Plant Assessment Form

Washingtonia robusta

Synonyms: W. filamentosa (often mistaken for native W. filifera)

Common Names: Mexican fan palm; Washington palm; skyduster; thread palm

Evaluated on: 1/4/05

List committee review date: 11/03/2005

Re-evaluation date:


Elizabeth Brusati, project manager
California Invasive Plant Council
1442A Walnut St. #462, Berkeley, CA 94709

List commitee members

Joe DiTomaso
John Randall
Carla Bossard

General Comments

No general comments for this species

Table 2. Criteria, Section, and Overall Scores

Overall Score? Moderate
Alert Status? Alert
Documentation? 3 out of 5
Score Documentation
1.1 ?Impact on abiotic ecosystem processes C Other Published Material
Four-part score CBBC Total Score
1.2 ?Impact on plant community B. Moderate Other Published Material
1.3 ?Impact on higher trophic levels B. Moderate Observational
1.4 ?Impact on genetic integrity C. Minor/Low Other Published Material
2.1 ?Role of anthropogenic and natural disturbance in establishment B. Moderate Reviewed Scientific Publication
Total Points
14 Total Score B
2.2 ?Local rate of spread with no management B. Increases less rapidly Anecdotal
2.3 ?Recent trend in total area infested within state B. Increasing less rapidly Anecdotal
2.4 ?Innate reproductive potential
(see Worksheet A)
C. Low Other Published Material
2.5 ?Potential for human-caused dispersal A. High Other Published Material
2.6 ? Potential for natural long-distance dispersal A. Frequent Reviewed Scientific Publication
2.7 ?Other regions invaded C. Already invaded Other Published Material
3.1 ?Ecological amplitude/Range
(see Worksheet C)
C. Limited Other Published Material
Total Score C
3.2 ?Distribution/Peak frequency
(see Worksheet C)
D. Very low Observational

Table 3. Documentation

Scores are explained in the "Criteria for Categorizing Invasive Non-Native Plants that Threaten Wildlands".

Section 1: Impact
Question 1.1 Impact on abiotic ecosystem processes? C Other Published Material
Identify ecosystem processes impacted:

Increased fire danger Dead fronds hanging on tree are a fire hazard and in some areas are required by law to be removed.

Sources of information:

Gilman, E. F., and D. W. Watson. 1994. Washingtonia robusta. Washington palm. Fact Sheet ST-670. Environmental Horticulture Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Gainesville, FL.

Question 1.2 Impact on plant community composition,
structure, and interactions?
B Other Published Material
Identify type of impact or alteration:

Can convert riparian communities into monospecific stands (1). W. robusta forms dense thickets (2) that can grow to 80 ft. tall (3). However, the shade it produces is not as dense as other trees.

Sources of information:

1. Tu, M., and J. M. Randall. 2002. Red Alert! New Introductions and Recent Expansions in California. Proceedings, California Exotic Pest Plant Council Symposiums 2000, 2001, 2002.
2. Daehler, C. No date. Washingtonia robusta (Mexican fan palm). Australian/New Zealand Weed Risk Assessment adapted for Hawai'i. Kaulunani Urban Forestry Program and U.S. Forest Service.
3. Miller, M.E., N.P. Maxwell, and J. Amador. 1980. Lethal decline of Phoenix canariensis and Phoenix dactylifera in the Rio Grande Valley Texas. Journal of the Rio Grande Valley Horticultural Society 34: 89-95.

Question 1.3 Impact on higher trophic levels? B Observational
Identify type of impact or alteration:

Possible increase in rodent populations (leading to increased predation on birds' nests?). Displaces native animal speces that cannot live in palm monoculture (2). Dead fronds are bedding roost for rodents (1). Dead leaves remain around the trunk for many years, forming a dense, thatchlike shroud that reaches almost to the ground (3). I'm extrapolating from that fact to a potential increase in predation pressure from those rodents.

Sources of information:

1. Gilman and Watson 1994
2. Tu and Randall 2002
3. Young, J.A. and C.G. Young. 1992. Seeds of woody plants in North America. Portland, Oregon: Dioscorides Press. Pp. 356-357.

Question 1.4 Impact on genetic integrity? C Other Published Material

Can hybridize with native California palm, W. filifera, to form hybrid Washingtonia x filabusta. Scoring as C because no information on how common these hybrids are outside of cultivation, although Sunset says they will readily hybridize when growing near each other.

Sources of information:

Starr, F., K. Starr, and L. Loope. 2003. USGS Biological Resources Division, Hawaiian Ecosystems at Risk. Haleakala, HI.
Brenzel, K. N. 2001. Sunset Western Garden Book. Sunset Publishing Company, Menlo Park, CA.

Section 2: Invasiveness
Question 2.1 Role of anthropogenic and natural disturbance
in establishment?
B Anecdotal
Describe role of disturbance:

Most spread seems to occur in disturbed areas. Found in undisturbed habitat with available water source (3,4) . Occasionally found in disturbed areas near planted landscapes in southern California. A couple of palms were found in undisturbed desert washes in southern California (1). In Hawaii, prolific near urban water sources such as irrigation ditches or ponds (2).

Sources of information:

1. Cornett, J. W., J. Stewart, and T. Glenn. 1986. Washingtonia robusta naturalized in southern California. Bulletin of the Southern California Academy of Sciences. 85:56-57
2. Starr et al. 2003
3. Hicks, B.F. 1989. Prehistoric development and dispersal of the desert fan palm. Principes 33(1): 33-39.
4. Knapp, J. 2004. Catalina Invasive Plant Ranking Plan for the Catalina Island Conservancy. Unpublished.

Question 2.2 Local rate of spread with no management? B Anecdotal
Describe rate of spread:

Spreading in southern California.

Sources of information:

Question 2.3 Recent trend in total area infested within state? B Anecdotal
Describe trend:

Spreading in southern California.,

Sources of information:

Question 2.4 Innate reproductive potential? C Other Published Material
Describe key reproductive characteristics:

Reproduces by seed. Fruits are drupes.
Self-compatible, does not require specialist pollinators (1).
In Australia, 9 years or more to reproductive maturity (2). Does not produce coppices or resprouts (2), but does resprout when cut completely (3). Resistant to fire damage (4). Seed production lasts two months (5).

Sources of information:

1. Anonymous. no date. Risk Assessment Results - Washingtonia robusta. USFS. Institute of Pacific Islands Forestry. Pacific Island Ecosystems at Risk. Accessed 1/4/05
2. Brown, K. and K. Brooks. 2002. Bushland weeds _ a practical guide to their management. Environmental Weeds Action Network. Greenwood, Australia. Pp. 88-89.
3. Knapp, J.J. 2002. Personal observation of palm control efforts on Catalina Island, CA. (310) 510-1299.
4 Hicks 1989
5. Young, J.A. and C.G. Young. 1992. Seeds of woody plants in North America. Portland, Oregon: Dioscorides Press. Pp. 356-357.

Question 2.5 Potential for human-caused dispersal? A Other Published Material
Identify dispersal mechanisms:

Escape from cultivation. Listed by numerous references (internet and books) as a popular plant for large gardens and as a street tree (1, 2). Has escaped from gardens on Maui (3). Related W. filifera spread from plantings at picnic sites into remote springs in Nevada (4).

Sources of information:

1. Brenzel 2001
2. Gilman and Watson 1994
3. Starr et al. 2003
4. Pers. comm. E-mail from Curt Deuser, Lake Mead Exotic Plant Mgmt. Team, Boulder City, NV to Carolyn Martus, California Native Plant Society - San Diego. 10/4/04

Question 2.6 Potential for natural long-distance dispersal? A Reviewed Scientific Publication
Identify dispersal mechanisms:

Seeds could disperse by water where it invades riparian areas, or with birds (1, 2) or coyotes (1). Birds such as mountain bluebirds, cedar waxwing, and house finch are also considered primary dispersal agents (3). Birds often perch in the branches, but the information on dispersal is observational only. Gilman and Watson say the fruits are not attractive to wildlife.

Sources of information:

1. Cornett et al. 1986
2. Starr et al. 2003
3. Hicks 1989
4. Gilman and Watson 1994

Question 2.7 Other regions invaded? C Other Published Material
Identify other regions:

Hawaii (1), Australia (1). Naturalized in Florida (2). Starr et al. list it as invasive in Florida, but Gilman and Watson (3) lists it with little invasive potential. Mostly a riparian problem.

Sources of information:

1. Starr et al. 1986
2. USDA, NRCS. 2004. The PLANTS Database, Version 3.5 ( National Plant Data Center, Baton Rouge, LA 70874-4490 USA
3. Gilman and Watson 1994.

Section 3: Distribution
Question 3.1 Ecological amplitude/Range? C Other Published Material

Southern California riparian areas (1). Capable of growing in semi-arid, desert regions, usually forming colonies near water (2). Sunset lists Washingtonia as appropriate for gardens in warmer areas of zones 8, 9, and 10 (3). In San Diego area, present in wetlands, canyons, creeks, and coastal lagoons (4). W. robusta was introduced to California by the mission fathers as early as the 18th century (5).

Sources of information:

1. Tu and Randall 2002
2. Starr et al. 2002
3. Brenzel 2001
4. E-mail from Carolyn Martus, California Native Plant Society, forwarded 1/9/05.
5. Deardorff, D. 1976. Plant portraits: Washingtonia robusta the Mexican fan palm. Lasca Leaves 26(2): 43-45

Question 3.2 Distribution/Peak frequency? D Observational
Describe distribution:

see 3.1

Sources of information:

Worksheet A - Innate reproductive potential

Reaches reproductive maturity in 2 years or less No
Dense infestations produce >1,000 viable seed per square meter No
Populations of this species produce seeds every year. Yes
Seed production sustained over 3 or more months within a population annually No
Seeds remain viable in soil for three or more years Unknown
Viable seed produced with both self-pollination and cross-pollination Yes
Has quickly spreading vegetative structures (rhizomes, roots, etc.) that may root at nodes No
Fragments easily and fragments can become established elsewhere No
Resprouts readily when cut, grazed, or burned Yes
Total points: 3
Total unknowns: 1
Total score: C?

Related traits:

Worksheet B - Arizona Ecological Types is not included here

Worksheet C - California Ecological Types

(sensu Holland 1986)
Major Ecological Types Minor Ecological Types Code?
Marine Systemsmarine systems
Freshwater and Estuarine lakes, ponds, reservoirs
Aquatic Systemsrivers, streams, canals
Scrub and Chaparralcoastal bluff scrub
coastal scrub
Sonoran desert scrub
Mojavean desert scrub (incl. Joshua tree woodland)
Great Basin scrub
chenopod scrub
montane dwarf scrub
Upper Sonoran subshrub scrub
Grasslands, Vernal Pools, Meadows, and other Herb Communitiescoastal prairie
valley and foothill grassland
Great Basin grassland
vernal pool
meadow and seep
alkali playa
pebble plain
Bog and Marshbog and fen
marsh and swamp
Riparian and Bottomland habitatriparian forest
riparian woodlandD, < 5%
riparian scrub (incl.desert washes)D, < 5%
Woodlandcismontane woodland
piñon and juniper woodland
Sonoran thorn woodland
Forestbroadleaved upland forest
North Coast coniferous forest
closed cone coniferous forest
lower montane coniferous forest
upper montane coniferous forest
subalpine coniferous forest
Alpine Habitatsalpine boulder and rock field
alpine dwarf scrub
Amplitude (breadth): C
Distribution (highest score): D

Infested Jepson Regions

Click here for a map of Jepson regions

  • Cascade Range
  • Central West
  • Great Valley
  • Northwest
  • Sierra Nevada
  • Southwest