Alternantera philoxeroides, or Alligator weed, is common perennial throughout the southeastern United States and California.
Its leaves are thin and oblong, and its spiky white flowers top leafless stalks. An invasive to North America, it is considered a noxious weed in many places, where it forms dense mats in standing water that become a haven for mosquitoes. At the same time, it tolerates the dry sandy soils of coastal dunes.
Introduced to Alabama in 1897, it reached Texas in 1975.
https://nas.er.usgs.gov/queries/factsheet.aspx?SpeciesID=227
