The dominant vegetation in this zone is woody and the more important species are Quercus stellata (post oak), Quercus marilandica (blackjack oak), Juniperus virginiana (eastern red cedar), Ulmus spp. (elm) and Populus deltoides (cottonwood). The pollen from all of these species are known to cause hay fever problems. The post oak-blackjack oak community is often referred to as the Cross-Timbers due to the density of the vegetation. The Post Oak-Blackjack Oak community intergrades with the Tallgrass Prairie in central Oklahoma.
Juniperus ashei (mountain cedar), which is restricted to the Arbuckle Mountains, is found in this zone and causes a serious type of winter hay fever.
Close to 80% of the major causes for hay fever in Oklahoma occur in this zone. The moisture level also allows for herbaceous forms such as Cynodon dactylon (bermuda grass), Festuca pratensis (meadow fescue), Phleum pratensis (timothy), Rumex acetosella (sheep sorrel) and R. crispus (curly or yellow dock), all of which are major allergens. Species of Ambrosia and other weeds can be abundant in the area.