California Invasive Plant Council

Alternanthera philoxeroides

Synonyms: Bucholzia philoxeroides Mart., Telanthera philoxeroides (Mart.)

Common names: alligatorweed

Alternanthera philoxeroides (alligatorweed) is a noxious herbaceous aquatic perennial (family Amaranthaceae), that forms dense floating mats. This weed grows in the San Joaquin Valley (Tulare and Kings counties), Los Angeles County, and the Sacramento Delta. It invades lakes, streams, canals, ponds and irrigation ditches. It was previously used in the aquarium trade and is currently rapidly expanding in waterways across California.

Cal-IPC Rating: High?

Explanation of Cal-IPC ratings

• High – These species have severe ecological impacts on physical processes, plant and animal communities, and vegetation structure. Their reproductive biology and other attributes are conducive to moderate to high rates of dispersal and establishment. Most are widely distributed ecologically.
• Moderate – These species have substantial and apparent-but generally not severe-ecological impacts on physical processes, plant and animal communities, and vegetation structure. Their reproductive biology and other attributes are conducive to moderate to high rates of dispersal, though establishment is generally dependent upon ecological disturbance. Ecological amplitude and distribution may range from limited to widespread.
• Limited – These species are invasive but their ecological impacts are minor on a statewide level or there was not enough information to justify a higher score. Their reproductive biology and other attributes result in low to moderate rates of invasiveness. Ecological amplitude and distribution are generally limited, but these species may be locally persistent and problematic.
• Alert – An Alert is listed on species with High or Moderate impacts that have limited distribution in California, but may have the potential to spread much further.
• Watch – These species have been assessed as posing a high risk of becoming invasive in the future in California.

CDFA Rating: A*?

Explanation of CDFA ratings

• A – A pest of known economic or environmental detriment and is either not known to be established in California or it is present in a limited distribution that allows for the possibility of eradication or successful containment.
• B – A pest of known economic or environmental detriment and, if present in California, it is of limited distribution.
• C – A pest of known economic or environmental detriment and, if present in California, it is usually widespread.
• D – An organism known to be of little or no economic or environmental detriment, to have an extremely low likelihood of weediness, or is known to be a parasite or predator.
• Q – An organism or disorder suspected to be of economic or environmental detriment, but whose status is uncertain because of incomplete identification or inadequate information.
• * – An asterisk next to the rating indicates that a plant is included in the CCR Section 4500 list of California State Noxious Weeds.
• -* – This plant is included in the CCR Section 4500 list of California State Noxious Weeds, but is otherwise not rated.

More details

Assessment(s)

Plant Assessment Form - Information gathered by Cal-IPC on the impacts, rate of spread, and distribution of invasive plants in California. Does not include management information.

Special Reports

Weed Management Notes

• Management Notes - Information on management techniques and effectiveness from the University of California Cooperative Extension’s Weed Research & Information Center.

Cal-IPC Newsletter Articles

• Aquatic weed management in the Delta. Johnson, Doug. Vol 31, Issue 1
• Weed biological control agents approved for California. Pitcairn, Michael J.; Smith, Lincoln; Moran, Patrick. Vol 22, Issue 1

Cal-IPC Symposium Presentations

Presentations are linked where available. Where a presentation is not available, find more information by reading the abstract in the Cal-IPC Symposia Archive.

• Biological control of aquatic weeds in California: Status update and a look to the future. Pratt, Paul (2023)
• Cross-county collaboration on weed management for success on Delta habitat restorations. Davis, Elizabeth; Weise, Ben; Williams, Amy (2022)
• Continued management and monitoring of alligatorweed (Alternanthera philoxeroides) in the Sacramento-San Joaquin Delta and Suisun Marsh. Kwong, Michael; Earl-Gould, Leslie; Grewell, Brenda; Martinez, Jose (2020)
• The role of public gardens as sentinels of plant invasion. Culley, Theresa; Dreisilker, Kurt; Ryan, Clair; Landel, Hans; Cavallin, Nadia; Arcate Schuler, Jessica; Shultz, Brittany; Gettig, Roger; Havens, Kayri (2020)
• The sky is not the limit – controlling invasive plants with survey and spray drones. Takekawa, John; Edmunds, Tim; Reynolds, Bill; Potter, Chris; Chappell, Steven (2020)
• Weed alerts and other invasive plant highlights for 2020. Burger, Jutta; Price, Robert (2020)
• Alligatorweed in the Delta an early detection rapid response story. O'Brien, Jon (2018)
• New weeds in California: An update. Kelch, Dean (2017)
• Hydrilla: A case study: The state of California’s noxious weed eradication programs. O'Connell, Ross A. (1997)
• Use of biologically based methods to control pest plants:Issues related to federal research, regulation, and implementation. Chornesky, Elizabeth A.; Palmer, Cynthia M. (1995)

Other Alternanthera philoxeroides Information

• Bugwood - National database from the Center for Invasive Species and Ecosystem Health at the University of Georgia.
• CalPhotos - Images of plants taken mostly in California.
• Calflora - Distribution map and records of this species in California.
• CalWeedMapper - Distribution map of this species in California with ability to determine regional priorities.
• EDDMapS - Distribution of this species in North America.
• GBIF - Global distribution information.
• Jepson Interchange - Information on this plant's taxonomy, biology, and distribution from UC Berkeley's Jepson Herbarium.
• USDA PLANTS Database - Information on identification and distribution, with links to websites in individual states.

Additional photos