humilis suitable hosts for the red palm weevil, Rhynchophorus ferrugineus (Col. Curculionidae)?"
- Moapa – foreword
- Moapa – part1
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- Patencio, Francisco. 1943. Stories and Legends of the Palm Springs Indians. As told to Margaret Boynton. Palm Springs Desert Museum, Palm Springs, CA. (Numerous myths and legends documented in the 1930s.) OCLC 4020904
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- Johnson (1998). Washingtonia filifera. 2006. IUCN Red List of Threatened Species. IUCN 2006. www.iucnredlist.org. Retrieved on 11 May 2006.
- Floridata.com: Washingtonia filifera
- C. Michael Hogan. 2009. California Fan Palm: Washingtonia filifera, GlobalTwitcher.com, ed. Nicklas Stromberg
- Scanpalm – Washingtonia filifera
- Interactive Distribution Map for Washingtonia filifera
- Joshua Tree National Park: California Fan Palm oases
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