Observation of the Month: Alligatorweed (Alternanthera philoxeroides) Amaranthaceae It’s on the official list of California noxious weeds, so the discovery of Alligatorweed ( Alternanthera Alligator weed Alligator weed is most commonly found spread across the surface of a body of water described in a sprawling fashion. It can be found in terrestrial areas around gardens or in
Observation of the Month: Alligatorweed (Alternanthera philoxeroides) Amaranthaceae
It’s on the official list of California noxious weeds, so the discovery of Alligatorweed (Alternanthera philoxeroides) in San Diego County was not welcome news. The observation posted by Jorge Ayon (chrysaetos) a few months ago brought the plant to the attention of Jon Rebman and with Jorge’s assistance, he obtained a voucher specimen to confirm the identification and document the plant’s presence in our county. Early detection of invasive species such as Alligatorweed is important to allow land managers to gain control of the population before it grows to the point where it cannot be eradicated locally and before it spreads to other locations.
Alligatorweed can form dense floating mats when it invades lakes, ponds, streams, and irrigation ditches. It is native to South America. According to Cal-IPC, it was introduced in California when it was previously used in the aquarium trade. Efforts are underway to eradicate it in the San Joaquin Valley and Sacramento Delta areas.
The leaves of Alligatorweed are elliptic, less than an inch wide and 5 inches long, and are opposite with entire margins. The inflorescence is spherical about ½ inch in diameter with papery white flowers. Outside its native range, Alligatorweed is not known to produce seeds, but it still reproduces and spreads very easily by vegetative fragments.
Observations posted to iNat can be the source of crucial information about the presence of new invasive species. The only known location of Alligatorweed in San Diego County is the Otay Lake area. But be on the lookout and if you spot any plants that resemble Alligatorweed, be sure to take multiple photos and post them to iNat with accurate GPS coordinates.
Thank for sharing @milliebasden
Very interesting read. It’s great to know iNat is being used to keep tabs on invasive species in our county.
Unfortunately, the problem is worse than I had originally suspected and Alligatorweed has spread to other parts of the lake: https://www.inaturalist.org/observations/67029749
Thanks to Jorge for posting it on iNat and getting the word out. Is there any attempt to eradicate it from Otay Lakes?
Sorry to hear there is more of it. I don’t know what the plan is for tackling it.
Alligator weed is most commonly found spread across the surface of a body of water described in a sprawling fashion. It can be found in terrestrial areas around gardens or in between rows of crops with sufficient moisture present. Stems are pink and hollow and can reach lengths of 1 m with opposite narrow elliptical leaves. Flowers are reduced and white in color, have thin petals, and are on stems that extend 4-5 inches away from the plant.
Alligator weed grows in thick dense mats along the shoreline of lakes and streams creating difficulty for wildlife to access the edge of the water. Alligator weed doesn’t provide a sufficient food source or shelter for aquatic wildlife. By preventing native plants from growing, alligator weed removes necessary food sources and shelter for native animals.
Alligator weed is able to spread and reproduce rapidly through stems or leaf cuttings making it difficult to eradicate in areas once established because it can grow from small portions of the plant left behind. Alligator weed propagates most commonly from stolons vegatatively with each individual node capable of propagating allowing for rapid spread and propagation of the plant.
Alligator grass originated in South America, but was transferred to the United States through water ways accidentally. The exact date when the weed was transferred to the United States is not known. It became noticeable on a pest status in 1959 when the U.S. Army Corps of Engineers investigated the damage potential caused by propagation of alligator weed. Investigation into damage potential of alligator weed resulted in placement on the federally noxious weed list for several states preventing further propagation and distribution.
U.S. Present: Alligator weed is found in the southwest United States from San Joaquin Valley south to Los Angeles and across the south and east portions of the continent south to Central America.
U.S. Habitat: Alligator weed can grow in a variety of habitats from dry to immersed in water, but the preferred habitat is aquatic. In the United States alligator weed is most often found growing along the surface of streams and ponds at the shores edge.
Physical removal of alligator weed is possible, but not usually 100% successful in eradicating the weed because the plant is able to re-grow and propagate from stem fragments alone. There are currently no biological control methods of eradication rather than goats which can keep the plant under control by feeding on the weed. Chemical control has been found to be the most successful when containing fluridone or imazapyr. Other chemical treatments have been found slightly less successful, but still effective when containing: 2,4-D, glyphosate, triclopyr, and imazamox. Systematic herbicides such as Navigate and Weedar 64 are successful chemical treatments as well.
Andres, L. A. 1977. The economics of biological control of weeds. Aquatic Botany 3: 111-123.
Barreto, R., R. Charudattan, A. Pomella, and R. Hanada. 2000. Biological control of neotropical aquatic weeds with fungi. Crop Protection 19: 697-703.
Buckingham, G. R. 1996. Biological control of alligatorweed, Alternanthera philoxeroides, the world’s first aquatic weed success story. Castanea 61: 231-243.
Holcomb, G. E. 1978. Alternaria alternantherae from alligatotorweed is also pathogenic on ornamental Amaranthaceae species. Phytopathology 68: 265-266.
Pemberton, R. W. 2000. Predictable risk to native plants in weed biological control. Oecologia 125: 489-494.
Zeiger, C. F. 1967. Biological control of alligatorweed with Agasicles n. sp. in Florida. Hyacinth Control J. 6: 31-34.