The spring tree pollen season continues and reaches peak levels in April. Because of variety of trees that are pollinating by mid-April, the total airborne pollen concentrations are higher than any other month of the year. Late April sees the start of pollen release in hickory/pecan, black walnut, and Osage orange trees. The following are the most important tree species that typically pollinate in April:
Sycamore (Platanus americana)
Ash (Fraxinus americana and F. pennsylvanica)
Cottonwood (Populus deltoides and P. alba)
Hackberry (Celtis laevigata and C. occidentalis)
River birch (Betula nigra)
Mulberry (Morus alba and M. rubra)
Pine (Pinus spp.)
Willow (Salix spp.)
Sweetgum (Liquidamber styraciflua)
Hickory and Pecan (Carya spp.)
Black walnut (Juglans nigra)
Osage orange (Maclura pomifera)
From this list special attention should be paid to oak. There are 26 oak species native to Oklahoma. In addition, other oak species are used as ornamentals. Oak pollen is allergenic, and it is frequently the most abundant pollen on a cumulative yearly total.
In addition to all the tree pollen, we begin seeing some grass pollen and some weed pollen (sedges, dock, and nettle) present in the atmosphere.