Oak-Hickory-Pine
In the eastern portion of the state are primarily oak-hickory-pine similar to the vegetation of Arkansas and southern Missouri. Of the 26 oak species reported for Oklahoma, 21 are found in or right outside this zone. Identifying the pollen as oak is sufficient when testing for allergens because inbreeding is so prevalent in this genus.
The hickory of greatest significance in hay fever is Carya illoensis, or pecan. It is found along streams and in moist bottomlands. After the spring rains help clear the pollen from the air, this tree begins its brief reproductive period.
Recent literature would suggest that Ulmus americana (American elm) is of moderate importance. Due to Dutch elm disease, in many areas nearly 80% of American elms have died and were almost put on the endangered species list. In Oklahoma, however, American elms are still a major hay fever problem and the pollen is abundant.
In Oklahoma, 11 species of grasses are considered major allergens, all occurring in zone one. Some were introduced and have become established with the native flora, while others are cultivated in lawns and prairies. The most important grasses in this area are Cynodon dactylon (bermuda grass) and Fetuca pratensis (meadow fescue). Artemisia ludoviciana (white sage) and Amaranthus spp (pigweed) and Ambrosia spp (ragweed) are weeds mostly associated with the numerous disturbed habitats, the edges of forests and clearings.