The trees of major allergenic significance occurring in this zone are Populus deltoides (cottonwood) and Prosopis juliflora (mesquite). The more important allergenic grasses and weeds in Zone 4 are found here as well. Juniperus monosperma (one-seed juniper) and J. scopulorum (Rocky Mountain juniper) are found in the foothills of the Rockies in Cimarron County. Both species are more common west of Oklahoma and release abundant pollen in the spring. These are well known as major allergy-causing plants in New Mexico, Colorado, and other states.