Coastal Prairie is located in southern Texas and across the Rio Grande in Mexico. These two prairie associations have many dominants and sub-dominants in common. The northwestern margin of the coastal prairie joins the mixed prairie in a broad ecotone and on the west it also joins with the desert plains grassland in a broad ecotone which extends southward into Mexico. The coastal prairie is the wettest of our prairie associations and, except for the desert plains grassland, is the warmest. Hence, many species occur here that are not found to the north or west.
Grasses grow taller here than in cooler, drier regions.
The coastal prairie has a very large number of dominants, some of its own and some in common with each adjacent association. Heavy rainfall tends to obscure the difference between mid and tall grasses. Hence, little bluestem may reach 2 meters and become, in fact, a tallgrass. The coastal prairie is richer in forbs than in True Prairie, containing most True Prairie species, and many additional ones. Spartina, Sporobolus, and other wetland grasses dominate.