Observation of the Month: Alligatorweed (Alternanthera philoxeroides) Amaranthaceae

https://www.inaturalist.org/observations/58476888 by @chrysaetos

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.

Posted on January 11, 2021 10:58 PM by milliebasden