Flaveria trinervia is a species of flowering plant in the aster family known by the common names clustered yellowtops, speedyweed, and yellow twinstem. It is native to parts of the Americas, including the southeastern and southwestern United States (Florida, Texas, Arizona, New Mexico), most of Mexico, Belize, and parts of the Caribbean, but it is known in many other places as an introduced species and often a noxious weed, such as in Hawaii.
Flaveria trinervia grows easily in many types of wet habitats, including saline and alkaline soils and highly disturbed habitat. This is an annual herb growing erect and known to exceed two meters (7 feet) in height. The lance-shaped to oval leaves are each up to 15 centimeters (8 inches) long and arranged oppositely in pairs around the stem, their bases sometimes fused together. The edges of the leaves generally have tiny widely spaced teeth. The inflorescence is a large dense cluster of many very small flower heads, sometimes over 300 in one cluster. Each flower head contains 0-1 yellow or whitish ray floret and 0-2 yellow disc florets.
This plant exhibits C4 carbon fixation.
The Australian species F. australasica is very similar and possibly closely related to F. trinervia. Some authors treat the two as distinct species while others consider F. australasica as a synonym of F. trinervia.