White Oak flowers in the spring, at about the same time as the leaves emerge.

All members of the Artemisia genus produce large amounts of highly allergenic pollen during the mid-summer and into the fall.

This hickory is distinct because of its bark, sections of which peel away from the trunk (giving the species its common name).

Sheep Sorrel is small, creeping weed that occurs in all US states, and many states list it as an invasive species. It has a reputation as a medicinal herb for various ailments, but its main health impact is as an allergenic plant.

Ragweed pollen is a severe allergen – as little as 10 pollen grains per cubic meter can cause problems in an allergic individual.

Stemphylium solani is a plant fungus found throughout the world. It is a particular concern for growers of tomatoes, potatoes, peppers and cotton.

Sweet Gum is a very distinct species that becomes a large tree under suitable growing conditions, which typically involve moist soils.

Timothy produces very large amounts of highly allergenic pollen. A given flower head will produce pollen only over two weeks or so, but flower heads may bloom at different times, so the actual pollen season may be much longer.

Rough Marsh Elder is rather closely related to the Ragweeds and superficially resembles Giant Ragweed, but the bracts extending from the flowering spike easily distinguish it.