Due to the presence of Prosopis juliflora (mesquite), this area has been placed into a separate zone. The boundary of this zone is obscure because of an extensive mesquite dominated association covering a large area of Texas. P. juliflora is reported to be the major cause of hay fever in west Texas.
Herbacious species include Buchloe dactyloides (buffalo grass), Bouteloua spp (grama grasses), Artemisia spp (sages) and Ambrosia spp (ragweeds), are found in this zone and are all important allergens.
Hay Fever Map