The spring tree pollen season winds down in May. Air samples will record some oak, mulberry, and other April pollen types but at much lower concentrations than in April. Black walnut, hickories, and Osage orange pollen which began to appear in the air during mid to late April, typically reach peak levels in May. Grass pollen is an important contributor to airborne pollen levels in May as many of the native grasses are flowering. The yearly peak of the grass pollen season occurs in late May; however, grass pollen will continue to be found in air samples as many grasses continue to release pollen during summer and into the fall. Weeds that releasing pollen in May include sedges (Cyperaceae), dock (Rumex sp) and nettle (Urticaceae). For more information see the May Pollen page.