Alligatorweed | Alternanthera philoxeroides

California Pest Rating for
white flower with green leaves
Alligatorweed | Alternanthera philoxeroides
Family:  Amaranthaceae
Pest Rating: A | Proposed Seed Rating: R

Initiating Event:

There was a recent find of alligatorweed in Solano County; this is the first detection in northern California in many years.

History & Status:

Background: Alligatorweed is a perennial herb with horizontal to ascending stems to 1 m long, rooting at the nodes. The flowers are small and borne in small heads with white floral bracts. Like many aquatic emergent, it has distinctive submerged and emersed forms. The submerged form has hollow, floating, emergent and submerged stems. Terrestrial plants have solid stems. Typically, plants grow rooted in soil in shallow water and form dense, interwoven floating mats that extend over the surface of deeper water. Mats can become quite dense and nearly impenetrable. The floating mats can break away and follow currents to colonize new sites. Mats disrupt the natural ecology of a site by reducing light penetration and crowding out native species. Serious infestations can create anoxic, disease-promoting, and mosquito-breeding conditions.

Worldwide Distribution: This weed is found in wet, disturbed areas. It is also a weed of rice and sugar cane fields in tropical and subtropical regions. Native to southern Asia, alligatorweed is now found in tropical and subtropical regions around the world. It is considered an invasive species in Australia, China, New Zealand, and Thailand. Alligatorweed has been introduced throughout the southeastern United States from Virginia to Texas.

Official Control: Alligatorweed has had a CDFA rating of A as a pest in California for decades. The population in Los Angeles County has been managed intermittently over the years by the county, but it still persists.  It has official status as a weed in Alabama, Arizona, Arkansas, California, Florida, South Carolina, and Texas.

California Distribution:  Alligatorweed occurs in several southern California counties. It also has been detected in Contra Costa and Kings Counties, where it is eradicated. There was a recent find of 2 colonies in southern Solano County.

California Interceptions: Alligatorweed has been sent to CDFA by land managers.

The risk Alternanthera philoxeroides (alligatorweed) poses to California is evaluated below.

Consequences of Introduction:

1) Climate/Host Interaction: Risk is Medium (2), as the plant occurs in wetlands such as the Delta and creeks and rivers, as well as irrigation canals and watering ponds. These habitats are limited but widely distributed in California.

Evaluate if the pest would have suitable hosts and climate to establish in California:

– Low (1) Not likely to establish in California; or likely to establish in very limited areas.

Medium (2) may be able to establish in a larger but limited part of California.

– High (3) likely to establish a widespread distribution in California.

2) Known Pest Host Range: Most plants do not require any one host, but grow wherever ecological conditions are favorable. It receives a High (3) in this category.

Evaluate the host range of the pest.

– Low (1) has a very limited host range.

– Medium (2) has a moderate host range.

High (3) has a wide host range.

3) Pest Dispersal Potential: Risk is Medium (2). Alligatorweed can spread rapidly via water movement and on boats and equipment as stem fragments. It is also grown as an aquarium plant and occasionally discarded into waterways. Seeds evidently are not produced in the United States.

Evaluate the natural and artificial dispersal potential of the pest.

– Low (1) does not have high reproductive or dispersal potential.

Medium (2) has either high reproductive or dispersal potential.

– High (3) has both high reproduction and dispersal potential.

4) Economic Impact: Risk is High (3), as the plant can lower crop yields in rice fields, trigger state or international quarantines, and force changes in cultural practices by blocking canals. It has spread widely in the southeast, and has proven difficult to eradicate both there and in California. Its mats can improve habitat for mosquito larvae, leading to larger mosquito populations.

Evaluate the economic impact of the pest to California using the criteria below.

Economic Impact: A, C, D, E, G

A. The pest could lower crop yield.

B. The pest could lower crop value (includes increasing crop production costs).

C. The pest could trigger the loss of markets (includes quarantines).

D. The pest could negatively change normal cultural practices.

E. The pest can vector, or is vectored, by another pestiferous organism.

F. The organism is injurious or poisonous to agriculturally important animals.

G. The organism can interfere with the delivery or supply of water for agricultural uses.

Economic Impact Score: 3

– Low (1) causes 0 or 1 of these impacts.

– Medium (2) causes 2 of these impacts.

High (3) causes 3 or more of these impacts.

5) Environmental Impact: Risk is High (3) as alligatorweed could further invade the water systems of California, disrupt natural wetland communities and potentially lower biodiversity by invading wetlands. The dense growth impedes water movement, blocks the growth of native plants, and reduces available habitat for water birds and fish. Its invasion in the Delta and its tributaries could degrade habitat of rare species such as Mason’s lilaeopsis (Lilaeopsis masonii), Sacramento River Chinook salmon (Oncorhynchus tshawytscha),  and Giant garter snake (Thamnophis gigas).  Its presence would trigger additional control measures.

Evaluate the environmental impact of the pest on California using the criteria below.

Environmental Impact: A, C, D

A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.

B. The pest could directly affect threatened or endangered species.

C. The pest could impact threatened or endangered species by disrupting critical habitats.

D. The pest could trigger additional official or private treatment programs.

E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Environmental Impact: Score: 3

– Low (1) causes none of the above to occur.

– Medium (2) causes one of the above to occur.

High (3) causes two or more of the above to occur.

Consequences of Introduction to California for Alternanthera philoxeroides (alligatorweed) : High (13)

-Low = 5-8 points

-Medium = 9-12 points

High = 13-15 points

6) Post Entry Distribution and Survey Information: Alligatorweed currently is known from 3-4 populations in northern and southern California. It receives a Medium (-2) in this category.

Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included:

Score: -2

-Not established (0) Pest never detected in California, or known only from incursions.

-Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).

Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.

-High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Final Score:

The final score is the consequences of introduction score minus the post entry distribution and survey information score: Medium (11)


Uncertainty is low, as alligatorweed has established in wetlands in California and other states. There is some uncertainty as to the actual distribution of this plant in California, as, like some other aquatic weeds, it is likely to be overlooked.

Conclusion and Rating Justification:

Conclusions of the harm(s) associated with this pest to California using all of the evidence presented above:

Proposed Rating: Despite its limited ability to disperse between watersheds, this is a potentially significant weed in California of both natural wetlands and irrigation canals. Because of its potential economic impacts, it deserves an A rating, as it has proven tenacious and is actively spreading.


Baldwin, B. G., D. H. Goldman, D. J. Keil, R. Patterson, T. J. Rosatti, and D. H. Wilken, editors. 2012. The Jepson manual: vascular plants of California, second edition. University of California Press, Berkeley.

Consortium of California Herbaria. Accessed 10/3/2017:

Florida Dept of Agriculture Weed of the Month: Alternanthera sessilis. Accessed 10/3/2017:

Invasive Plant Atlas of the Mid-South. alligatorweed. Accessed 10/3/2017:

Responsible Party:

Dean G. Kelch, Primary Botanist; California Department of Food and Agriculture; 1220 N Street, Sacramento, CA 95814; Tel. (916) 403-6650.[@]


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Pest Rating: A | Proposed Seed Rating: R

Posted by ls