Due to the presence of mesquite, Prosopis juliflora (also known as Neltuma juliflora), this area has been placed into a separate zone. Mesquite plants are small trees or shrubs in the Fabaceae (bean family). 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. Juniperus pinchotii (red-berry juniper) is also found in this area.
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.