Ammannia robusta is a species of flowering plant in the loosestrife family known by the common name grand redstem.

It is widespread across much of North America (from British Columbia and Ontario to Guatemala) with additional populations in southeastern Brazil.

[3][4] Ammannia robusta grows in moist places, such as ditches and pond margins.

It is an annual herb reaching up to 100 cm in height when growing erect.

The flowers are urn-shaped with light lavender petals about 5 mm long and protruding stamens with yellow anthers.