|Washingtonia filifera in native grove near Twentynine Palms, California.|
Washingtonia filifera (common name desert fan ) is a species of flowering plant in the palm family Arecaceae, native to southwestern North America. Growing to 15–20 m (49–66 ft) tall by 3–6 m (10–20 ft) broad, it is technically an evergreen monocot (not actually a tree) but 'tree-like' since it presents with a sturdy columnar trunk it has waxy fan-shaped leaves which are properly described in Botanical terms as palmate.
Other common names include desert fan palm, California palm, fanpalm, petticoat palm, cotton palm, Arizona fan palm. The specific epithet filifera means "thread-bearing".
Washingtonia filifera is the only palm native to the Western United States, and the country's largest native palm (though most palms in Los Angeles and San Diego are specimens of the closely related and very similar W. robusta).
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. 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 also introduced in northern Baja California. It is a 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.
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 (11.5–13 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 petiole 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 its not a pure filifera but a fila-busta hybrid.
Washingtonia filifera can live from 80 to 250 years or more.
Fan palms provide a habitat for desert 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.
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.
Today, due to urbanization and ground water depletion, palm oases are shrinking and disappearing. Increased agricultural irrigation has lowered aquifers, reducing or removing water availability at palm oases. This creates a threat to the species and the organisms which rely on its habitat to survive.
Fossils of this palm are known to exist as far north as Colorado, Wyoming and Oregon. This would make the use of the description 'introduced species' a semantical technicality since most areas of the western United States anciently supported this palm species. The palm apparently reached its current form by at least 50–70 million years BP making it one of the oldest known palms identifiable to taxon through morphological characteristics of the leaves, petioles and other fossilized parts.
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 as well as other Southern Paiutes have written memories of using this palm's seed, fruit or leaves for various purposes including starvation food.
Washingtonia 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 will survive 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 warm temperate climates with dry summers and wetter winters. Specimens outside of Mediterranean climates rarely exceed 15 metres (49 ft).
- Genera Palmarum - A Classification of Palms: Natalie Uhl and John Dransfield. -The L.H.Bailey Hortorium and The International Palm Society, Allen Press, Lawrence KS 1987 - ISBN 0-935868-305 Lib of Congress No: 87-81063 pp. 287-288
- 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
-  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
- "RHS Plant Selector - Washingtonia filifera". Retrieved 9 June 2013.
- 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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